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Why don’t my batteries charge well?

2014 September 1

Dear Boat Doctor,

I use an invertor to charge the house batteries at the dock and the engine alternator to charge them away from the dock. The inverter seems to charge just fine, but the alternator is not working well. I checked the voltage level of the batteries while running the engine the other day,  and the engine bank was at almost 13 volts but the house was only at 12.1 volts. I never really seem to get a good charge underway.

Roger Farnham

Baltimore, Maryland

Dear Roger,

It sounds like you’re using a battery combiner to connect your banks for charging. Although these devices work, they drop almost a volt in operation. You can see this when you compare the voltage between your house and engine banks while running the engine. By only charging the house bank with 12 volts, charging will be very slow. I would replace the battery combiner with an automatic charge relay like the Blue Seas 7610 (www.bluesea.com, 800-222-7617).

I also suspect that your alternator is connected to your engine battery. An alternator uses a regulator to control its output, but a regulator reduces the output of the alternator as the battery charges. Since the regulator is seeing your fully charged engine battery it reduces the alternator output and little current gets to the house bank. Simply connecting your alternator to your house bank will solve this problem.