We have been getting a lot of questions about what to do with your boat batteries during the winter, especially in small trailered boats, batteries can be expensive, dual purpose 24 series from $130.00 each to an 8D marine battery that West Marine sells for $889.00!
A great resource for all types of information is Boat US, this is what they recommend.
Winter Battery Care
On small boats that aren’t left in the water, you may want to take your batteries home and put them on a trickle charger. If you chose to keep them aboard, here are some tips: Top up wet-cell batteries with electrolyte. Make sure battery cable connections are tight and free of corrosion — clean them if necessary with a pot scrubber or emery board. Coat the connections with a corrosion inhibitor like Boeshield T-9. Leave the batteries hooked up to a marine charger that has a float setting or leave them unplugged but charge them up completely at least once a month.
Batteries left on an automotive trickle charger for long periods of time run the risk of boiling off the electrolyte and, at the extreme, exploding. Today’s batteries do best when charged using a marine “smart charger” that varies the charge based on differences in battery chemistry and matches charging voltage to what the battery can accept at different stages of the charging cycle. When buying a marine battery charger, look for the following features:
•A three-stage charger with bulk, absorption, and float stages (or a four-stage charger with an additional pre-float stage) and battery type selection
•Output of 25-40 percent of the battery bank capacity in amp hours
•Temperature sensing at the batteries for automatic adjustment of charger output
•Equalization phase for use with flooded cell batteries
•Ignition protection if installed in a gasoline engine room space.