Overwintering batteries depends on their technology. Conventional lead-acid batteries have a self-discharge rate of up to 10% per month. Wintering for 6 months is enough to over-discharge and damage the battery. With this type of battery, you should therefore plan to recharge at least every 2 months.
New battery technologies such as lead/calcium, gel or AGM have a self-discharge rate of around 2% per month. In this case, there's no need to plan a winter recharge.
When it's time to winterize, make sure the batteries are properly charged, using a battery charger if possible. Then disconnect the batteries via the circuit breaker. Ideally, we use a bipolar circuit-breaker, i.e. one that cuts off both the positive (+) and negative (-) sides of the battery. You can also disconnect the terminals, which also allows you to maintain them and grease them well.
When wintering on board, never leave the charger plugged in all winter. It's not bad for the battery, but you're exposing yourself to the risk of overheating and fire.
A small 20 W solar panel is enough to cover a battery's self-discharge during winter. Small kits with panel, regulator and support are even available for this purpose. In particular, we found the Uniteck Kit available at ?239.
A battery loves the cold!
Contrary to popular belief, you shouldn't keep your batteries warm in winter. Self-discharge is lower when it's cold. So you can leave the batteries in your boat over winter. On the other hand, unloading your battery may make it easier to recharge in mid-winter...
Moisture in the air encourages self-discharge. Lubricating the studs well insulates them from humidity and limits the phenomenon.
Don't forget the bilge pump!
If you winterize your boat in the water, the bilge pump must be able to start automatically in your absence. This means leaving a battery running during the winter. That's why you need a small solar panel to maintain the charge...
