Battery fluid (4)

I believe the answer is:
acid
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">battery fluid</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for acid that I've seen before include "Polluted precipitation" , "Etcher's fluid" , "Sour-tasting" , "Corrosive liquid; drug" , "Bitter or sour" .)