Originally Posted by 10Boomer
I have 2 deep cycle 6 volt batteries, 2nd year since newly installed. I use 1.5 watt solar panel I mount on TT tongue. It maintains the charge at 13.4 volts when stored. I tried a 4.5 watt earlier and it over charged the batteries. Just remember to check water levels in the batteries. Good luck
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This is why a solar charge controller is needed to replace battery capacity lost.
Small panels without integral controllers can overcharge because current will always flow when the panel in in partial to full sun; whether the battery can handle the influx or not.
Your 1.5 watt panel is producing 13.4 volts at (.112 amps) 112 milliamps (1.5 watts) in full sun; less current if not. That is about what parasite draw plus internal loss is; so capacity remains about the same when the panel is connected. Much more continuous current than that will definitely overcharge and ruin the battery over time as you discovered.
A solar charge controller will reduce charging current to a trickle as the battery fills; a good one will also vary voltage output based on battery condition, like your camper's staged converter, to "make hay while the sun shines" and efficiently recharge a partially discharged battery.