Not 100% sure, but my mind is telling me that i have heard "flat battery" in an American accent, which would most likely be from a film or TV series. I can't imagine them saying "out of gas" instead though, as that is a different thing altogether. As to the question of where the expression comes from, no idea, but i'd be interested to hear as it is odd!.

Ask Susie Dent!

May be an analogy with a flat tyre?

"I doubt there’s a definitive answer to this question, but here’s some speculation. The word “flat” can mean (according to the Google dictionary) “lacking interest or emotion; dull and lifeless.” This definition fits pretty well, in a figurative sense, with a flat battery. Fizzy drinks can also be flat, meaning they’re not fizzing any more, and I wouldn’t be surprised if the idea of a “flat battery” is at least in part inspired by a flat drink—in both cases, all the “energy” or “pep” is gone."

Going (a long way) back to my youth, we had "accumulators"/ batteries made of glass to run the radio. You took these to the "radio" shop to be recharged - and the acid tended to bubble

See EverReady battery here:

www.radiomuseum.org/forum/mcmichael_a_super_range_...l

Edited by colinh on 10/08/2018 at 19:56