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Old 09-30-2001
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Tom Wood is on a distinguished road
Batteries in Winter

In Winter Storage, Joy Smith suggests that up north the batteries should be removed and a trickle charger attached, but no reasons are offered as to why. Can you explain it?

  1. Why is it bad to leave them in place? Or is it simply an urban legend?
  2. Would it be bad to leave them outside (in the boat) with a trickle charger left on year round?

Tom Wood responds:
I’m continually amazed that this question comes up every season. Even though cars up north survive the winter outside each year, and indeed many inexpensive automotive batteries last three or four years, their more expensive marine and RV cousins die an unnaturally premature death at the age of two. But the car gets started and used, throwing a load and recharge onto the battery every day. Try letting your car sit in the driveway from Labor Day until spring and see if it starts after Easter.

Here are the facts. Wet-cell batteries have a self-discharge rate of about one percent per day at 75 degrees Fahrenheit. This rate goes down with the temperature, but it rises to two percent per day at 95 degrees. AGM and gel-cell batteries have a slower self-discharge rate. But in any event, a battery left fully charged in October will be dead or dying by New Year’s Eve even though it wasn’t used.

What is happening as the battery loses its charge is that the specific gravity of the acid is diminishing.  A battery that is 100-percent charged won’t freeze because the electrolyte is dense, with a freezing temperature around minus 75 degrees Fahrenheit. A battery that is zero-percent charged, however, has acid with a specific gravity approaching that of water, and it will freeze at only 27 degrees above zero.

When the electrolyte freezes, a battery’s life is over, and many times the case will crack from the expansion, driving the point home with an unmistakable acid spill.

A slower, more painful battery death is usually ascribed to sulfation. Don Casey covered it in some depth at SailNet in an article entitled "A Little Help from the Sun." Basically, when a battery sits unused and undercharged, some of the sulphuric acid adheres to the lead plates as a coating of sulfates. This causes such a portion of the plates to have reduced effectiveness in transferring the chemical to electrical (and reverse) process in the future. Sometimes the sulfates can be "blown" off the plates by "equalizing" the batteries—that is charging at an unusually high voltage, but each time the battery sits unused, the sulfation becomes a little worse. Letting a battery sit over the winter without stirring the electrolyte by frequent charging will reduce its life by approximately 50 percent.

The answer: Disconnect the battery entirely to avoid any stray electrical leaks draining the battery, make sure that the cells are topped with water monthly, and give a small charge weekly. It’s a lot easier to do this in the garage than up a ladder at the boatyard in the snow. A trickle charger without a full shut-off regulator such as the little portables found in many automotive stores does as much damage as not charging at all—buy a good one. I frankly wish that someone would invent a machine on a timer that would throw a small load on the battery for an hour and then charge it back up automatically once a week. Batteries could be left in storage indefinitely that way.

Oh, by the way, do you remember when you bought a battery in the old days and you had to wait two hours while the store added the acid and charged it the first time? How sulfated do you think many brand new batteries are today when you buy them off the dealer’s shelf where they have been sitting with the acid in them all winter?

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