The most common and most likely answer to this crossword clue is the 6 plus 7 letter word DIRECT CURRENT
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More clues leading to the result DIRECT CURRENT
Buddy explains!

The straight part
We know that "battery output" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case battery output = direct current.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
DIRECT CURRENT noun- An electric current in which the electrons flow in one direction, but may vary with time.
- An electric current which does not vary with time.
- An electric circuit in which voltage and current do not vary with time.