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ITFrequently Asked Questions · Batteries
What does self-discharge mean for batteries?
It is the natural loss of charge when the battery is not in use. For AGM batteries it is 2–3% per month.
Self-discharge is caused by slow chemical reactions inside the battery and happens even when it is not connected to anything. The quoted rate applies at room temperature, around 20°C. Heat speeds self-discharge up considerably, while cold slows it down.
At 2–3% per month, an AGM battery stored fully charged will have lost about 12–18% of its charge after six months in a cool place, and more if the place is warm. The problem is not just the lost energy: when a lead-acid battery stays half-empty for a long time, hard lead sulphate crystals form (sulphation), which permanently reduce its capacity.
In a UPS that is permanently plugged in, self-discharge is compensated automatically by the float charge. It matters when the UPS or spare batteries are left disconnected.
A common mistake: a UPS sits in storage or unplugged for months and, when it is needed, its backup time is much shorter or the battery no longer accepts a charge. Fully charge batteries before storage and recharge them periodically.