I wouldn't do that. Lead-acid batteries require regular trickle-charging during long-term storage if you ever expect to use them to start your engine again.
Sorry but this is just not accurate, especially in the climate
we are discussing in regards to Fallard's post..
On of the only detrimental things that happens during long periods of not charging in colder weather is electrolyte stratification
. However the vast majority of battery chargers do nothing to prevent this and are really poor at preventing stratification. A float voltage will not prevent stratification. Perhaps 90% of "smart chargers" will remain in float indefinitely which is pretty useless because this alone won't prevent stratification. Only a charger that periodically returns to an absorption cycle with enough voltage to "roll the electrolyte" will help prevent this. You are far safer
to leave the charger OFF, when stored on the hard & unattended, and periodically, perhaps every 30-45 days, cycle the charger on for an hour or two and hit a good gassing voltage.
The best advice is to remove the batteries from the boat, store them someplace they won't freeze and put them on a trickle-charger (even a 5Watt solar panel from a camping store will suffice) to ensure they're still ok next season.
That is the best advice
if you want disc surgery...
A properly winterized battery will NOT FREEZE until -70F or colder... Leaving batteries to "trickle" unattended is far more dangerous than leaving them connected. Just ask me about how many faulty chargers I see on a yearly basis... LOTS!!!!
I had a customer destroy a bank of golf cart batteries with a single 10W solar panel... By spring they were cooked dry because the 10W panel had no controller. If he had left them equalized and 100% disconnected, as I left them, they would have still been in service to this day.
I have also had customers leave solar panels on during winter but then become occluded with snow. The batteries drained well below the safe turn on voltage for the controller and it could not recharge them. The controller and phantom loads then murdered the bank.
In over 30 years of leaving batteries on boats, disconnected, but charged to 100% and equalized in the fall, if applicable, cause any of them to die prematurely. Seen PILES of boats left charging, while unattended, with dead batteries come spring.....
This may be of use to some:
Effect of Winter on Self Discharge (LINK)