I have a 12V, 1/2 amp charger. Can it be used on a 6V system/ battery?
No! You need a 6v charger. Walmart has a 2 amp trickle charger on their web site for $17. I use one of those with great results.
Mike, if you have time, look at:
Especially the post on July 11,, 2009 by John Regan for a way to build a battery maintainer for "under $5.00"