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How to prolong lithium-based batteries

Battery research is focusing heavily on lithium chemistries, so much so that one could presume that all portable devices will be powered with lithium-ion batteries in the future. In many ways, lithium-ion is superior to nickel and lead-based chemistries and the applications for lithium-ion batteries are growing as a result.

Lithium-ion has not yet fully matured and this chemistry is being improved continuously. New metal and chemical combinations are being tried every six months to increase energy density and prolong service life. The improvements in longevity after each change will not be known for a few years.

A lithium-ion battery provides 300 to 500 discharge/charge cycles. The battery prefers a partial rather than a full discharge. Full discharges should be avoided when possible. Instead, charge the battery more often or use a larger battery. There is no concern of memory when applying unscheduled charges.

Aging of lithium-ion is an issue that is often ignored. lithium-based batteries have a lifetime of two to three years. The clock starts ticking as soon as the battery comes off the manufacturing line. The capacity loss manifests itself in increased internal resistance caused by oxidation. Eventually, the cell resistance will reach a point where the pack can no longer deliver the stored energy, although the battery may still contain ample charge.

The speed by which lithium-ion ages is governed by temperature and state-of-charge. Figure 1 illustrates the capacity loss as a function of these two parameters.

There are no remedies to restore lithium-ion once worn out. A momentarily improvement in performance is noticeable when heating up the battery but the high internal resistance will revert to its former state with normal temperature.

If possible, store the battery in a cool place at about a 40% state-of-charge. This reserve charge is needed to keep the battery and its protection circuit active during prolonged storage. The most harmful combination is full charge at high temperature. This is the case when running a laptop computer on the mains with the battery installed. While the battery is kept fully charged, the inside operating temperature rises to 45°C (113°F). A cell phone battery placed in a hot car will also suffer heat-related aging.

A large number of lithium-ion batteries for cell phones are being discarded under the warranty return policy. Some failed batteries are sent to service centers or the manufacturer, where they are refurbished.. Studies show that 80% to 90% of the returned batteries can be repaired and returned to service.

Some lithium-ion batteries fail due to excessive low discharge. If discharged below 2.5 volts per cell, the internal safety circuit opens and the battery appears dead. A charge with the original charger is no longer possible. The Cadex battery analyzers feature a boost function that reactivates the protection circuit of a failed battery and enables a recharge. However, 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 because of safety concerns. To avoid failure, never store the battery fully discharged. Apply some charge, and then charge fully before use.
Simple Guidelines

· Avoid full discharges; recharge lithium-ion more often. Repetitive random charge does not harm the battery. There is no memory.
· Keep the lithium?ion battery cool but do not freeze. Avoid a hot car. For prolonged storage, keep the battery at a 40% charge level.
· If your laptop runs without a battery and fixed power is used most of the time, remove and store the battery in a cool place.
· Avoid purchasing spare lithium?ion batteries for later use. Observe manufacturing date. Do not buy old stock, even if sold at clearance prices.

Figure 1

Figure 1

Permanent capacity loss of lithium-ion as a function of temperature and charge level.
High charge levels and elevated temperatures hasten permanent capacity loss. Improvements in chemistry have increased the storage performance of lithium-ion batteries.


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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