Swapping the battery in an old line-interactive UPS

2026-09-14

The little UPS under the desk started beeping every few minutes last week. The self-test said the battery was fine; the runtime estimate said otherwise. Four years is about what you get from a sealed lead-acid cell that lives next to a warm switch.

The replacement was a standard 12 V 9 Ah block with F2 terminals. The case opens with two screws, the old battery slides out, and the only trap is the polarity of the spade connectors: photograph them before you pull anything.

After the swap I ran a full discharge test with the NAS and the router on it. Nineteen minutes, which is more than enough for a clean shutdown. The NUT client on the NAS now triggers the shutdown at 30 % instead of the default low-battery signal, because the old firmware reports it far too late.