Surrette Battery Co. Ltd. Bulletin #506
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 o F (70oC). Below shows temperatures at which electrolyte, in various states of charge, starts to freeze.

Specific Gravity( 80oF/26C) Freezing Temp
1.280 Spec. Grav.( 80oF/26C) -92F (-69C)
1.265 Spec. Grav.( 80oF/26C) -72.3F (-57.4C)
1.250 Spec Grav.( 80oF/26C) -62F (-52.2C)
1.200 Spec Grav.( 80oF/26C) -16F (-26.7C)
1.150 Spec. Grav.( 80oF/26C) +5F (-15C)
1.100 Spec. Grav.( 80oF/26C) +19F (-7.2C)



A 3/4 charged battery is in no danger of freezing. Therefore, batteries should be kept at least 3/4 charged, especially during winter weather.

The frequency of checking batteries depends greatly on temperature. The effect of temperature on self discharge for the average fully charged, new, conventional battery in good condition is approximately as follows:

  1. At 100oF (37.8oC) .0025 Sp.Gr. per day
  2. At 80oF (26.7oC) .001 Sp.Gr. per day
  3. At 50oF (10oC) .0003 Sp. Gr. per day

A fully charged battery stored at 80oF (26.7oC) will take 30 days before it self discharges 25 percent. At 50oF (10oC) the time period increases to 100 days. This will give you an idea of how often a battery should be checked.

Some makes of batteries will have a higher and some a lower rate of self discharge. This depends on the method of manufacture and purity of materials used.