Me, I make my own battery tender/chargers from small Lambda 8a 15v power supplies. I add a diode on the output to prevent discharge through the power supply when it isn't powered; I connect the sense input after the diode to take care of the diode drop; and I adjust the supply to exactly 13.8v. Then I install it permanently. It floats the battery at 13.8v whenever there is AC from either shore power or the generator, and I never have to worry about the battery when stationary for months. I once bought a box of ten such little Lambda supplies, brand new, for less than $50 on eBay.
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-- Carl
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