baterio

Two suffixes seem to be on offer here, and neither one is. Baterio ends in -erio, so the eye tries bat- (the root of bati, to beat) plus -er- plus -io. There is no such build: -er- means a fragment of a mass, -io names countries and sciences, and neither would produce anything like a power cell. Bateri- is one indivisible borrowed root.

As with parfumo, though, the history behind the false seam is true. The French batterie really does come from battre, to beat, because a battery was first a row of cannon beating at a wall. Franklin took the military word for a row of Leyden jars wired together, and the meaning drifted from "several things acting as one" to "the thing in your torch". The beating is genuinely in there; it is just Latin history rather than Esperanto grammar.

Both senses survive. Baterio is the everyday power cell, but also a battery of guns, and a baterio da demandoj is a barrage of questions. Akumulatoro is the more technical word for a rechargeable one, though most speakers say baterio for everything and let context sort it out.