Boat battery over winter: leave it aboard or take it home?

Every battery loses charge over the winter, even with everything switched off. Lead acid, AGM and gel batteries self discharge, and small loads such as an automatic bilge pump switch, an alarm or a tracker keep drawing current. A flat lead battery is the one at risk in a hard frost: the more it is discharged, the more the acid turns to water, and that freezes early. The case splits and the plates buckle. Fully charged, the same battery copes with a serious frost. Lithium batteries hardly self discharge, but most makers want them stored part charged and never charged below freezing.

Whether the battery stays aboard usually depends on power in the yard. If it stays, charge it fully before lay up, turn the isolator off and disconnect the negative terminal so nothing drains it unnoticed. Then top it up every few weeks, or connect a maintenance charger with the right charging profile for the battery type if the yard allows shore power on boats ashore. With no power, take it home: clean and grease the terminals, store it dry and frost free, check the voltage once a month. A charger always sits on a solid, non combustible surface, never on a cover, wood or cushions.

For insurance, what killed the battery matters. Hull wordings commonly name fire as an insured peril, including in winter storage; ours does in every tier. Many also exclude wear and tear, poor maintenance and frost unless the frost follows an insured event, which is why a frozen or worn out battery is rarely a claim. Electric and hybrid boats are often subject to their own conditions: wordings may require the owner to keep the battery charged during lay up and exclude deep discharge and fires caused by it. Read the battery section of your policy before the boat comes out of the water, and ask your broker if the lay up period is covered at all.

Worth remembering

Ten minutes of charging a month costs less than a new battery.