Better than wood is a thick sheet of expanded polystyrene foam. Or a couple of old ceiling tiles if you are, like me, old enough to remember those horrors.
Wood can be as damp as concrete, EPS sheet can't.
This I found, it is a really good explanation of how batters work and what ails them.
Batteries: A short life...! Quite why Biggin Hill Airfield is posting on the subject I can't explain...!
