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ITFrequently Asked Questions · Batteries
What is deep discharge and why is it dangerous?
It means using the battery until it is almost completely empty. It can cause irreparable damage.
Every lead-acid battery has a minimum voltage it should not go below. For a 12 V battery, the end-of-discharge limit is usually around 10.5 V under load, depending on the discharge current. Below that, the plates are heavily stressed, the electrolyte becomes very dilute and sulphation progresses quickly. A single such discharge can take away part of the capacity, and a few repetitions are enough to ruin the battery.
UPS units have low-voltage protection that shuts the inverter down before the battery reaches a dangerous level. Deep discharge happens in other ways, though: when the UPS is stored for months, when it is left without power for days after an outage and self-discharge keeps draining the batteries, or when batteries are connected to improvised setups without protection.
A deeply discharged battery may no longer accept a proper charge from the UPS. Never leave it empty, and charge it as soon as possible. If after charging the backup time is very short, or the battery swells or gets hot, replace it rather than keep using it.
