Sizing an Alternator for House Batteries and Loads


Hi Jeff, I would like assistance with purchasing the correct alternator.  Boat use: 6 months sailing around the south and north islands, limited shore power time, 6 months at dock on shore power (living on boat). Changing setup to separate start bank and house bank, with all loads and charging to house bank. 200watts of solar and ProNautic1260P shore charger, echo charger for start batteries. House bank USL16HC 420Amps. Recommended to only charge at constant current of 10% and 14.7Volts in the 1st stage.

So thinking a 105 to 120amp is plenty with external regulation so can control the stages of charging to maximize battery life. Thinking base current use is 3-10amps, but want to purchase a Balmar SG200 to help verify that and possibly help with alternator sizing.  - Roy


Hi Roy, thanks for contacting PYS about an alternator for your boat. Based on the info you provided, a 100 – 120 amp alternator with external regulator would be a good size for your battery bank and your draws while running the engine. You’ve probably seen this, but here is a link to size a Balmar alternator for your engine

Good idea on a battery monitor to accurately measure the depth of discharge and the current going in and out of your house battery.  - Jeff

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