Telephone central exchanges used to have enormous lead acid battery (like automobile batteries, the starter motor not the EV kind) arrays if they lost power.
It was amazing to see the battery array discharge ammeter at 5000 amps when utility power went out.
some ISP's still do (or did, last time I was in one), at least one I worked for a few years back. Had a room full of lead acid batteries on shelves and another with a big spinning wheel to smooth out the flow.
It was amazing to see the battery array discharge ammeter at 5000 amps when utility power went out.