I guess it depends on what you currently have, Amazon has lots of chargers so you may need to narrow that down just a little.
My advice would be go to a local battery source, someone that knows batteries not Walmart or one of the chain auto parts stores, a dedicated battery supplier should give better information. I have a small charger, from a local battery source, that will charge 6 and 12 volt batteries, it will float or not overcharge, and it is selectable for three different battery chemistries and I have had good luck with it.
I don't remember the name of the charger but if I make it out to the toy box today I will snap a pic and post it up.