Battery Knowledge

Monday 25 april 1 25 /04 /Apr 04:02

We often get puzzled by announcements of new batteries that are said to offer very high energy densities, deliver 1000 charge/discharge cycle and are paper-thin. Are they real?  Perhaps — but not in one and the same battery. While one battery type may be designed for small size and long runtime, this pack will not last and wear out prematurely. Another battery may be built for long life, but the size is big and bulky. A third battery may provide all the desirable attributes, but the price would be too high for commercial use.

Battery manufacturers are well aware of customer needs and have responded by offering packs that best suit the specific applications. The mobile phone industry is an example of clever adaptation. Emphasis is placed on small size, high energy density and low price. Longevity comes in second.

The inscription of NiMH on a battery pack does not automatically guarantee high energy density. A prismatic Nickel-Metal Hydride battery for a mobile phone, for example, is made for slim geometry. Such a pack provides an energy density of about 60Wh/kg and the cycle count is around 300. In comparison, a cylindrical NiMH offers energy densities of 80Wh/kg and higher. Still, the cycle count of this battery is moderate to low. High durability NiMH batteries, which endure 1000 discharges, are commonly packaged in bulky cylindrical cells. The energy density of these cells is a modest 70Wh/kg.

Compromises also exist on lithium-based batteries. Li‑ion packs are being produced for defense applications that far exceed the energy density of the commercial equivalent. Unfortunately, these super-high capacity Li‑ion batteries are deemed unsafe in the hands of the public and the high price puts them out of reach of the commercial market.

In this article we look at the advantages and limitations of the commercial battery. The so-called miracle battery that merely live in controlled environments is excluded. We scrutinize the batteries not only in terms of energy density but also longevity, load characteristics, maintenance requirements, self-discharge and operational costs. Since NiCd remains a standard against which other batteries are compared, we evaluate alternative chemistries against this classic battery type.

Nickel Cadmium (NiCd) — mature and well understood but relatively low in energy density. The NiCd is used where long life, high discharge rate and economical price are important. Main applications are two-way radios, biomedical equipment, professional video cameras and power tools. The NiCd contains toxic metals and is environmentally unfriendly.

Nickel-Metal Hydride (NiMH) — has a higher energy density compared to the NiCd at the expense of reduced cycle life. NiMH contains no toxic metals. Applications include mobile phones and laptop computers.

Lead Acid — most economical for larger power applications where weight is of little concern. The lead acid battery is the preferred choice for hospital equipment, wheelchairs, emergency lighting and UPS systems.

Lithium Ion (Li‑ion) — fastest growing battery system. Li‑ion is used where high-energy density and lightweight is of prime importance. The technology is fragile and a protection circuit is required to assure safety. Applications include notebook computers and cellular phones.

Lithium Ion Polymer (Li‑ion polymer) — offers the attributes of the Li-ion in ultra-slim geometry and simplified packaging. Main applications are mobile phones.

Figure 1 compares the characteristics of the six most commonly used rechargeable battery systems in terms of energy density, cycle life, exercise requirements and cost. The figures are based on average ratings of commercially available batteries at the time of publication.

  NiCd NiMH Lead Acid Li-ion Li-ion polymer Reusable
Alkaline
Gravimetric Energy Density(Wh/kg) 45-80 60-120 30-50 110-160 100-130 80 (initial)
Internal Resistance 
(includes peripheral circuits) in mΩ
100 to 2001
6V pack
200 to 3001
6V pack
<1001
12V pack
150 to 2501
7.2V pack
200 to 3001
7.2V pack
200 to 20001
6V pack
Cycle Life (to 80% of initial capacity) 15002 300 to 5002,3 200 to 
3002
500 to 10003 300 to 
500
503 
(to 50%)
Fast Charge Time 1h typical 2-4h 8-16h 2-4h 2-4h 2-3h
Overcharge Tolerance moderate low high very low low moderate
Self-discharge / Month (room temperature) 20%4 30%4 5% 10%5 ~10%5 0.3%
Cell Voltage(nominal) 1.25V6 1.25V6 2V 3.6V 3.6V 1.5V
Load Current
-    peak
-    best result

20C
1C

5C
0.5C or lower

5C
0.2C

>2C
1C or lower

>2C
1C or lower

0.5C
0.2C or lower
Operating Temperature(discharge only) -40 to 
60°C
-20 to 
60°C
-20 to 
60°C
-20 to 
60°C
0 to 
60°C
0 to 
65°C
Maintenance Requirement 30 to 60 days 60 to 90 days 3 to 6 months9 not req. not req. not req.
Typical Battery Cost
(US$, reference only)
$50
(7.2V)
$60
(7.2V)
$25
(6V)
$100
(7.2V)
$100
(7.2V)
$5
(9V)
Cost per Cycle(US$)11 $0.04 $0.12 $0.10 $0.14 $0.29 $0.10-0.50
Commercial use since 1950 1990 1970 1991 1999 1992

Figure 1: Characteristics of commonly used rechargeable batteries

  1. Internal resistance of a battery pack varies with cell rating, type of protection circuit and number of cells. Protection circuit of Li‑ion and Li-polymer adds about 100mΩ.
  2. Cycle life is based on battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.
  3. Cycle life is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges.
  4. The discharge is highest immediately after charge, then tapers off. The NiCd capacity decreases 10% in the first 24h, then declines to about 10% every 30 days thereafter. Self-discharge increases with higher temperature.
  5. Internal protection circuits typically consume 3% of the stored energy per month.
  6. 1.25V is the open cell voltage. 1.2V is the commonly used value. There is no difference between the cells; it is simply a method of rating.
  7. Capable of high current pulses.
  8. Applies to discharge only; charge temperature range is more confined.
  9. Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.
  10. Cost of battery for commercially available portable devices.
  11. Derived from the battery price divided by cycle life. Does not include the cost of electricity and chargers.

Observation: It is interesting to note that NiCd has the shortest charge time, delivers the highest load current and offers the lowest overall cost-per-cycle, but has the most demanding maintenance requirements.

The Nickel Cadmium (NiCd) battery

The NiCd prefers fast charge to slow charge and pulse charge to DC charge. All other chemistries prefer a shallow discharge and moderate load currents. The NiCd is a strong and silent worker; hard labor poses no problem. In fact, the NiCd is the only battery type that performs well under rigorous working conditions. It does not like to be pampered by sitting in chargers for days and being used only occasionally for brief periods. A periodic full discharge is so important that, if omitted, large crystals will form on the cell plates (also referred to as memory) and the NiCd will gradually lose its performance.

Among rechargeable batteries, NiCd remains a popular choice for applications such as two-way radios, emergency medical equipment and power tools. Batteries with higher energy densities and less toxic metals are causing a diversion from NiCd to newer technologies.

Advantages and Limitations of NiCd Batteries

Advantages

Fast and simple charge — even after prolonged storage.

High number of charge/discharge cycles — if properly maintained, the NiCd provides over 1000 charge/discharge cycles.

Good load performance — the NiCd allows recharging at low temperatures.

Long shelf life – in any state-of-charge.

Simple storage and transportation — most airfreight companies accept the NiCd without special conditions.

Good low temperature performance.

Forgiving if abused — the NiCd is one of the most rugged rechargeable batteries.

Economically priced — the NiCd is the lowest cost battery in terms of cost per cycle.

Available in a wide range of sizes and performance options — most NiCd cells are cylindrical.

Limitations

Relatively low energy density — compared with newer systems.

Memory effect — the NiCd must periodically be exercised to prevent memory.

Environmentally unfriendly — the NiCd contains toxic metals. Some countries are limiting the use of the NiCd battery.

Has relatively high self-discharge — needs recharging after storage.

Figure 2: Advantages and limitations of NiCd batteries. 

The Nickel-Metal Hydride (NiMH) battery

Research of the NiMH system started in the 1970s as a means of discovering how to store hydrogen for the nickel hydrogen battery. Today, nickel hydrogen batteries are mainly used for satellite applications. They are bulky, contain high-pressure steel canisters and cost thousands of dollars per cell.

In the early experimental days of the NiMH battery, the metal hydride alloys were unstable in the cell environment and the desired performance characteristics could not be achieved. As a result, the development of the NiMH slowed down. New hydride alloys were developed in the 1980s that were stable enough for use in a cell. Since the late 1980s, NiMH has steadily improved.

The success of the NiMH has been driven by its high energy density and the use of environmentally friendly metals. The modern NiMH offers up to 40 percent higher energy density compared to NiCd. There is potential for yet higher capacities, but not without some negative side effects.

The NiMH is less durable than the NiCd. Cycling under heavy load and storage at high temperature reduces the service life. The NiMH suffers from high self-discharge, which is considerably greater than that of the NiCd.

The NiMH has been replacing the NiCd in markets such as wireless communications and mobile computing. In many parts of the world, the buyer is encouraged to use NiMH rather than NiCd batteries. This is due to environmental concerns about careless disposal of the spent battery.

Experts agree that the NiMH has greatly improved over the years, but limitations remain. Most of the shortcomings are native to the nickel-based technology and are shared with the NiCd battery. It is widely accepted that NiMH is an interim step to lithium battery technology.

Advantages and Limitations of NiMH Batteries

Advantages

30 – 40 percent higher capacity over a standard NiCd. The NiMH has potential for yet higher energy densities.

Less prone to memory than the NiCd. Periodic exercise cycles are required less often.

Simple storage and transportation — transportation conditions are not subject to regulatory control.

Environmentally friendly — contains only mild toxins; profitable for recycling.

Limitations

Limited service life — if repeatedly deep cycled, especially at high load currents, the performance starts to deteriorate after 200 to 300 cycles. Shallow rather than deep discharge cycles are preferred.

Limited discharge current — although a NiMH battery is capable of delivering high discharge currents, repeated discharges with high load currents reduces the battery’s cycle life. Best results are achieved with load currents of 0.2C to 0.5C (one-fifth to one-half of the rated capacity).

More complex charge algorithm needed — the NiMH generates more heat during charge and requires a longer charge time than the NiCd. The trickle charge is critical and must be controlled carefully.

High self-discharge — the NiMH has about 50 percent higher self-discharge compared to the NiCd. New chemical additives improve the self-discharge but at the expense of lower energy density.

Performance degrades if stored at elevated temperatures — the NiMH should be stored in a cool place and at a state-of-charge of about 40 percent.

High maintenance — battery requires regular full discharge to prevent crystalline formation.

About 20 percent more expensive than NiCd — NiMH batteries designed for high current draw are more expensive than the regular version.

Figure 3: Advantages and limitations of NiMH batteries

The Lead Acid battery

Invented by the French physician Gaston Planté in 1859, lead acid was the first rechargeable battery for commercial use. Today, the flooded lead acid battery is used in automobiles, forklifts and large uninterruptible power supply (UPS) systems.

During the mid 1970s, researchers developed a maintenance-free lead acid battery that could operate in any position. The liquid electrolyte was transformed into moistened separators and the enclosure was sealed. Safety valves were added to allow venting of gas during charge and discharge.

Driven by different applications, two battery designations emerged. They are the small sealed lead acid (SLA), also known under the brand name of Gelcell, and the large valve regulated lead acid (VRLA). Technically, both batteries are the same. (Engineers may argue that the word ‘sealed lead acid’ is a misnomer because no lead acid battery can be totally sealed.) Because of our emphasis on portable batteries, we focus on the SLA.

Unlike the flooded lead acid battery, both the SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging would cause gassing and water depletion. Consequently, these batteries can never be charged to their full potential.

The lead acid is not subject to memory. Leaving the battery on float charge for a prolonged time does not cause damage. The battery’s charge retention is best among rechargeable batteries. Whereas the NiCd self-discharges approximately 40 percent of its stored energy in three months, the SLA self-discharges the same amount in one year. The SLA is relatively inexpensive to purchase but the operational costs can be more expensive than the NiCd if full cycles are required on a repetitive basis.

The SLA does not lend itself to fast charging — typical charge times are 8 to 16 hours. The SLA must always be stored in a charged state. Leaving the battery in a discharged condition causes sulfation, a condition that makes the battery difficult, if not impossible, to recharge.

Unlike the NiCd, the SLA does not like deep cycling. A full discharge causes extra strain and each cycle robs the battery of a small amount of capacity. This wear-down characteristic also applies to other battery chemistries in varying degrees. To prevent the battery from being stressed through repetitive deep discharge, a larger SLA battery is recommended.

Depending on the depth of discharge and operating temperature, the SLA provides 200 to 300 discharge/ charge cycles. The primary reason for its relatively short cycle life is grid corrosion of the positive electrode, depletion of the active material and expansion of the positive plates. These changes are most prevalent at higher operating temperatures. Cycling does not prevent or reverse the trend.

The optimum operating temperature for the SLA and VRLA battery is 25°C (77°F). As a rule of thumb, every 8°C (15°F) rise in temperature will cut the battery life in half. VRLA that would last for 10 years at 25°C will only be good for 5 years if operated at 33°C (95°F). The same battery would endure a little more than one year at a temperature of 42°C (107°F).

Among modern rechargeable batteries, the lead acid battery family has the lowest energy density, making it unsuitable for handheld devices that demand compact size. In addition, performance at low temperatures is poor.

The SLA is rated at a 5-hour discharge or 0.2C. Some batteries are even rated at a slow 20-hour discharge. Longer discharge times produce higher capacity readings. The SLA performs well on high pulse currents. During these pulses, discharge rates well in excess of 1C can be drawn.

In terms of disposal, the SLA is less harmful than the NiCd battery but the high lead content makes the SLA environmentally unfriendly.

Advantages and Limitations of Lead Acid Batteries

Advantages

Inexpensive and simple to manufacture — in terms of cost per watt hours, the SLA is the least expensive.

Mature, reliable and well-understood technology — when used correctly, the SLA is durable and provides dependable service.

Low self-discharge —the self-discharge rate is among the lowest in rechargeable batterysystems.

Low maintenance requirements — no memory; no electrolyte to fill.

Capable of high discharge rates.

Limitations

Cannot be stored in a discharged condition.

Low energy density — poor weight-to-energy density limits use to stationary and wheeled applications.

Allows only a limited number of full discharge cycles — well suited for standby applications that require only occasional deep discharges.

Environmentally unfriendly — the electrolyte and the lead content can cause environmental damage.

Transportation restrictions on flooded lead acid — there are environmental concerns regarding spillage in case of an accident.

Thermal runaway can occur with improper charging.

Figure 4: Advantages and limitations of lead acid batteries. 

The Lithium Ion battery

Pioneer work with the lithium battery began in 1912 under G.N. Lewis but it was not until the early 1970s that the first non-rechargeable lithium batteries became commercially available. Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density per weight.

Attempts to develop rechargeable lithium batteries followed in the 1980s, but failed due to safety problems. Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density than lithium metal, the Li‑ion is safe, provided certain precautions are met when charging and discharging. In 1991, the Sony Corporation commercialized the first Li‑ion battery. Other manufacturers followed suit. Today, the Li‑ion is the fastest growing and most promising battery chemistry.

The energy density of the Li‑ion is typically twice that of the standard NiCd. Improvements in electrode active materials have the potential of increasing the energy density close to three times that of the NiCd. In addition to high capacity, the load characteristics are reasonably good and behave similarly to the NiCd in terms of discharge characteristics (similar shape of discharge profile, but different voltage). The flat discharge curve offers effective utilization of the stored power in a desirable voltage spectrum.

The high cell voltage allows battery packs with only one cell. Most of today’s mobile phones run on a single cell, an advantage that simplifies battery design. To maintain the same power, higher currents are drawn. Low cell resistance is important to allow unrestricted current flow during load pulses.

The Li‑ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery’s life. In addition, the self-discharge is less than half compared to NiCd, making the Li‑ion well suited for modern fuel gauge applications. Li‑ion cells cause little harm when disposed.

Despite its overall advantages, Li‑ion also has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the cell temperature is monitored to prevent temperature extremes. The maximum charge and discharge current is limited to between 1C and 2C. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.

Aging is a concern with most Li‑ion batteries and many manufacturers remain silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. Over two or perhaps three years, the battery frequently fails. It should be noted that other chemistries also have age-related degenerative effects. This is especially true for the NiMH if exposed to high ambient temperatures.

Storing the battery in a cool place slows down the aging process of the Li‑ion (and other chemistries). Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the battery should be partially charged during storage.

Manufacturers are constantly improving the chemistry of the Li‑ion battery. New and enhanced chemical combinations are introduced every six months or so. With such rapid progress, it is difficult to assess how well the revised battery will age.

The most economical Li-ion battery in terms of cost-to-energy ratio is the cylindrical 18650 cell. This cell is used for mobile computing and other applications that do not demand ultra-thin geometry. If a slimmer pack is required (thinner than 18 mm), the prismatic Li‑ion cell is the best choice. There are no gains in energy density over the 18650, however, the cost of obtaining the same energy may double.

For ultra-slim geometry (less than 4 mm), the only choice is Li‑ion polymer. This is the most expensive system in terms of cost-to-energy ratio. There are no gains in energy density and the durability is inferior to the rugged 18560 cell.

Advantages and Limitations of Li-ion Batteries

Advantages

High energy density — potential for yet higher capacities.

Relatively low self-discharge — self-discharge is less than half that of NiCd and NiMH.

Low Maintenance — no periodic discharge is needed; no memory.

Limitations

Requires protection circuit — protection circuit limits voltage and current. Battery is safe if not provoked.

Subject to aging, even if not in use — storing the battery in a cool place and at 40 percent state-of-charge reduces the aging effect.

Moderate discharge current.

Subject to transportation regulations — shipment of larger quantities of Li-ion batteries may be subject to regulatory control. This restriction does not apply to personal carry-on batteries.

Expensive to manufacture — about 40 percent higher in cost than NiCd. Better manufacturing techniques and replacement of rare metals with lower cost alternatives will likely reduce the price.

Not fully mature — changes in metal and chemical combinations affect battery test results, especially with some quick test methods.

Figure 5: Advantages and limitations of Li-ion batteries

The Lithium Polymer battery

The Li-polymer differentiates itself from other battery systems in the type of electrolyte used. The original design, dating back to the 1970s, uses a dry solid polymer electrolyte. This electrolyte resembles a plastic-like film that does not conduct electricity but allows an exchange of ions (electrically charged atoms or groups of atoms). The polymer electrolyte replaces the traditional porous separator, which is soaked with electrolyte.

The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile geometry. There is no danger of flammability because no liquid or gelled electrolyte is used. With a cell thickness measuring as little as one millimeter (0.039 inches), equipment designers are left to their own imagination in terms of form, shape and size.

Unfortunately, the dry Li-polymer suffers from poor conductivity. Internal resistance is too high and cannot deliver the current bursts needed for modern communication devices and spinning up the hard drives of mobile computing equipment. Heating the cell to 60°C (140°F) and higher increases the conductivity but this requirement is unsuitable for portable applications.

To make a small Li-polymer battery conductive, some gelled electrolyte has been added. Most of the commercial Li-polymer batteries used today for mobile phones are a hybrid and contain gelled electrolyte. The correct term for this system is Lithium Ion Polymer. For promotional reasons, most battery manufacturers mark the battery simply as Li-polymer. Since the hybrid lithium polymer is the only functioning polymer battery for portable use today, we will focus on this chemistry.

With gelled electrolyte added, what then is the difference between classic Li‑ion and Li‑ion polymer? Although the characteristics and performance of the two systems are very similar, the Li‑ion polymer is unique in that solid electrolyte replaces the porous separator. The gelled electrolyte is simply added to enhance ion conductivity.

Technical difficulties and delays in volume manufacturing have deferred the introduction of the Li‑ion polymer battery. In addition, the promised superiority of the Li‑ion polymer has not yet been realized. No improvements in capacity gains are achieved — in fact, the capacity is slightly less than that of the standard Li‑ion battery. For the present, there is no cost advantage. The major reason for switching to the Li-ion polymer is form factor. It allows wafer-thin geometries, a style that is demanded by the highly competitive mobile phone industry.

Advantages and Limitations of Li-ion Polymer Batteries

Advantages

Very low profile — batteries that resemble the profile of a credit card are feasible.

Flexible form factor — manufacturers are not bound by standard cell formats. With high volume, any reasonable size can be produced economically.

Light weight – gelled rather than liquid electrolytes enable simplified packaging, in some cases eliminating the metal shell.

Improved safety — more resistant to overcharge; less chance for electrolyte leakage.

Limitations

Lower energy density and decreased cycle count compared to Li-ion — potential for improvements exist.

Expensive to manufacture — once mass-produced, the Li-ion polymer has the potential for lower cost. Reduced control circuit offsets higher manufacturing costs.

By www. charger-battery.ca - Posted in: Battery Knowledge
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Monday 25 april 1 25 /04 /Apr 04:01
In 1995, Li-polymer surprised the battery world with a radical new design, the pouch cell. Rather than using a metallic cylinder and glass-to-metal electrical feed-through for insulation, conductive foil tabs welded to the electrode and sealed to the pouch carry the positive and negative terminals to the outside. Figure 1 illustrates such a pouch cell.

Figure 1: The pouch cell
The pouch cell offers a simple, flexible and lightweight solution to battery design. Exposure to high humidity and hot temperature can shorten service life.

Courtesy of Cadex
The pouch cell makes the most efficient use of space and achieves a 90 to 95 percent packaging efficiency, the highest among battery packs. Eliminating the metal enclosure reduces weight but the cell needs some alternative support in the battery compartment. The pouch pack finds applications in consumer, military, as well as automotive applications. No standardized pouch cells exist; each manufacturer builds the cells for a specific application.
Pouch packs are normally Li-polymer. The energy density can be lower and be less durable than Li-ion in the cylindrical package. Swelling as a result of gas generation during charge and discharge is a concern. Battery manufacturers insist that Li-ion batteries do not generate excess gases that can lead to swelling when properly used. Nevertheless, some swelling can occur and most is due to faulty manufacturing. The pressure from swelling can crack a battery cover open and in some cases break the display or electronic circuit. Manufacturers say that an inflated cell is safe. While this may be true, do not puncture a swollen cell in close proximity of fire or heat; the escaping gases can ignite. Figure 2 illustrates a pouch cell that has swelled.

Figure 2: Swelling pouch cell
Swelling can occur as part of gas generation. Battery manufacturers are at odds why this occasionally happens.

Courtesy of Cadex

To prevent swelling, the manufacturer adds excess film to create a “gas bag” outside the cell. During the first charge, gases escape into the gasbag, which is then cut off and the pack is resealed as part of the finishing process. Gas buildup on subsequent charges is minimal; nevertheless, when designing the battery compartment for pouch cells, provision must be made to allow for some expansion. It is best not to stack pouch cells but to lay them flat side by side. The battery compartment must be made to protect the cell from mechanical stress and be free of sharp edges.

Summary of Packaging Advantages and Disadvantages

  • A cell in a cylindrical metallic case has good cycling ability, offers a long calendar life, is economical to manufacture, but is heavy and has low packaging density.
  • The prismatic metallic case has improved packaging density but can be more expensive to manufacture, is less efficient in thermal management and may have a shorter cycle life.
  • The prismatic pouch pack is light and cost-effective to manufacture. Exposure to high humidity and hot temperature can shorten service life.
By www. charger-battery.ca - Posted in: Battery Knowledge
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Saturday 23 april 6 23 /04 /Apr 02:43

For many years, nickel-cadmium had been the only suitable battery for portable equipment from wireless communications to mobile computing. Nickel-metal-hydride and lithium-ion emerged In the early 1990s, fighting nose-to-nose to gain customer's acceptance. Today, lithium-ion is the fastest growing and most promising battery chemistry.

The lithium-ion battery

Pioneer work with the lithium battery began in 1912 under G.N. Lewis but it was not until the early 1970s when the first non-rechargeable lithium batteries became commercially available. lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density for weight.
 
Attempts to develop rechargeable lithium batteries failed due to safety problems. Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density than lithium metal, lithium-ion is safe, provided certain precautions are met when charging and discharging. In 1991, the Sony Corporation commercialized the first lithium-ion battery. Other manufacturers followed suit. 
 
The energy density of lithium-ion is typically twice that of the standard nickel-cadmium. There is potential for higher energy densities. The load characteristics are reasonably good and behave similarly to nickel-cadmium in terms of discharge. The high cell voltage of 3.6 volts allows battery pack designs with only one cell. Most of today's mobile phones run on a single cell. A nickel-based pack would require three 1.2-volt cells connected in series.
 
Lithium-ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery's life. In addition, the self-discharge is less than half compared to nickel-cadmium, making lithium-ion well suited for modern fuel gauge applications. lithium-ion cells cause little harm when disposed.
 
Despite its overall advantages, lithium-ion has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the cell temperature is monitored to prevent temperature extremes. The maximum charge and discharge current on most packs are is limited to between 1C and 2C. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.
 
Aging is a concern with most lithium-ion batteries and many manufacturers remain silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. The battery frequently fails after two or three years. It should be noted that other chemistries also have age-related degenerative effects. This is especially true for nickel-metal-hydride if exposed to high ambient temperatures. At the same time, lithium-ion packs are known to have served for five years in some applications. 
 
Manufacturers are constantly improving lithium-ion. New and enhanced chemical combinations are introduced every six months or so. With such rapid progress, it is difficult to assess how well the revised battery will age. 
 
Storage in a cool place slows the aging process of lithium-ion (and other chemistries). Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the battery should be partially charged during storage. The manufacturer recommends a 40% charge.
 
The most economical lithium-ion battery in terms of cost-to-energy ratio is the cylindrical 18650 (18 is the diameter and 650 the length in mm). This cell is used for mobile computing and other applications that do not demand ultra-thin geometry. If a slim pack is required, the prismatic lithium-ion cell is the best choice. These cells come at a higher cost in terms of stored energy.

Advantages

  • High energy density - potential for yet higher capacities.
  • Does not need prolonged priming when new. One regular charge is all that's needed.
  • Relatively low self-discharge - self-discharge is less than half that of nickel-based batteries.
  • Low Maintenance - no periodic discharge is needed; there is no memory.
  • Specialty cells can provide very high current to applications such as power tools.

Limitations

  • Requires protection circuit to maintain voltage and current within safe limits.
  • Subject to aging, even if not in use - storage in a cool place at 40% charge reduces the aging effect.
  • Transportation restrictions - shipment of larger quantities may be subject to regulatory control. This restriction does not apply to personal carry-on batteries.
  • Expensive to manufacture - about 40 percent higher in cost than nickel-cadmium.
  • Not fully mature - metals and chemicals are changing on a continuing basis.

The lithium polymer battery

The lithium-polymer differentiates itself from conventional battery systems in the type of electrolyte used. The original design, dating back to the 1970s, uses a dry solid polymer electrolyte. This electrolyte resembles a plastic-like film that does not conduct electricity but allows ions exchange (electrically charged atoms or groups of atoms). The polymer electrolyte replaces the traditional porous separator, which is soaked with electrolyte.

The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile geometry. With a cell thickness measuring as little as one millimeter (0.039 inches), equipment designers are left to their own imagination in terms of form, shape and size. 

Unfortunately, the dry lithium-polymer suffers from poor conductivity. The internal resistance is too high and cannot deliver the current bursts needed to power modern communication devices and spin up the hard drives of mobile computing equipment. Heating the cell to 60°C (140°F) and higher increases the conductivity, a requirement that is unsuitable for portable applications.

To compromise, some gelled electrolyte has been added. The commercial cells use a separator/ electrolyte membrane prepared from the same traditional porous polyethylene or polypropylene separator filled with a polymer, which gels upon filling with the liquid electrolyte. Thus the commercial lithium-ion polymer cells are very similar in chemistry and materials to their liquid electrolyte counter parts. 

Lithium-ion-polymer has not caught on as quickly as some analysts had expected. Its superiority to other systems and low manufacturing costs has not been realized. No improvements in capacity gains are achieved - in fact, the capacity is slightly less than that of the standard lithium-ion battery. Lithium-ion-polymer finds its market niche in wafer-thin geometries, such as batteries for credit cards and other such applications.

Advantages

  • Very low profile - batteries resembling the profile of a credit card are feasible.
  • Flexible form factor - manufacturers are not bound by standard cell formats. With high volume, any reasonable size can be produced economically.
  • Lightweight - gelled electrolytes enable simplified packaging by eliminating the metal shell.
  • Improved safety - more resistant to overcharge; less chance for electrolyte leakage.

Limitations

  • Lower energy density and decreased cycle count compared to lithium-ion.
  • Expensive to manufacture.
  • No standard sizes. Most cells are produced for high volume consumer markets.
  • Higher cost-to-energy ratio than lithium-ion

Restrictions on lithium content for air travel

Air travelers ask the question, "How much lithium in a battery am I allowed to bring on board?" We differentiate between two battery types: Lithium metal and lithium-ion. 
Most lithium metal batteries are non-rechargeable and are used in film cameras. Lithium-ion packs are rechargeable and power laptops, cellular phones and camcorders. Both battery types, including spare packs, are allowed as carry-on but cannot exceed the following lithium content: 
- 2 grams for lithium metal or lithium alloy batteries 
- 8 grams for lithium-ion batteries 

Lithium-ion batteries exceeding 8 grams but no more than 25 grams may be carried in carry-on baggage if individually protected to prevent short circuits and are limited to two spare batteries per person. 

How do I know the lithium content of a lithium-ion battery? From a theoretical perspective, there is no metallic lithium in a typical lithium-ion battery. There is, however, equivalent lithium content that must be considered. For a lithium-ion cell, this is calculated at 0.3 times the rated capacity (in ampere-hours). 

Example: A 2Ah 18650 Li-ion cell has 0.6 grams of lithium content. On a typical 60 Wh laptop battery with 8 cells (4 in series and 2 in parallel), this adds up to 4.8g. To stay under the 8-gram UN limit, the largest battery you can bring is 96 Wh. This pack could include 2.2Ah cells in a 12 cells arrangement (4s3p). If the 2.4Ah cell were used instead, the pack would need to be limited to 9 cells (3s3p).

Restrictions on shipment of lithium-ion batteries

  • Anyone shipping lithium-ion batteries in bulk is responsible to meet transportation regulations. This applies to domestic and international shipments by land, sea and air. 
  • Lithium-ion cells whose equivalent lithium content exceeds 1.5 grams or 8 grams per battery pack must be shipped as "Class 9 miscellaneous hazardous material." Cell capacity and the number of cells in a pack determine the lithium content. 
  • Exception is given to packs that contain less than 8 grams of lithium content. If, however, a shipment contains more than 24 lithium cells or 12 lithium-ion battery packs, special markings and shipping documents will be required. Each package must be marked that it contains lithium batteries.
  • All lithium-ion batteries must be tested in accordance with specifications detailed in UN 3090 regardless of lithium content (UN manual of Tests and Criteria, Part III, subsection 38.3). This precaution safeguards against the shipment of flawed batteries. 
  • Cells & batteries must be separated to prevent short-circuiting and packaged in strong boxes.
By www. charger-battery.ca - Posted in: Battery Knowledge
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Saturday 23 april 6 23 /04 /Apr 02:42

Batteries have a mind of their own. Their stubborn and unpredictable behavior has left many battery users in awkward situations. And yet, the battery is our steady travel companion that allows us to carry out our activities disconnected from home and office. In this paper we observe the battery in personal use and fleet applications.

The personal battery user

It is interesting to observe that batteries cared for by a single user generally last longer than those operating in an open fleet environment where everyone has access to but no one is accountable for them. A personal user is one who operates a mobile phone, a laptop or a video camera for pleasure or business. He or she will likely follow the recommended guidelines in caring for the battery. When the runtime gets low, the battery gets serviced or is replaced. Critical failures are rare because the owner adjusts to the performance of the battery and lowers the expectation as the battery ages.

The fleet battery user

The fleet user, on the other hand, has little personal interest in the battery and has no tolerance for a pack that is less than perfect. He simply grabs a battery from the charger and expects it to last through the shift. The battery is returned to the charger at the end of the day, ready for the next person. Regular battery maintenance is minimal and performance often starts to degrade after one year of service.

How can fleet batteries be made to last longer? I examined the US and the Dutch Army, both of which use fleet batteries. The US Army issues batteries with no maintenance program. If the battery fails, another pack is released, no questions asked. Little or no care is given and the failure rate is high.

The Dutch Army, on the other hand, has moved away from the open fleet system by making the soldiers responsible for their batteries. This change was made in an attempt to reduce operational costs and improve reliability. The batteries are issued to the soldiers and become part of their personal belongings. The results are startling. Since adapting this new regime, the failure rate has dropped considerably and the battery performance has increased. Unexpected down time has almost been eliminated.

It should be noted that the Dutch Army uses exclusively nickel-cadmium batteries. Each pack receives periodic maintenance on a battery analyzer (Cadex) to prolong service life. Batteries that do not meet the 80% target capacity setting are reconditioned; those that fall below target are replaced. The US Army, on the other hand, uses nickel-metal-hydride, a battery that has higher energy density but is less durable. The US army is evaluating lithium-ion batteries for the next generation battery.

What lack of battery maintenance can do

Batteries get checked when they no longer hold charge or the equipment is sent in for repair. In an effort to improve reliability and cut replacement costs, many organizations have adapted some type of battery maintenance. 

A user may feel that his or her battery works adequately during routine days, not knowing that the pack holds only half the capacity. A system must be fit to operate in unforeseen circumstances and emergencies where every watt of battery power is needed. Breakdowns during these critical moments are all too common and weak batteries are often to blame. The loss of adequate battery power is as detrimental as any other malfunction in the system.
I have recorded a number of stories in which lack of battery maintenance was evident:

Fire brigade - A fire brigade had chronic communication problems with two-way radios. The problems were most acute during call-outs lasting two hours and longer. Although their radios functioned on receive, the transmissions broke up and the calls did not get through. 
The fire brigade acquired a battery analyzer (Cadex) and all batteries were serviced through exercise and recondition methods. Batteries that did not recover to a set target capacity were replaced.

Shortly thereafter, the firefighters were summoned to a ten-hour call that demanded heavy radio traffic. To their astonishment, none of the radios failed. The success of this operation was credited to the good performance of their batteries. The following day, the captain of the fire brigade personally contacted the manufacturer of the battery analyzer and enthusiastically endorsed the use of the device.

Emergency response - A Cadex representative was allowed to view the State Emergency Management Facility of a large US city. In the fortified underground bunker, 1400 batteries were kept in chargers. The green lights glowed, indicating that the batteries where ready at a moment's notice. The officer in charge stood erect and confidently said, "We are prepared for any emergency".

The representative then asked the officer to hand over a battery to check the state-of-health. Within seconds, the battery analyzer detected a fail condition. In an effort to make good, the officer grabbed another battery from the charger but it failed too. Subsequent batteries also fell short.

nickel-based batteries placed on prolonged standby become inoperable due to memory in as little a three months. Scenarios such as these are common. Political hurdles and lack of funding often stand in the way of a quick solution. The only thing the officer can do is pray that no emergency will occur.

Army - Defense organizations take great pride in employing the highest quality and best performing equipment. When it comes to rechargeable batteries, however, there are exceptions. The battery often escapes the scrutiny of a full military inspection and only its visual appearance is checked. Maintenance is frequently ignored and little effort is made in keeping track of the battery's state of health, cycle count and age. In time, the soldiers begin carrying rocks instead of batteries. 

Figure 1: Results of battery neglect. 
The soldiers begin carrying rocks instead of batteries. Maintenance helps to keep deadwood out of military arsenal.

Batteries fooled the British Army during the Falkland War in 1982. The army assumed that a battery would always follow the rigid military specifications, even after long neglect. Not so. When the order was given to launch the portable missiles, nothing happened and the missiles did not fly that day. The batteries were dead.

Government services - An organization continually experienced failures with nickel-cadmium batteries. Although the batteries performed at 100% when new, the capacity dropped to 20% and lower in only one year. We discovered that their two-way radios were under-utilized; yet the batteries received a full recharge after each short field use.
After replacing the batteries, we advised the organization to exercise the batteries once per month through a full discharge. The first exercise occurred only after four month of service. Here is what we found:

The capacity on half of the batteries had dropped to 70-75%. With exercise and recondition (deep cycle), all batteries were fully restored (100%). Had maintenance been omitted for much longer, the probability of a full recovery would have been jeopardized.

Construction - I noticed fewer battery problems on two-way radios with construction workers than security guards. The construction workers often did not bother turning off their two-way radios at the end of the shift. As a result, the nickel-cadmium batteries got their needed exercise and kept performing until they fell apart from old age, often held together with duct tape.

In comparison, the security guards pampered their batteries to death by giving them light duty and plenty of recharge. These batteries still looked new when they had to be discarded after only 12 months of service. Because of the advanced memory, recondition was no longer effective.

Memory only occurs on nickel-based batteries, a phenomenon that can be corrected with periodic discharge cycles.

By www. charger-battery.ca - Posted in: Battery Knowledge
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Saturday 23 april 6 23 /04 /Apr 02:39
Li-ion batteries come in many varieties but all have one thing in common — the catchword “lithium-ion.” Although strikingly similar at first glance, these batteries vary in performance, and it’s mostly the cathode material that gives then their unique personality.

Unless you are a chemist, the names of the materials in a lithium-ion battery can get confusing. This article provides clarity by listing six of the most common lithium-ion batteries and giving examples of typical uses. Their full chemical names and colloquial short names are summarized in Table 1 below. 
 

Chemical name

Material

Abbreviation

Short form

Applications

Lithium Cobalt Oxide* Also Lithium Cobalate or lithium-ion-cobalt)

LiCoO2
(60% Co)

LCO

Li-cobalt

Cell phone laptop, camera

Lithium 
Manganese Oxide* 

Also Lithium Manganate 
or lithium-ion-manganese

LiMn2O4

LMO

Li-manganese, or spinel

 

Power tools, 
e-bikes, EV, medical, hobbyist.

Lithium 
Iron Phosphate*

LiFePO4

LFP

Li-phosphate

Lithium Nickel Manganese Cobalt Oxide*, also lithium-manganese-cobalt-oxide

LiNiMnCoO2
(10–20% Co)

NMC

NMC

Lithium Nickel Cobalt Aluminum Oxide*

LiNiCoAlO2
9% Co)

NCA

NCA

Gaining importance 
in electric powertrain and grid storage

Lithium Titanate**

Li4Ti5O12

LTO

Li-titanate

Table 1: Summary of names given to Li-ion batteries. The article will use the short form when appropriate.      *   Cathode material             **  Anode material

To learn more about the unique characters and limitations of the six lithium-ion families, we examine the batteries on hand of spider charts. We begin with Li-cobalt, the most common variety used in cellular phones and laptops. We then move to Li-manganese and Li- phosphate, batteries deployed in power tools, and finally address the newer players such as NME, NCA and Li-titanate.

Lithium Cobalt Oxide  (LiCoO2)

Li-cobalt is the most popular consumer battery. Its high specific energy provides satisfactory runtime for cell phones, laptops and digital cameras. The battery consists of a cobalt oxide cathode and a graphite carbon anode. The cathode has a layered structure and during discharge lithium ions move from the anode to the cathode. The flow reverses on charge. The drawback of Li-cobalt is a relatively short life span and limited load capabilities (specific power). Figure 2 illustrates the structure.
 
Li-cobalt structure

Figure 2Li-cobalt structure
The cathode has a layered structure. During discharge the lithium ions move from the anode to the cathode; on charge the flow is from anode to cathode.

Courtesy of Cadex

Li-cobalt cannot be charged and discharged at a current higher than its rating. This means that an 18650 cell with 2,400mAh can only be charged and discharged at 2,400mA. Forcing a fast charge or applying a load higher than 2,400mA causes overheating and undue stress. For optimal fast charge, the manufacturer recommends a C-rate of 0.8C or 1920mA. The mandatory battery protection circuit limits the charge and discharge rate to a safe level of about 1C.

Figure 3 summarizes the performance of Li-cobalt in terms of specific energy, or capacity; specific power, or the ability to deliver high current; safety; performance at hot and cold temperatures; life span reflecting cycle life and longevity; and cost. The hexagonal spider web provides a quick and easy performance analysis of the battery characteristics.
 
Snapshot of an average Li-cobalt battery

Figure 3Snapshot of an average Li-cobalt battery
Li-cobalt excels on high specific energy but offers only moderate specific power, safety and life span.

Courtesy of Cadex

Lithium Manganese Oxide (LiMn2O4)

Lithium insertion in manganese spinels was first published in the Materials Research Bulletin in 1983. In 1996, Moli Energy commercialized a Li-ion cell with lithium manganese oxide as a cathode material. The architecture forms a three-dimensional spinel structure that improves ion flow on the electrode, which results in lower internal resistance and improves current handling. A further advantage of spinel is high thermal stability and enhanced safety, but the cycle and calendar life is limited.

Low internal cell resistance is key to fast charging and high-current discharging. In an 18650 package, Li-manganese can be discharged at currents of 20–30A with moderate heat buildup. It is also possible to apply one-second load pulses of up to 50A. A continuous high load at this current would cause heat buildup and the cell temperature cannot exceed 80°C (176°F). Li-manganese is used for power tools, medical instruments, as well as hybrid and electric vehicles. 

Figure 4 shows the crystalline formation of the cathode in a three-dimensional framework. This spinel structure, which is usually composed of diamond shapes connected into a lattice, appears after initial formation.
 
Li-manganese structure

Figure 4: Li-manganese structure
The cathode crystalline formation of lithium manganese oxide has a three-dimensional framework structure that appears after initial formation. Spinel provides low resistance but has a more moderate specific energy than cobalt. 

Courtesy of Cadex

Li-manganese has a capacity that is roughly one-third lower compared to Li-cobalt but the battery still offers about 50 percent more energy than nickel-based chemistries. Design flexibility allows engineers to maximize the battery for either optimal longevity (life span), maximum load current (specific power) or high capacity (specific energy). For example, the long-life version in the 18650 cell has a moderate capacity of 1,100mAh; the high-capacity version is 1,500mAh but has a reduced service life. Laptop manufacturers would likely choose the high capacity version for maximum runtime; where as the maker of cars with the electric powertrain would take the long-life version with high specific power and sacrifice on runtime. 

Figure 5 shows the spider web of a typical Li-manganese battery. In this chart, all characteristics show as marginal, however, newer designs have improved in terms of specific power, safety and life span.
 
Snapshot of a typical Li-manganese battery

Figure 5: Snapshot of a typical Li-manganese battery 
Although moderate in overall performance, newer designs of Li-manganese offer improvements in specific power, safety and life span.

Courtesy of BCG research

Lithium Iron Phosphate (LiFePO4)

In 1996, the University of Texas (and other contributors) discovered phosphate as cathode material for rechargeable lithium batteries. Li-phosphate offers good electrochemical performance with low resistance. This is made possible with nano-scale phosphate cathode material. The key benefits are enhanced safety, good thermal stability, tolerant to abuse, high current rating and long cycle life. Storing a fully charged battery has minimal impact on the life span. As trade-off, the lower voltage of 3.3V/cell reduces the specific energy to slightly less than Li-manganese. In addition, cold temperature reduces performance, and elevated storage temperature shortens the service life (better than lead acid, NiCd or NiMH). Li-phosphate has a higher self-discharge than other Li-ion batteries, which can cause balancing issues with aging. Figure 6 summarizes the attributes of Li-phosphate.
 
Snapshot of a typical Li-phosphate battery

Figure 6: Snapshot of a typical Li-phosphate battery 
Li-phosphate has excellent safety and long life span but moderate specific energy and elevated self-discharge.

Courtesy of BCG research

Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2)

Leading battery manufacturers focus on a cathode combination of nickel-manganese-cobalt (NMC). Similar to Li-manganese, these systems can also be tailored to high specific energy or high specific power, but not both. For example, NMC in an 18650 cell for consumer use can be tweaked to 2,250mAh, but the specific power is moderate. NMC in the same cell optimized for high specific power has a capacity of only 1,500mAh. A silicon-based anode in will be able to go to 4,000mAh; however, the specific power and the cycle life may be compromised. 

The secret of NMC lies in combining nickel and manganese. An analogy of this is table salt in which the main ingredients of sodium and chloride are toxic on their own but mixing them serves as seasoning salt and food preserver. Nickel is known for its high specific energy but low stability; manganese has the benefit of forming a spinel structure to achieve very low internal resistance but offers a low specific energy. Combining the metals brings out the best in each.

NMC is the battery of choice for power tools and powertrains for vehicles. The cathode combination of one-third nickel, one-third manganese and one-third cobalt offers a unique blend that also lowers raw material cost due to reduced cobalt content. Striking the right balance is important and manufacturers keep their recipes a well-guarded secret. Figure 7 demonstrates the characteristics of the NMC.
 
Snapshot of NMC

Figure 7: Snapshot of NMC 
NMC has good overall performance and excels on specific energy. This battery is the preferred candidate for the electric vehicle.

Courtesy of BCG research

Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2)

The Lithium Nickel Cobalt Aluminum Oxide battery, or NCA, is less commonly used in the consumer market, however high specific energy and specific power, as well as a long life span, get the attention of the automotive industry. Less flattering are safety and cost. Figure 8 demonstrates the strong points against areas for further development.
 
Snapshot of NCA

Figure 8: Snapshot of NCA 
High energy and power densities, as well as good life span, make the NCA 
a candidate for EV powertrains. High cost and marginal safety are negatives.

Courtesy of BCG research

Lithium Titanate (Li4Ti5O12)

Batteries with lithium titanate anodes have been known since the 1980s. Li-titanate replaces the graphite in the anode of a typical lithium-ion battery and the material forms into a spinel structure. Li-titanate has a nominal cell voltage of 2.40V, can be fast-charged and delivers a high discharge current of 10C, or 10 times the rated capacity. The cycle count is said to be higher than that of a regular Li-ion; the battery is safe, has excellent low-temperature discharge characteristics and obtains a capacity of 80 percent at –30°C (–22°F). At 65Wh/kg, the specific energy is low. Li-titanate charges to 2.80V/cell, and the end of discharge is 1.80V/cell. Figure 9 illustrates the characteristics of the Li-titanate battery.
 
Snapshot of Li-titanate

Figure 9: Snapshot of Li-titanate 
Li-titanate excels in safety, low-temperature performance and life span. Efforts are being made to improve the specific energy and lower cost.

Courtesy of BCG research

Figure 10 compares the specific energy of lead, nickel- and lithium-based systems. While Li-cobalt is the clear winner by being able to store more capacity than other systems, this only applies to specific energy. In terms of load characteristics and thermal stability, Li-manganese and Li-phosphate are superior. As we move towards electric powertrains, safety and cycle life will become more important than capacity.
 
Typical energy densities of lead, nickel- and lithium-based batteriesFigure 10: Typical energy densities of lead, nickel- and lithium-based batteries 
Lithium-cobalt enjoys the highest specific energy, however, manganese and phosphate are superior in terms of specific power and thermal stability. Courtesy of Cadex.

Never was the competition to find an ideal battery more intense than it is today. Manufacturers see huge potential for automotive propulsion systems, as well as stationary and grid storage applications, also knows as load leveling. At time of writing, the battery industry speculates that the Li-manganese and/or NMC might be the winners for the electric powertrain. 

The author’s battery experience has mostly been in portable applications, and the long-term suitability of batteries for automotive use is still unknown. A clear assessment of the cycle life, performance and long-term operating cost will only be known after having gone through a few generations of batteries for vehicles with electric powertrains, and more is known about customer’s behavior and climate conditions under which the batteries are exposed.

 

By www. charger-battery.ca - Posted in: Battery Knowledge
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Friday 22 april 5 22 /04 /Apr 03:09

Most lithium-ion batteries for portable applications are cobalt-based. The system consists of a cobalt oxide positive electrode (cathode) and a graphite carbon in the negative electrode (anode). One of the main advantages of the cobalt-based battery is its high energy density. Long run-time makes this chemistry attractive for cell phones, laptops and cameras.

The widely used cobalt-based lithium-ion has drawbacks; it offers a relatively low discharge current. A high load would overheat the pack and its safety would be jeopardized. The safety circuit of the cobalt-based battery is typically limited to a charge and discharge rate of about 1C. This means that a 2400mAh 18650 cell can only be charged and discharged with a maximum current of 2.4A. Another downside is the increase of the internal resistance that occurs with cycling and aging. After 2-3 years of use, the pack often becomes unserviceable due to a large voltage drop under load that is caused by high internal resistance. Figure 1 illustrates the crystalline structure of cobalt oxide.
Cathode crystalline of lithium cobalt oxide has 'layered' structures
Figure 1: Cathode crystalline of lithium cobalt oxide has 'layered' structures. The lithium ions are shown bound to the cobalt oxide. During discharge, the lithium ions move from the cathode to the anode. The flow reverses on charge.
Cathode crystalline of lithium manganese oxide In 1996, scientists succeeded in using lithium manganese oxide as a cathode material. This substance forms a three-dimensional spinel structure that improves the ion flow between the electrodes. High ion flow lowers the internal resistance and increases loading capability. The resistance stays low with cycling, however, the battery does age and the overall service life is similar to that of cobalt. Spinel has an inherently high thermal stability and needs less safety circuitry than a cobalt system.Low internal cell resistance is the key to high rate capability. This characteristic benefits fast-charging and high-current discharging. A spinel-based lithium-ion in an 18650 cell can be discharged at 20-30A with marginal heat build-up. Short one-second load pulses of twice the specified current are permissible. Some heat build-up cannot be prevented and the cell temperature should not exceed 80°C.
Figure 2: Cathode crystalline of
lithium manganese oxide
 has a 
'three-dimensional framework structure'. 
This spinel structure, which is usually composed of diamond shapes connected into a lattice, appears after initial formation. This system provides high conductivity but lower energy density.

The spinel battery also has weaknesses. One of the most significant drawbacks is the lower capacity compared to the cobalt-based system. Spinel provides roughly 1200mAh in an 18650 package, about half that of the cobalt equivalent. In spite of this, spinel still provides an energy density that is about 50% higher than that of a nickel-based equivalent.
  Format of 18650 cell Figure 3: Format of 18650 cell. 
The dimensionsof this commonly used cell are: 18mm in diameter and 65mm in length.

Types of lithium-ion batteries

Lithium-ion has not yet reached full maturity and the technology is continually improving. The anode in today's cells is made up of a graphite mixture and the cathode is a combination of lithium and other choice metals. It should be noted that all materials in a battery have a theoretical energy density. With lithium-ion, the anode is well optimized and little improvements can be gained in terms of design changes. The cathode, however, shows promise for further enhancements. Battery research is therefore focusing on the cathode material. Another part that has potential is the electrolyte. The electrolyte serves as a reaction medium between the anode and the cathode. 

The battery industry is making incremental capacity gains of 8-10% per year. This trend is expected to continue. This, however, is a far cry from Moore's Law that specifies a doubling of transistors on a chip every 18 to 24 months. Translating this increase to a battery would mean a doubling of capacity every two years. Instead of two years, lithium-ion has doubled its energy capacity in 10 years.

 

Today's lithium-ion comes in many "flavours" and the differences in the composition are mostly related to the cathode material. Table 1 below summarizes the most commonly used lithium-ion on the market today. For simplicity, we summarize the chemistries into four groupings, which are Cobalt, Manganese, NCM and Phosphate.

 

Chemical name

Material

Abbreviation

Short form

Notes

Lithium Cobalt Oxide1AlsoLithium Cobalate or lithium-ion-cobalt)

LiCoO2
(60% Co)

LCO

Li-cobalt

High capacity; for cell phone laptop, camera

Lithium
Manganese Oxide
1
Also Lithium Manganate
or lithium-ion-manganese

LiMn2O4

LMO

Li-manganese, or spinel

Most safe; lower capacity than Li-cobalt but high specific power and long life.

Power tools,
e-bikes, EV, medical, hobbyist.

Lithium
Iron Phosphate
1

LiFePO4

LFP

Li-phosphate

Lithium Nickel Manganese Cobalt Oxide1, also lithium-manganese-cobalt-oxide

LiNiMnCoO2
(10–20% Co)

NMC

NMC

Lithium Nickel Cobalt Aluminum Oxide1

LiNiCoAlO2
9% Co)

NCA

NCA

Gaining importance
in electric powertrain and grid storage

Lithium Titanate2

Li4Ti5O12

LTO

Li-titanate

Table 1: Reference names for Li-ion batteries.We willuse the short form when appropriate.

1  Cathode material        

2  Anode material

The cobalt-based lithium-ion appeared first in 1991, introduced by Sony. This battery chemistry gained quick acceptance because of its high energy density. Possibly due to lower energy density, spinel-based lithium-ion had a slower start. When introduced in 1996, the world demanded longer runtime above anything else. With the need for high current rate on many portable devices, spinel has now moved to the frontline and is in hot demand. The requirements are so great that manufacturers producing these batteries are unable to meet the demand. This is one of the reasons why so little advertising is done to promote this product. E-One Moli Energy (Canada) is a leading manufacturer of the spinel lithium-ion in cylindrical form. They are specializing in the 18650 and 26700 cell formats. Other major players of spinel-based lithium-ion are Sanyo, Panasonic and Sony.


Sony is focusing on the nickel-cobalt manganese (NCM) version. The cathode incorporates cobalt, nickel and manganese in the crystal structure that forms a multi-metal oxide material to which lithium is added. The manufacturer offers a range of different products within this battery family, catering to users that either needs high energy density or high load capability. It should be noted that these two attributes could not be combined in one and the same package; there is a compromise between the two. Note that the NCM charges to 4.10V/cell, 100mV lower than cobalt and spinel. Charging this battery chemistry to 4.20V/cell would provide higher capacities but the cycle life would be cut short. Instead of the customary 800 cycles achieved in a laboratory environment, the cycle count would be reduced to about 300.

The newest addition to the lithium-ion family is the A123 System in which nano-phosphate materials are added in the cathode. It claims to have the highest power density in W/kg of a commercially available lithium-ion battery. The cell can be continuously discharged to 100% depth-of-discharge at 35C and can endure discharge pulses as high as 100C. The phosphate-based system has a nominal voltage of about 3.3V/cell and peak charge voltage is 3.60V. This is lower than the cobalt-based lithium-ion and the battery will require a designated charger. Valance Technology was the first to commercialize the phosphate-based lithium-ion and their cells are sold under the Saphionâ name.

In Figure 4 we compare the energy density (Wh/kg) of the three lithium-ion chemistries and place them against the traditional lead acid, nickel-cadmium, nickel-metal-hydride. One can see the incremental improvement of Manganese and Phosphate over older technologies. Cobalt offers the highest energy density but is thermally less stable and cannot deliver high load currents.

Energy densities of common battery chemistries Figure 4: Energy densities of common battery chemistries.

 

Definition of Energy Density and Power Density

Energy Density (Wh/kg) is a measure of how much energy a battery can hold. The higher the energy density, the longer the runtime will be. Lithium-ion with cobalt cathodes offer the highest energy densities. Typical applications are cell phones, laptops and digital cameras.
Power Density (W/kg) indicates how much power a battery can deliver on demand. The focus is on power bursts, such as drilling through heavy steel, rather than runtime. Manganese and phosphate-based lithium-ion, as well as nickel-based chemistries, are among the best performers. Batteries with high power density are used for power tools, medical devices and transportation systems. 

An analogy between energy and power densities can be made with a water bottle. The size of the bottle is the energy density, while the opening denotes the power density. A large bottle can carry a lot of water, while a large opening can pore it quickly. The large container with a wide mouth is the best combination.

Confusion with voltages

For the last 10 years or so, the nominal voltage of lithium-ion was known to be 3.60V/cell. This was a rather handy figure because it made up for three nickel-based batteries (1.2V/cell) connected in series. Using the higher cell voltages for lithium-ion reflects in better watt/hours readings on paper and poses a marketing advantage, however, the equipment manufacturer will continue assuming the cell to be 3.60V.
The nominal voltage of a lithium-ion battery is calculated by taking a fully charged battery of about 4.20V, fully discharging it to about 3.00V at a rate of 0.5C while measuring the average voltage. 

Because of the lower internal resistance, the average voltage of a spinel system will be higher than that of the cobalt-based equivalent. Pure spinel has the lowest internal resistance and the nominal cell voltage is 3.80V. The exception again is the phosphate-based lithium-ion. This system deviates the furthest from the conventional lithium-ion system

Prolonged battery life through moderation

Batteries live longer if treated in a gentle manner. High charge voltages, excessive charge rate and extreme load conditions have a negative effect on battery life. The longevity is often a direct result of the environmental stresses applied. The following guidelines suggest ways to prolong battery life.

-The time at which the battery stays at 4.20/cell should be as short as possible. Prolonged high voltage promotes corrosion, especially at elevated temperatures. Spinel is less sensitive to high voltage.

-3.92V/cell is the best upper voltage threshold for cobalt-based lithium-ion. Charging batteries to this voltage level has been shown to double cycle life. Lithium-ion systems for defense applications make use of the lower voltage threshold. The negative is a much lower capacity.

-The charge current of Li-ion should be moderate (0.5C for cobalt-based lithium-ion). The lower charge current reduces the time in which the cell resides at 4.20V. A 0.5C charge only adds marginally to the charge time over 1C because the topping charge will be shorter. A high current charge tends to push the voltage into voltage limit prematurely.

-Do not discharge lithium-ion too deeply. Instead, charge it frequently. Lithium-ion does not have memory problems like nickel-cadmium batteries. No deep discharges are needed for conditioning.

-Do not charge lithium-ion at or below freezing temperature. Although accepting charge, an irreversible plating of metallic lithium will occur that compromises the safety of the pack.

Not only does a lithium-ion battery live longer with a slower charge rate; moderate discharge rates also help. Figure 5 shows the cycle life as a function of charge and discharge rates. Observe the improved laboratory performance on a charge and discharge rate of 1C compared to 2 and 3C.

Longevity of lithium-ion as a function of charge and discharge rates Figure 5: Longevity of lithium-ion as a function of charge and discharge rates.
Lithium-cobalt enjoys the highest energy density. Manganese and phosphate systems are terminally more stable and deliver high load currents than cobalt.

Battery experts agree that the longevity of lithium-ion is shortened by other factors than charge and discharge rates. Even though incremental improvements can be achieved with careful use, our environment and the services required are not always conducive for optimal battery life. In this respect, the battery behaves much like us humans - we cannot always live a life that caters to achieve maximum life span.

By www. charger-battery.ca - Posted in: Battery Knowledge
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