I've got two 'BatteryMinders', that I have used for a few years.
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200332201_200332201Won't over charge the batteries, won't 'cook' them. You can make your own harness to charge up to 4 batteries at once. I did that, and keep the 4Wheelers in the trailer, all winter, hooked up and ready to go (three of them). Made a 2 battery harness with the other, and keep it on the Valk, and my boat battery.
They really work, and the price is right too.
Black Dog