I have a large (group 31) AGM deep cycle battery I use as my house battery. It is about 2 years old and has had what I would call moderate use. So far it has the winch, fridge, water pump, and charging ports hooked up to it. Drove about 7 hours last week to destination. Shut everything down except fridge. Battery was only indicating a charge of around 12 volts. Fridge ran fine until battery hits about 10.5 volts, at which point the fridge is designed to cut out, which it did. Fridge only took about 24 hours to reach shut off this last trip, whereas before it would run for days. I put a trickle charger on the battery last night and it charged overnight to 14.4 volts. I would have thought the alternator would have done the same thing on a 7 hour drive. Any theories or advice are welcome, as I am not that experienced with deep cycles or battery charging physics. Is my alternator defective? Battery aging means it won't accept charges of certain characteristics? My starting battery is AGM but not deep cycle, and is charging off the identical alternator. I checked both batteries with the multimeter last night at the same time.