Batteries in a Portable World 2^nd Ed.
       A Handbook on Rechargeable Batteries for Non-Engineers

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10.5 How to Prolong Lithium-based Batteries

Today’s battery research is heavily focused on lithium chemistries, so much so that one could assume that all future batteries will be lithium systems. Lithium-based batteries offer many advantages over nickel and lead-based systems. Although maintenance free, no external service is known that can restore the battery’s performance once degraded.

In many respects, Li-ion provides a superior service to other chemistries, but its performance is limited to a defined lifespan. The Li-ion battery has a time clock that starts ticking as soon as the battery leaves the factory. The electrolyte slowly ‘eats up’ the positive plate and the electrolyte decays. This chemical change causes the internal resistance to increase. In time, the cell resistance raises to a point where the battery can no longer deliver the energy, although it may still be retained in the battery. Equipment requiring high current bursts is affected most by the increase of internal resistance.

Battery wear-down on lithium-based batteries is caused by two activities: actual usage or cycling, and aging. The wear-down effects by usage and aging apply to all batteries but this is more pronounced on lithium-based systems.

The Li-ion batteries prefer a shallow discharge. Partial discharges produce less wear than a full discharge and the capacity loss per cycle is reduced. A periodic full discharge is not required because the lithium-based battery has no memory. A full cycle constitutes a discharge to 3V/cell. When specifying the number of cycles a lithium-based battery can endure, manufacturers commonly use an 80 percent depth of discharge. This method resembles a reasonably accurate field simulation. It also achieves a higher cycle count than doing full discharges.

In addition to cycling, the battery ages even if not used. The amount of permanent capacity loss the battery suffers during storage is governed by the SoC and temperature. For best results, keep the battery cool. In addition, store the battery at a 40 percent charge level. Never fully charge or discharge the battery before storage. The 40 percent charge assures a stable condition even if self-discharge robs some of the battery’s energy. Most battery manufacturers store Li-ion batteries at 15°C (59°F) and at 40 percent charge.

Simple Guidelines

• Charge the Li-ion often, except before a long storage. Avoid repeated deep discharges.
• Keep the Li-ion battery cool. Prevent storage in a hot car. Never freeze a battery.
• If your laptop is capable of running without a battery and fixed power is used most of the time, remove the battery and store it in a cool place.
• Avoid purchasing spare Li-ion batteries for later use. Observe manufacturing date when purchasing. Do not buy old stock, even if sold at clearance prices.

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