You might like this battery charger it is small enough to make into an onboard charger plus it automaticaly changes into a float charger. If you dont like the spring clips you can change them out for ring terminals.
harborfreight.com/6-12-volt-electronic-battery-charger-and-maintainer-with-lcd-display-65834.html]6/12 Volt Electronic Battery Charger and Maintainer With LCD Display
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