By who? I used to be a professional musican and I've never in my life heard anyone call it that. Also the Wikipedia page for "drum kit" doesn't contain the word battery at all, it's not a listed definition in online dictionaries or in my copy of the Oxford Concise, and Googling "drum battery" just brings up a load of Metallica drum covers. The only reference I can find for them being synonymous is that one Wikipedia page you linked. For all intents and purposes, that is not what the word means.