What's the Non-Correctable Battery Problems(3)?

An SLA self-discharges at a rate of only five percent per month or 50 percent per year. Repeated deep cycling increases the self-discharge. When deep cycling, the electrolyte is drawn into the separator, resulting in a crystalline formation similar to that of a NiCd battery.

The self-discharge of a battery is best measured with a battery analyzer. The procedure starts by charging the battery. The capacity is read by applying a controlled discharge. The battery is then recharged and put on a shelf for 24 hours, after which the capacity is measured again. The discrepancy between the capacity readings reveals the level of self-discharge.

More accurate self-discharge measurements can be obtained by allowing the battery to rest for at least 72 hours before taking the reading. The longer rest period compensates for the relatively high self-discharge immediately after charge. At 72 hours, the self-discharge should be between 15 and 20 percent. The most uniform self-discharge readings are obtained after seven days. On some battery analyzers, the user may choose to adjust the desired rest periods in which the self-discharge is measured.

Research is being conducted to find a way to measure the self-discharge of a battery in minutes, if not seconds. The accuracy and repeatability of such technology is still unknown. The challenge is finding a formula that applies to all major batteries and includes the common chemistries.

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