I leave my vessel plugged to shore power and the refrigeration on all the time (I like a cold drink when I go to the docks). It seems my house bank (two 4Ds) goes through a lot of water. My charger, a Flyback 3-20 seems to be working ok. I've wondered if the 12V refrigeration keeps a constant load on the batters/charger thus taking out the water. If this is the case, could it be possible to issolate the refrigeration from the house bank and provide dock side power from a small 110V/12V transformer? Anyone tried this or some other solution, or am I all "cold" on this one?
I had similar problems. I had an older, small capacity battery charger (8 amp - way too small) to support the batteries, and left refrigeration running.
Two seasons running, the batteries used what I considered way too much water, but I also noticed that the battery charger, even with fully charged batteries attached and NO load, was always registering 3-4 amps of charging. Check out the charger with the refrigeration off and the batteries fully charged to see if you're overcharging and driving the water off.
I did two things:
1) Replaced the old, undersized charger with a Statpower Truecharge 20+, and the batteries with brand new group 27's. (this was an upgrade I was in dire need of anyhow)
2) I Placed a 110VAC/12VDC Power supply to feed the fridge. Mine came form an old project at work, but you could get a 240W switching supply capable of 18A @ 12VDC for about $100. Look at Astrodyne and or Jameco. Then I installed a relay so whenever the power supply saw 110VAC, the relay switched power to the output of the supply (on the N.O. contacts, and the relay coil energized by 100VAC). When I unplug to leave the dock, it looks for its power from the DC distribution panel (N.C. Contacts). Relay is rated at 30A/pole.
One consideration: I sail in Fresh water. You may want a more robust, better sealed power supply if you sail in salt water. The supply will likely have a fan on it, so you cannot just seal it in an enclosure.
One person's solution . . .
"Tranquilizer"
1986 MKI #91