Using a 36V to 12V converter to keep a 12V battery charged?

I have a friend that just re-batteried her cart. She replaced all 6 6V batteries and the 12V battery used for the stereo and lights. She is not mindful to plug in 2 separate chargers and I was thinking of adding a 36 to 12 volt converter from the cart bank to the accessory battery to keep a charge on it.

Has anyone done this and is it a good or bad idea?

I am an electronics person, and can wire it all for her. I just don't know if it is good or bad, or if there is a better solution than adding a $25 converter.



Thanks in advance,
Mike