Originally Posted by Tim M
Will a 12v transformer charge a battery? If yes, how beefy does it need to be?
Transformer by itself, no. It outputs AC at 12V RMS (which should give you the required peak voltage once you rectify the sinewave.)
Begging for problems, though: Battery chargers are usually current limited (in case of a completely drained battery or somesuch) whereas the jury rigged transformer wouldn't be.
Small smart chargers aren't all that expensive. Larger automotive types (not smart - stupid, perhaps?) are even less.