Solar charger damages batteies - what's good one?

I've just discovered the Morningstar Sunsaver 6 charge controller I've
been using with my 20 watt panel is a two stage controller, and does not
have a "float" stage. This appears to have damaged my batteries more in
one year than several batteries I've used for 3 years or more, but were
charged with a good 3 stage AC charger.

Are there recommendations for a more suitable charge controller? When
charging, the controller is placed in the trailer, or in the gear well
if the glider is tied out, so the battery and controller temperatures
are the same.

--
Eric Greenwell - Washington State, USA
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