We keep one on our RV, since it's parked most of the time... Buy extras for boats, motorcycles... They sell all sizes...
http://batterytender.com/products/automotive/battery-tender-plus-12v-at-1-25a.html
The site will explain it all... After years of using the RV charger and it boiling off the batteries over the winter, I mounted one of these permanently in the stowage area, it just works.. every couple years, check the water level, rarely need any additional.. But I'm using Interstate Deep cycle golf cart batteries, but it's the same for auto 12v setups...
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