Lead-acid batteries will self-discharge when not in use.  The rate of self-discharge increases as ambient temperature rises.  As a precaution, stored batteries should be recharged every 3 months until put into service to avoid over-discharge, sulfation and possible freezing in cold temperatures.


Winter Storage

 

Prior to placing batteries into winter storage make certain the electrolyte level is approximately 1/2" (13mm) above the top of the separators. The electrolyte level in very cold batteries will be lower than normal, so let batteries warm to a normal temperature before judging electrolyte levels.

Once the electrolyte level is correct ensure that the batteries are fully charged. Ensure that the battery tops are clean and dry.

 

Now the choice is whether to leave the batteries aboard your boat or remove and store in a cool dry area. If the batteries are stored aboard the boat, disconnect the terminal cables. This will prevent premature discharge of the batteries due to a ground in the electrical circuits or failure to turn a piece of electrical equipment off.

If the batteries become discharged, the electrolyte can freeze when stored below +20° F (70° C). Below shows temperatures at which electrolyte, in various states of charge, starts to freeze.
 

Specific Gravity

(cor. to 80° F/26° C)
Freezing Temp
1.280 -92° F (-69° C)
1.265 -72.3° F (-57.4° C)
1.250 -62° F (-52.2° C)
1.200 -16° F (-26.7° C)
1.150 +5° F (-15° C)
1.100 +19° F (-7.2° C