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Why won’t my house batteries hold a charge when powering?

2020 March 1

Dear Boat Doctor,

I just got a new-to-me boat and I have a charging problem: the house bank won’t stay charged when I run the engine. When the engine is running, there is 13.5 volts at the alternator, 13.4 volts at the engine battery and 12.7 volts at the house bank. I am guessing the problem is 12.7 volts, which seems low, but I don’t know what the problem is.

Gerald de Thame

Norfolk, Virginia

Dear Gerald,

Your alternator is putting out a decent voltage level, which will vary depending on the state of the batteries, but 13.5 volts is in the ballpark. I am assuming that the alternator is connected to the engine bank and the house bank is connected through a combiner of some sort. Battery combiners (isolators) were traditionally diode-based, and diodes will drop about .7 volts, this is the difference between the 13.4 volts at the engine battery and the 12.7 volts at the house.

A fully charged 12-volts battery will have a voltage of about 12.8 volts but takes more voltage than that to charge it efficiently. Your charging voltage of 12.7 volts will take a very long time to charge the house bank.

I would recommend a couple of changes. With your alternator connected to the engine bank, your alternator will see the typically well-charged engine bank and greatly reduce its output to not overcharge. I would connect the output of the alternator to the house bank so that it can get an accurate reading of the needs of this typically much more discharged bank.

I would also look into a lossless battery combiner. A Blue Seas Automatic Charge Relay would work well, or you could use one of the modern lossless isolators. These new isolators have an input for your alternator and an output for each battery bank.

In either case, you will want to be sure to remove the wire connecting your alternator and starter. This will allow the engine to start from one bank, and the alternator to independently charge the other. You’ll want the engine bank connected directly to the starter and potentially a new cable connecting the alternator to the house bank or isolator.