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ITFrequently Asked Questions · Batteries
What is the risk of prolonged discharge?
Excessive discharge permanently reduces capacity and increases internal resistance.
When a lead-acid battery discharges, lead sulphate forms on its plates. If it is recharged soon, this material returns to its original form. But if it stays empty or half-empty for days or weeks, the crystals harden (sulphation) and no longer dissolve with normal charging. The result is less active material, meaning lower capacity, and higher internal resistance, which makes the voltage drop sharply as soon as a load is applied.
In practice, a UPS with such a battery may show as charged but shut down within seconds or minutes at the first outage. A typical scenario: a long outage drains the batteries, and then the UPS stays switched off or without power for days, for example in a holiday home or a shop closed for the holidays.
Practical tip: after every long outage, leave the UPS plugged in and switched on so it can fully recharge the batteries, which can take several hours. If backup time has dropped noticeably, run a battery test and consider replacement, because a sulphated battery rarely recovers fully.