Is lithium-ion the ideal battery?

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. (See last section)
   
• 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
   

Lithium content for purpose of shipment

The question is often asked what is the typical amount of lithium (in grams) of a lithium-ion
rechargeable battery for laptops and other portable devices. This question is asked in compliance to US Department of Transportation requirements.

From a "theoretical" perspective, there is no metallic lithium in a typical lithium-ion battery. However, from a transportation viewpoint there is an "equivalent lithium content" that must be considered. Transportation authorities include the following exception statement:

The "equivalent lithium content" of a lithium-ion cell (in grams) is calculated at 0.3 times the rated capacity (in ampere-hours). The lithium-equivalent content of a battery equals the sum of the grams of lithium-equivalent content contained in the component cells of the battery."

Example: A 2Ah 18650 Li-ion cell has 0.6g of lithium-equivalent content and a typical laptop battery with 8 cells (4 in series and 2 in parallel) has 4.8g. To stay under the 8g UN limit, the largest battery you can build using the 2.2Ah 18650 is 12 cells (4s3p). The largest pack using the 2.4Ah is 9 cells (3s3p).
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Created: April 2003, Last edited: August 2006
 

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About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC.
Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.

Charging lithium-ion batteries

There is only one way to charge lithium-based batteries. The so-called 'miracle chargers', which claim to restore and prolong batteries, do not exist for lithium chemistries. Neither does super-fast charging apply. Manufacturers of lithium-ion cells have very strict guidelines in charge procedures and the pack should be charged as per the manufacturers "typical" charge technique.

Lithium-ion is a very clean system and does not need priming as nickel-based batteries do. The 1st charge is no different to the 5th or the 50th charge. Stickers instructing to charge the battery for 8 hours or more for the first time may be a leftover from the nickel battery days.

Most cells are charged to 4.20 volts with a tolerance of +/?0.05V/cell. Charging only to 4.10V reduced the capacity by 10% but provides a longer service life. Newer cell are capable of delivering a good cycle count with a charge to 4.20 volts per cell. Figure 1 shows the voltage and current signature as the lithium-ion cell passes through the charge stages.

 

Figure 1: Charge stages of a lithium-ion battery. Increasing the charge current on a lithium?ion charger does not shorten the charge time by much. Although the voltage peak is reached quicker with higher current, the topping charge will take longer.

The charge time of most chargers is about 3 hours. Smaller batteries used for cell phones can be charged at 1C; the larger 18650 cell used for laptops should be charged at 0.8C or less. The charge efficiency is 99.9% and the battery remains cool during charge. Full charge is attained after the voltage threshold has been reached and the current has dropped to 3% of the rated current or has leveled off.

Increasing the charge current does not shorten the charge time by much. Although the voltage peak is reached quicker with higher charge current, the topping charge will take longer.

Some chargers claim to fast-charge a lithium-ion battery in one hour or less. Such a charger eliminates stage 2 and goes directly to 'ready' once the voltage threshold is reached at the end of stage 1. The charge level at this point is about 70%. The topping charge typically takes twice as long as the initial charge.

No trickle charge is applied because lithium-ion is unable to absorb overcharge. A continuous trickle charge above 4.05V/cell would causes plating of metallic lithium that could lead to instabilities and compromise safety. Instead, a brief topping charge is provided to compensate for the small self-discharge the battery and its protective circuit consume. Depending on the battery, a topping charge may be repeated once every 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05V/cell and turns off at a high 4.20V/cell.

What happens if a battery is inadvertently overcharged? lithium-ion is designed to operate safely within their normal operating voltage but become unstable if charged to higher voltages. When charging above 4.30V, the cell causes plating of metallic lithium on the anode; the cathode material becomes an oxidizing agent, loses stability and releases oxygen. Overcharging causes the cell to heat up. If left unattended, the cell could vent with flame.

Much attention is focused to avoid over-charging and over-discharging. Commercial lithium ion packs contain a protection circuits that limit the charge voltage to 4.30V/cell, 0.10 volts higher than the voltage threshold of the charger. Temperature sensing disconnects the charge if the cell temperature approaches 90°C (194°F), and a mechanical pressure switch on many cells permanently interrupt the current path if a safe pressure threshold is exceeded. Exceptions are made on some spinel (manganese) packs containing one or two small cells.

Extreme low voltage must also be prevented. The safety circuit is designed to cut off the current path if the battery is inadvertently discharged below 2.50V/cell. At this voltage, most circuits render the battery unserviceable and a recharge on a regular charger is not possible.
There are several safeguards to prevent excessive discharge. The equipment protects the battery by cutting off when the cell reaches 2.7 to 3.0V/cell. Battery manufacturers ship the batteries with a 40% charge to allow some self-discharge during storage. Advanced batteries contain a wake-up feature in which the protection circuit only starts to draw current after the battery has been activated with a brief charge. This allows prolonged storage.

In spite of these preventive measures, over-discharge does occur. Advanced battery analyzers (Cadex C7000 series) feature a 'boost' function that provides a gentle charge current to activate the safety circuit and re-energize the cells if discharged too deeply. A full charge and analysis follows.

If the cells have dwelled at 1.5V/cell and lower for a few days, however, a recharge should be avoided. Copper shunts may have formed inside the cells, leading a partial or total electrical short. The cell becomes unstable. Charging such a battery would cause excessive heat and safety could not be assured.

Battery experts agree that charging lithium-ion batteries is simpler and more straightforward than the nickel-based cousins. Besides meeting the tight voltage tolerances, the charge circuit can be designed with fewer variables to consider. Full-charge detection by applying voltage limits and observing the current saturations on full charge is simpler than analyzing many complex signatures, which nickel-metal-hydride produces. Charge currents are less critical and can vary. A low current still permits proper full charge detection. The battery simply takes longer to charge. The absence of topping and trickle charge also help in simplifying the charger. Best of all, there is no memory but aging issues are the drawback.

The charge process of a lithium-ion-polymer is similar to lithium-ion. These batteries use a gelled electrolyte to improve conductivity. In most cases, lithium-ion and lithium-ion-polymer share the same charger.

Preparing new lithium-ion for use

Unlike nickel and lead-based batteries, a new lithium-ion pack does not need cycling through charging and discharging. Priming will make little difference because the maximum capacity of lithium-ion is available right from the beginning. Neither does a full discharge improve the capacity of a faded pack. However, a full discharge/charge will reset the digital circuit of a 'smart' battery to improve the state-of-charge estimation

State-of-charge reading based on terminal voltage

The open circuit voltage can be used to estimate the battery state-of-charge of lithium, alkaline and lead-based batteries. Unfortunately, this method cannot be used for nickel-based packs.

On a lithium-ion cell, 3.8V/cell indicates a state-of-charge of about 50%. It must be noted that utilizing voltage as a fuel gauge function is inaccurate because cells made by different manufacturers produce a slightly different voltage profile. This is due to the electrochemistry of the electrodes and electrolyte. Temperature also affects the voltage. The higher the temperature, the lower the voltage will be.

Hints to long battery life

• Limit the time at which the battery stays at 4.20/cell. Prolonged high voltage promotes corrosion, especially at elevated temperatures. (Spinel is less sensitive to high voltage than cobalt-based systems).
  
   
• 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 reduced 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. It should be noted that 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 up and forces it into the voltage limit prematurely.
   

Note: In respect to fast-charging and topping charge, the charge behavior of lithium-ion is similar to lead acid. Here, the voltage threshold of 2.35V/cell during regular charge needs to be lowered to 2.27V/cell when the VRLA is on standby. Keeping the voltage at the high threshold would contribute to corrosion. A similar effect occurs with lithium-ion.

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Created: April 2003, Last edited: March 2006
 

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About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC.
Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.

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About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC. Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.

Lithium-ion safety concerns

When Sony introduced the first lithium-ion battery in 1991, they knew of the potential safety risks. A recall of the previously released rechargeable metallic lithium battery was a bleak reminder of the discipline one must exercise when dealing with this high energy-dense battery system.

Pioneering work for the lithium battery began in 1912, but is was not until the early 1970's when the first non-rechargeable lithium batteries became commercially available. Attempts to develop rechargeable lithium batteries followed in the eighties. These early models were based on metallic lithium and offered very high energy density. However, inherent instabilities of lithium metal, especially during charging, put a damper on the development. The cell had the potential of a thermal run-away. The temperature would quickly rise to the melting point of the metallic lithium and cause a violent reaction. A large quantity of rechargeable lithium batteries had to be recalled in 1991 after the pack in a cellular phone released hot gases and inflicted burns to a man's face.

Because of the inherent instability of lithium metal, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density, the lithium-ion system is safe, providing certain precautions are met when charging and discharging. Today, lithium-ion is one of the most successful and safe battery chemistries available. Two billion cells are produced every year.

Lithium-ion cells with cobalt cathodes hold twice the energy of a nickel-based battery and four-times that of lead acid. Lithium-ion is a low maintenance system, an advantage that most other chemistries cannot claim. There is no memory and the battery does not require scheduled cycling to prolong its life. Nor does lithium-ion have the sulfation problem of lead acid that occurs when the battery is stored without periodic topping charge. Lithium-ion has a low self-discharge and is environmentally friendly. Disposal causes minimal harm.

Long battery runtimes have always been the wish of many consumers. Battery manufacturers responded by packing more active material into a cell and making the electrodes and separator thinner. This enabled a doubling of energy density since lithium-ion was introduced in 1991.

The high energy density comes at a price. Manufacturing methods become more critical the denser the cells become. With a separator thickness of only 20-25µm, any small intrusion of metallic dust particles can have devastating consequences. Appropriate measures will be needed to achieve the mandated safety standard set forth by UL 1642. Whereas a nail penetration test could be tolerated on the older 18650 cell with a capacity of 1.35Ah, today's high-density 2.4Ah cell would become a bomb when performing the same test. UL 1642 does not require nail penetration. Lithium-ion batteries are nearing their theoretical energy density limit and battery manufacturers are beginning to focus on improving manufacturing methods and increasing safety.

Recall of lithium-ion batteries

With the high usage of lithium-ion in cell phones, digital cameras and laptops, there are bound to be issues. A one-in-200,000 failure rate triggered a recall of almost six million lithium-ion packs used in laptops manufactured by Dell and Apple. Heat related battery failures are taken very seriously and manufacturers chose a conservative approach. The decision to replace the batteries puts the consumer at ease and lawyers at bay. Let's now take a look at what's behind the recall.

Sony Energy Devices (Sony), the maker of the lithium-ion cells in question, says that on rare occasions microscopic metal particles may come into contact with other parts of the battery cell, leading to a short circuit within the cell. Although battery manufacturers strive to minimize the presence of metallic particles, complex assembly techniques make the elimination of all metallic dust nearly impossible.

 

Figure 1: Lithium-ion battery damages a laptop. Safety issues are enticing battery manufacturers to change the manufacturing process. According to Sony, contamination of Cu, Al, Fe and Ni particles during the manufacturing process may cause an internal short circuit.

A mild short will only cause an elevated self-discharge. Little heat is generated because the discharging energy is very low. If, however, enough microscopic metal particles converge on one spot, a major electrical short can develop and a sizable current will flow between the positive and negative plates. This causes the temperature to rise, leading to a thermal runaway, also referred to 'venting with flame.'

Lithium-ion cells with cobalt cathodes (same as the recalled laptop batteries) should never rise above 130°C (265°F). At 150°C (302°F) the cell becomes thermally unstable, a condition that can lead to a thermal runaway in which flaming gases are vented.

During a thermal runaway, the high heat of the failing cell can propagate to the next cell, causing it to become thermally unstable as well. In some cases, a chain reaction occurs in which each cell disintegrates at its own timetable. A pack can get destroyed within a few short seconds or linger on for several hours as each cell is consumed one-by-one. To increase safety, packs are fitted with dividers to protect the failing cell from spreading to neighboring cells.

Safety level of lithium-ion systems

There are two basic types of lithium-ion chemistries: cobalt and manganese (spinel). To achieve maximum runtime, cell phones, digital cameras and laptops use cobalt-based lithium-ion. Manganese is the newer of the two chemistries and offers superior thermal stability. It can sustain temperatures of up to 250°C (482°F) before becoming unstable. In addition, manganese has a very low internal resistance and can deliver high current on demand. Increasingly, these batteries are used for power tools and medical devices. Hybrid and electric vehicles will be next.

The drawback of spinel is lower energy density. Typically, a cell made of a pure manganese cathode provides only about half the capacity of cobalt. Cell phone and laptop users would not be happy if their batteries quit halfway through the expected runtime. To find a workable compromise between high energy density, operational safety and good current delivery, manufacturers of lithium-ion batteries can mix the metals. Typical cathode materials are cobalt, nickel, manganese and iron phosphate.

Let me assure the reader that lithium-ion batteries are safe and heat related failures are rare. The battery manufacturers achieve this high reliability by adding three layers of protection. They are: [1] limiting the amount of active material to achieve a workable equilibrium of energy density and safety; [2] inclusion of various safety mechanisms within the cell; and [3] the addition of an electronic protection circuit in the battery pack.

These protection devices work in the following ways: The PTC device built into the cell acts as a protection to inhibit high current surges; the circuit interrupt device (CID) opens the electrical path if an excessively high charge voltage raises the internal cell pressure to 10 Bar (150 psi); and the safety vent allows a controlled release of gas in the event of a rapid increase in cell pressure. In addition to the mechanical safeguards, the electronic protection circuit external to the cells opens a solid-state switch if the charge voltage of any cell reaches 4.30V. A fuse cuts the current flow if the skin temperature of the cell approaches 90°C (194°F). To prevent the battery from over-discharging, the control circuit cuts off the current path at about 2.50V/cell. In some applications, the higher inherent safety of the spinel system permits the exclusion of the electric circuit. In such a case, the battery relies wholly on the protection devices that are built into the cell.

We need to keep in mind that these safety precautions are only effective if the mode of operation comes from the outside, such as with an electrical short or a faulty charger. Under normal circumstances, a lithium-ion battery will simply power down when a short circuit occurs. If, however, a defect is inherent to the electrochemical cell, such as in contamination caused by microscopic metal particles, this anomaly will go undetected. Nor can the safety circuit stop the disintegration once the cell is in thermal runaway mode. Nothing can stop it once triggered.

What every battery user should know

A major concern arises if static electricity or a faulty charger has destroyed the battery's protection circuit. Such damage can permanently fuse the solid-state switches in an ON position without the user knowing. A battery with a faulty protection circuit may function normally but does not provide protection against abuse.

Another safety issue is cold temperature charging. Consumer grade lithium-ion batteries cannot be charged below 0°C (32°F). Although the packs appear to be charging normally, plating of metallic lithium occurs on the anode while on a sub-freezing charge. The plating is permanent and cannot be removed. If done repeatedly, such damage can compromise the safety of the pack. The battery will become more vulnerable to failure if subjected to impact, crush or high rate charging.

Asia produces many non-brand replacement batteries that are popular with cell phone users because of low price. Many of these batteries don't provide the same high safety standard as the main brand equivalent. A wise shopper spends a little more and replaces the battery with an approved model. Figure 1 shows a cell phone that was destroyed while charging in a car. The owner believes that a no-name pack caused the destruction.

 
Figure 2: A cell phone with a no-brand battery that vented with flame while charging in the back of a car.

To prevent the infiltration of unsafe packs on the market, most manufacturers sell lithium-ion cells only to approved battery pack assemblers. The inclusion of an approved safety circuit is part of the purchasing requirement. This makes it difficult for a hobbyist to purchase single lithium-ion cells off-the-shelf in a store. The hobbyist will have no other choice than to revert to nickel-based batteries. I would caution against using an unidentified lithium-ion battery from an Asian source, if such cells is available.

The safety precaution is especially critical on larger batteries, such as laptop packs. The hazard is so much greater than on a small cell phone battery if something goes wrong. For this reason, many laptop manufacturers secure their batteries with a secret code that only the matching computer can access. This prevents non-brand-name batteries from flooding the market. The drawback is a higher price for the replacement battery. Readers of www.BatteryUniversity.com often ask me for a source of cheap laptop batteries. I have to disappoint the shoppers by directing them to the original vendor for a brand name pack.

Considering the number of lithium-ion batteries used on the market, this energy storage system has caused little harm in terms of damage and personal injury. In spite of the good record, its safety is a hot topic that gets high media attention, even on a minor mishap. This caution is good for the consumer because we will be assured that this popular energy storage device is safe. After the recall of Dell and Apple laptop batteries, cell manufacturers will not only try packing more energy into the pack but will attempt to make it more bulletproof.

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Created:September 2006, Last edited: February 2007
 

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About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC.
Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.

How to store batteries

Batteries are perishable products that start deteriorating right from the moment they leave the factory. There are simple preventive measures that battery users can apply to slow the aging process. This paper provides guidelines to reduce age-related capacity losses and how to prime new and stored batteries.

The recommended storage temperature for most batteries is 15°C (59°F). While lead-acid batteries must always be kept at full charge, nickel and lithium-based chemistries should be stored at 40% state-of-charge (SoC). This level minimizes age-related capacity loss, yet keeps the battery in operating condition even with some self-discharge. While the open terminal voltage of nickel-based batteries cannot be used to determine the SoC accurately, voltage fuel gauging works well for lithium-ion cells. However, differences in the electrochemistry of the electrodes and electrolyte between manufacturers vary the voltage profile slightly. A SoC of 50% reads about 3.8V; 40% is 3.75V. Store lithium-ion at an open terminal voltage of 3.75-3.80V. Allow the battery to rest 90 minutes after charge before taking the voltage reading.

Figure 1 illustrates the recoverable capacity at various storage temperatures and charge levels over one year.


Figure 1: Non-recoverable capacity loss on lithium-ion and nickel-based batteries after storage. High charge levels and elevated temperatures hasten the capacity loss.

Among the lithium-ion family, cobalt has a slight advantage over manganese (spinel) in terms of storage at elevated temperatures. nickel-based batteries are also affected by elevated temperature but to a lesser degree than lithium-ion.

Lithium-ion powers most of today's laptop computers. The battery compartment on many laptops rises to about 45°C (113°F) during operation. The combination of high charge level and elevated ambient temperature presents an unfavorable condition for the battery. This explains the short lifespan of many laptop batteries.

Nickel-metal-hydride can be stored for about three years. The capacity drop that occurs during storage is permanent and cannot be reversed. Cool temperatures and a partial charge slows aging. Nickel-cadmium stores reasonably well. Field test reveled that NiCd batteries stored for five years still performed well after priming cycles. Alkaline and lithium batteries (primary) can be stored for up to 10 years. The capacity loss is minimal.

The sealed lead-acid battery can be stored for up to two years. A periodic topping charge, also referred to as 'refresh charge', is required to prevent the open cell voltage from dropping below 2.10V. (Some lead-acid batteries may allow lower voltage levels.) Insufficient charge induces sulfation, an oxidation layer on the negative plate that inhibits the current flow on charge and discharge. Topping charge and/or cycling may restore some of the capacity losses in the early stages.

Priming new batteries

Manufacturers recommend to trickle charge a nickel-based battery for 24 hours when new and after long storage. This service brings all cells to equal charge level and redistributes the electrolyte to remedy dry spots on the separator brought on by gravitation of the electrolyte. It is advisable to verify the capacity with a battery analyzer before use. This is especially important in critical applications.

Cycling (priming) is recommended to regain lost capacity after a nickel-based battery has been stored for 6 months or longer. A slow charge followed by one or several discharge/charge cycles will do this. The recovery rate is governed by the condition under which the battery was stored. The longer and warmer the storage temperature, the more cycles will be required. The Prime program of the Cadex battery analyzers automatically applies the number of cycles needed to regain full capacity.

Nickel-based batteries are not always fully formed when leaving the factory. Applying several charge/ discharge cycles through normal use or with a battery analyzer completes the forming. The number of cycles needed to attain full capacity differs between cell manufacturers. Quality cells perform to specification after 5-7 cycles. Those lacking formation may need 50 or more cycles to reach acceptable capacity levels.

What is the difference between priming and forming? For the user, both symptoms manifest themselves as insufficient capacity. The difference may be explained in that forming needs to be done only once when the battery is new, while priming must be repeated after each prolonged storage.

Lithium-ion batteries deliver full power after the initial charge. Manufacturers of lithium-ion cells insist that no priming is required. However, priming is beneficial as an initial start and to verify battery performance. Excessive cycling should be avoided because of wear-down effect.

The internal protection circuit of lithium-based batteries is known to cause some problems after a long storage. If the battery is left discharged after use, the self-discharge will further drain the pack and eventually drip the protection circuit at about 2.5 volts per cell. At this point, the charger will no longer recognize the battery and the pack appears dead. Advanced battery analyzers (Cadex) feature the Boost program that activates the protection circuit to enable a recharge. If the cell voltage has fallen below 1.5V/cell and has remained in that state for a few days, a recharge should be avoided for safety reasons.

To reduce the self-discharge on newly manufactured batteries, advanced lithium-ion packs feature a sleep mode that keeps the protection circuit off until activated by a brief charge. Once engaged, the battery remains operational and the advantage of the sleep mode no longer applies.

Lead-acid batteries should be primed by applying a full charge, followed by a discharge and recharge. Verifying the capacity through a discharge is important, especially if the battery is engaged in critical applications such as powering medical devices. Priming is also recommended after storing a battery for six months and longer. Battery analyzers provide the priming service automatically.

It is believed that a partial or full discharge applied once every six months or so enhances the performance of lead-acid batteries. Avoid too many full discharges, as this would wear down the battery unnecessarily.

While capacity loss during a battery's life cannot be eliminated, simple guidelines minimize the effect:

• Keep batteries in a cool and dry storage area. Refrigeration is recommended but freezers should be avoided. When refrigerated, the battery should be placed in a plastic bag to protect against condensation
   
• Do not fully charge lithium and nickel-based batteries before storage. Keep them partially charged and apply a full charge before use. Store lithium-ion at about 40% state-of-charge (3.75-3.80V/cell open terminal). Lead-acid batteries must be stored fully charged.
   
• Do not store lithium-ion fully depleted. If empty, charge for about 30 minutes before storage. Self-discharge on a depleted battery may cause the protection circuit to trip, preventing a recharge.
   
• Do not stockpile lithium-ion batteries; avoid buying dated stock, even if offered at a reduced price. Observe the manufacturing date, if available.
   
• Never leave a nickel-based battery sitting on a charger for more than a few days. Prolonged trickle charge causes crystalline formation (memory).
   
• Always store a lead acid battery in full-charge condition. Observe the open terminal voltage and recharge the battery every 6 months or as recommended by the manufacturer.

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Created: May 2003, Last edited: March 2004

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About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC.
Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.

FREE Battery Recycling for Versatile Customers

• Over 3 billion batteries are sold each year in the U.S.
• Batteries are are manufactured with toxic components including heavy metals like cadmium.
• Dumping batteries in our landfills is completely unnecessary because Versatile will recycle your batteries for free.
• We recycle batteries for all types of barcode scanners, handheld computers and portable printers.

Complete this form to receive recycling instructions.
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Recycling batteries

Modern batteries are often promoted on their environmental qualities. lithium-based batteries fall into this category. While nickel-cadmium presents an environmental problem on careless disposal, this chemistry continues to hold an important position among rechargeable batteries. Power tools are almost exclusively powered by nickel-cadmium. Lead-acid batteries continue to service designated market niches and these batteries also need to be disposed of in a proper manner. lithium-ion would simply be too fragile to replace many of these older, but environmentally unfriendly, battery chemistries.

Our quest for portability and mobility is steadily growing, so is the demand for batteries. Where will the mountains of batteries go when spent? The answer is recycling.

The lead-acid battery has led the way in recycling. The automotive industry should be given credit in organizing ways to dispose of spent car batteries. In the USA, 98% of all lead-acid batteries are recycled. In comparison, only one in six households in North America recycle batteries.

Careless disposal of nickel-cadmium is hazardous to the environment. If used in landfills, the cadmium will eventually dissolve itself and the toxic substance can seep into the water supply, causing serious health problems. Our oceans are already beginning to show traces of cadmium (along with aspirin, penicillin and antidepressants) but the source of the contamination is unknown.

Although nickel-metal-hydride is considered environmentally friendly, this chemistry is also being recycled. The main derivative is nickel, which is considered semi-toxic. nickel-metal-hydride also contains electrolyte that, in large amounts, is hazardous. If no disposal service is available in an area, individual nickel-metal-hydride batteries can be discarded with other household wastes. If ten or more batteries are accumulated, the user should consider disposing of these packs in a secure waste landfill.

Lithium (metal) batteries contain no toxic metals, however, there is the possibility of fire if the metallic lithium is exposed to moisture while the cells are corroding. Most lithium batteries are non-rechargeable and are used in cameras, hearing aids and defense applications. For proper disposal, the batteries must first be fully discharged to consume the metallic lithium content.
Lithium-ion batteries used for cell phones and laptops do not contain metallic lithium and the disposal problem does not exist. Most lithium systems contain toxic and flammable electrolyte.

In 1994, the Rechargeable Battery Recycling Corporation (RBRC) was founded to promote recycling of rechargeable batteries in North America. RBRC is a non-profit organization that collects batteries from consumers and businesses and sends them to recycling organizations. Inmetco and Toxco are among the best-known recycling companies in North America Europe and Asia have had programs to recycle spent batteries for many years. Sony and Sumitomo Metal in Japan have developed a technology to recycle cobalt and other precious metals from spent lithium-ion batteries.

Battery recycling plants require that the batteries be sorted according to chemistries. Some sorting must be done prior to the battery arriving at the recycling plants. nickel-cadmium, nickel-metal-hydride, lithium-ion and lead acid are placed in designated boxes at the collection point. Battery recyclers claim that if a steady stream of batteries, sorted by chemistry, were available at no charge, recycling would be profitable. But preparation and transportation add to the cost.

The recycling process starts by removing the combustible material, such as plastics and insulation, with a gas fired thermal oxidizer. Gases from the thermal oxidizer are sent to the plant's scrubber where they are neutralized to remove pollutants. The process leaves the clean, naked cells, which contain valuable metal content.

The cells are then chopped into small pieces, which are heated until the metal liquefies. Non-metallic substances are burned off; leaving a black slag on top that is removed with a slag arm. The different alloys settle according to their weights and are skimmed off like cream from raw milk.

Cadmium is relatively light and vaporizes at high temperatures. In a process that appears like a pan boiling over, a fan blows the cadmium vapor into a large tube, which is cooled with water mist. This causes the vapors to condense and produces cadmium that is 99.95 percent pure.

Some recyclers do not separate the metals on site but pour the liquid metals directly into what the industry refers to as 'pigs' (65 pounds) or 'hogs' (2000 pounds). The pigs and hogs are then shipped to metal recovery plants. Here, the material is used to produce nickel, chromium and iron re-melt alloy for the manufacturing of stainless steel and other high-end products.

Current battery recycling methods requires a high amount of energy. It takes six to ten times the amount of energy to reclaim metals from recycled batteries than it would through other means.

Who pays for the recycling of batteries? Participating countries impose their own rules in making recycling feasible. In North America, some recycling plants bill on weight. The rates vary according to chemistry. Systems that yield high metal retrieval rates are priced lower than those, which produce less valuable metals.

Nickel-metal-hydride yields the best return. It produces enough nickel to pay for the process. The highest recycling fees apply to nickel-cadmium and lithium-ion because the demand for cadmium is low and lithium-ion contains little retrievable metal.

Not all countries base the cost of recycling on the battery chemistry; some put it on tonnage alone. The flat cost to recycle batteries is about $1,000 to $2,000US per ton. Europe hopes to achieve a cost per ton of $300US. Ideally, this would include transportation, however, moving the goods is expected to double the overall cost. For this reason, Europe sets up several smaller processing locations in strategic geographic locations.

Significant subsidies are sill required from manufacturers, agencies and governments to support the battery recycling programs. This funding is in the form of a tax added to each manufactured cell. RBRC is financed by such a scheme.

Important: Under no circumstances should batteries be incinerated as this can cause explosion. If skin is exposed to electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.

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Created: May 2003, Last edited: July 2003

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About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC.
Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.