Self-discharge

Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes. Self-discharge decreases the shelf-life of batteries and causes them to have less charge than expected when actually put to use.

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Rates of Self-Discharge

How fast self-discharge in a battery occurs is dependent on the type of battery. Typically, lithium batteries suffer the least amount of self-discharge (around 2-3% discharge per month), while nickel-based batteries are more seriously affected by the phenomenon (nickel cadmium, 15-20% per month; nickel metal hydride, 30% per month).

Causes

Self-discharge is a chemical reaction, just as closed-circuit discharge is, and tends to occur more quickly at higher temperatures. Storing batteries at lower temperatures thus reduces the rate of self-discharge and preserves the initial energy stored in the battery. Self-discharge is also thought to be reduced over time as a passivity film develops on the electrodes.

The detailed chemical causes of self-discharge depend on the particular battery and are not well understood.

References

• Battery performance characteristics, MPower UK, 23 February 2007. Information on self-discharge characteristics of battery types
• Wu and White, "Self-Discharge Model of a Nickel-Hydrogen Cell." Journal of the Electrochemical Society, 147 (3) 901-909 (2000).