Here at work we’ve got a no-break that’s been untouched since it was bought 5 years ago. This wouldn’t be a major issue, except it is an enormous 10 kVA 3-phase no-break that uses 80 sealed lead acid (SLA) batteries.
A quick measurement shows us they have around 5-7 Volts, so they’re definitely “dead”. When searching for a way to recover them, I came across the video below that tells to open them up, refill with rain water, and then apply pulsed voltage to make them usable again. According to the video, the batteries “die” due to a chemical process called sulfation that happens inside the cells when the battery is left unused for a long time, and prevents them from being recharged.
Gutierrez PS 