Learn what you can do to prevent a Li-ion battery to fall asleep.
Li-ion batteries contain a protection circuit that shields the battery against abuse. This important safeguard has the disadvantage of turning the battery off if over-discharged. Storing a discharged battery for any length of time can do this as the self-discharge will gradually lower the voltage of a battery that is already low on charge. The protection circuit will eventually cut off between 2.20 and 2.90V/cell. (See BU-802b: Elevated Self-discharge)
Some battery chargers and analyzers (Cadex), feature a wake-up feature or “boost” to reactivate and recharge batteries that have fallen asleep. Without this provision, a charger would render these batteries unserviceable and the packs would be discarded. The boost feature applies a small charge current to first activate the protection circuit. If a normal cell voltage can be established, the charger will them start a normal charge. Figure 1 illustrates the “boost” function graphically.
Figure 1: Sleep mode of a lithium-ion battery
Some over-discharged batteries can be “boosted” to life again. Discard pack if the voltage does not rise to a normal level within a minute while on boost.
Do not boot lithium-based batteries back to life that have dwelled below 1.5V/cell for a week or longer. Copper shunts may have formed inside the cells that can lead to a partial or total electrical short. When recharging, such a cell might become unstable, causing excessive heat or showing other anomalies. The “boost” function by Cadex halts the charge if the voltage does not rise normally.
When boosting a battery, assure correct polarity. Advanced chargers and battery analyzers will not service a battery in reverse polarity. With a sleeping Li-ion, the voltage cannot read and the service must be done with this awareness. Li-ion is more delicate than other chemistries and a voltage applied in reverse may cause an electrical short or do other damages.
Storing lithium-ion batteries presents some uncertainty. On one end, manufacturers recommend to keep them at a state-of-charge of 40–50 percent, and on the other end there is the worry of losing them due to over-discharge. (See BU-702: How to Store Batteries) There is ample of bandwidth between these criteria, and if it doubt keep the battery at a higher charge in a cool place.
As part of a study, Cadex examined the cause of battery failure in mobile phones. Out of a large population of returned warranty batteries, 91 percent could easily be restored, of which 30 percent failed due to over-discharge. Only nine percent were non-serviceable. The restored batteries went back to service. Lack of test devices at customer service levels is in part to blame for the high exchange rate.
Last updated 4/29/2015
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