Self-discharge is a natural phenomenon of a chemical system. Similar to a spring under tension, a battery wants to return to a state of rest or revert to the lowest form of energy. Self-discharge is not a manufacturing fault per se, although poor manufacturing practices and improper use can accelerate this phenomenon.
Self-discharge of a normal cell is induced by spontaneous oxygen generation at the positive electrode, an activity that intensifies at higher temperatures. The amount of self-discharge differs with each system and cell design, and some manufacturers are more effective than others in keeping it low. Self-discharge is non linear and is most pronounced right after charge when the battery holds full capacity. High-performance batteries with enhanced electrode surface area and super conductive electrolyte are subject to higher self-discharge than low performance batteries.
The NiCd and NiMH battery chemistries exhibit an inherently high self-discharge. If left on the shelf, a new NiCd loses about 10% of its capacity in the first 24 hours after being removed from the charger. The rate of self-discharge settles to about 10% per month afterwards. At a higher temperature or with advanced age, the self-discharge rate increases substantially. Typically, the rate of self-discharge doubles with every 10° C (18° F). A problem arises if a battery with good capacity readings goes flat during use through excessive self-discharge rather than by providing power. Such an occurrence is quite common.
Self-discharge is increased by a damaged separator with metallic dendrites penetrating into it. The separator is a thin, delicate insulator that isolates the positive and negative cell plates (see figure below). Once marred, the separator can no longer be repaired by cycling the battery. External forces that harm the sensitive separator are uncontrolled crystalline growth caused by lack of maintenance and poorly designed chargers that "cook" the batteries.
Another contributor to high self-discharge is high cycle count and/or advanced age, a normal wear-down effect that causes the battery plates to swell. Once enlarged, the plates press more firmly against the separator, resulting in increased self-discharge.
Plate swelling can be reduced by loading less of the active materials onto the plates. This process improves the expansion and contraction while charging and discharging. In addition, the load characteristic is enhanced and the cycle life prolonged. The downside is lower capacity.
The self-discharge of a battery can best be measured with a battery analyzer by first charging the battery, then measuring its capacity through discharge. The battery is then recharged and put on a shelf for 24 hours after which the capacity is measured again.
While a capacity loss of 10% at room temperature over 24 hours is normal for a NiCd battery, a 15% to 20% loss can be expected at elevated temperatures (above 331/4C or 901/4F). If the loss approaches 30% at room temperature, the battery should be replaced. Expect a higher capacity loss with NiMH batteries. Cycling will not correct a high self-discharge condition.
More accurate self-discharge measurements can be obtained by allowing the battery to rest for 72 hours and more. The longer rest period compensates for the relatively high self-discharge immediately after the battery is removed from the charger. At 72 hours, the self-discharge should be between 15% and 20%. The most uniform self-discharge readings are obtained after seven days.
A good SLA self-discharges at a rate of only 5% per month or 50% per year. Deep cycling increases the self-discharge as it causes the electrolyte to be drawn into the separator, forming crystalline formation similar to that of a NiCd battery.
The Li-ion battery self-discharges 3 to 5% in the first 30 days, after which it settles to 1-2% per month. In addition, the electronic circuit used to control the Li-ion can draw as much as 3% per month.