#4
Basically, Batman names everything with “Bat” at the beginning—Batmobile, Batcave, Batwing, you name it. So when he tells Robin to check the “battery,” Robin assumes it’s another one of Batman’s Bat-named items.
He removes “Bat” from the word and ends up with “tery,” which confuses him completely, since “tery” isn’t a real word. That wordplay is the joke.