I have been using the Centech 1.5 amp on board battery charger maintainer from harbor Freight. I have a pigtail on each of my bikes and I wired up a pigtail onto the charger. They were just on sale with a coupon for about $12.00 each. I bought 2 at that price. My first one is still good at about 20.years old. The name brand on board charger that was similar overcharged batteries and I had to throw it away. I like that these are sealed and work well. I just move it as a regular charger, car, bikes, marine battery etc.