I would say off the top of my head. Battery Tender (brand). I think they are like $45. What kind of battery's do you have? If you have a standard flooded battery's the tender will work fine.
Chris: Yea the Battery Tender or the charger that Patrick posted look pretty much the same.
Trickle chargers work great for bringing you battery back to full charge, down side is they take along time. Example the Battery Tender I was talking about would take like 4 days to bring your 2 batterys from fully dead to fully charged. Im assuming you dont run your 2 batterys all the way down day after day so a charger like Im talking about will work perfict.
Patrick: If it took you over a month to decide what $49 trickel charger to get you need some more hobby's LOL J/K
"G" I research everything I buy in great detail. It is my OCD personality taking over, but I tend to learn quite a bit along the way. I work in a office cube so plenty of free time at work to search the web
this is the one i use. it works great. it reduces charge rate as batts get close and shuts down when they are full its about $150.i have not been to impressed with those energizer batts. i have alot of customers complain that they dont last for chit! and sams fights them about replacing them. hope they work out for you.