Running DC circuits while charging battery?


Feb 8, 2021
Catalina 310 108 Lake Guntersville
Dumb question but I have to ask. Can I run DC circuit equipment (lights, fridge, radio) while on shore power and having shore power charging the battery? Will this depend on the battery charger?
Feb 6, 1998
Canadian Sailcraft 36T Casco Bay, ME
If the charger is properly sized the answer is yes, absolutely.

The complete answer is slightly different, especially if your charger is not "properly sized". If your on-board DC loads are larger than your battery charger you can actually be discharging the battery even though the charger is running at full-bore. As an example we had a customer with a 10A charger on a 450Ah bank. On shore power he had an average 15.5A of load.. He was negative -5.5A even with the charger running at full-bore because it was not sized properly. A charger should be sized at a minimum of 10% of Ah capacity, after dockside loads have been accounted for, if that is how you will be using it.

Eg: 450Ah Bank + No Dockside Loads = 45A - 50A Charger

Eg: 450Ah Bank + 10A of Dockside DC Loads = 55A - 60A Charger

This article goes into much more detail:
Jan 4, 2006
Hunter 310 West Vancouver, B.C.
One example is that we often run the 12V refrigeration while on shore power at other marinas for a week at a time. Everybody happy :biggrin:.
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Oct 22, 2014
CAL 35 Cruiser moored EVERETT WA
How about turning on the lights to cook a meal while the charger is recharging the batteries. Not much difference between the AC charger running and the engine driving the Alternator charging... Do you run in the dark or shut down the electronics while motoring the boat? No.
Nov 16, 2012
Catalina 310, 2000, #31 31 Santa Cruz
One thing to watch for: if your DC equipment can’t handle 14+ volts. Most can, but it’s something to be aware of.
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