Battery Replacement - 4x6V To 2x12V

[dselec]

My 34mkII has 4 Trojan Mileage Master 105 6V batteries, age unknown. They are connected into two banks, each with a pair of batteries in series.  We bought the boat late in the 2013 season and I neglected to do any battery checking or maintenance, other than an occasional push-button check of bank voltages displayed on the analog meter of the electrical panel. Since we only used battery power for day sails and usually switched off the reefer when leaving the dock, I don't have any idea of the battery condition last season. All battery cables were disconnected in the fall.

This spring, the batteries measured 2.3V, 4.6V, 4.8V, and 4.9V on a DVM. The 2.3V battery had a split case and only one cell was functioning, so it is obviously toast. The 3 remaining batteries required over a gallon of distilled water to top them off.

I then connected each battery individually to the SOLAR PL2320 Pro-Logix 20 charger, 2 batteries quickly (15 min) charged to 95% and then stalled there. The cells checked very weak on the hydrometer.
The charger flagged a problem with the 3rd battery and was attempting to repair when I stopped charging at that point.

So, at least 3 of the 4 batteries are probably shot. Since I don't know for sure what killed them (age, lack of maintenance, malfunctioning on-board charger), I don't want to risk buying and potentially frying a new set of Trojan batteries at this time.

I would appreciate advice and/or opinion on the idea replacing the 4 Trojans with a pair of cheap 12V, deep-cycle batteries. I realize the amp-hour capacity will halve, but this should be adequate for day sailing and an occasional overnight.

[Stu Jackson]

With that amount of water required, the old batteries ARE toast.

The way you described your boat use answers the question already. 12V would work just fine for you.

What you really should consider is avoiding the two-equal-bank setup.  Buy two new 12V batteries (you can get 130 ah Group 31 DEEP CYCLE batteries) and put it one switch #1, and add a small starting or deep cycle one to switch #2.  You double your house bank capacity and get a reserve bank for backup.

With that size house bank you should be in fine shape for an overnight anchorout.

What charger do you have?

[dselec]

The charger is a Heart inverter/charger. I'm not at the boat, so I can't specify the model number. It is an older unit, rated at 1000W unit and has a black case.

If I understand correctly, you are suggesting two 12V, 130AH batteries in parallel on bank 1 and one smaller 12V battery on bank 2 for starting, correct?

[Stu Jackson]

Yes that's correct.

Your Heart is most likely a Freedom 10.  1000W inverter, 50A charger.  We have a Freedom 15 installed in 1998, works great.