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Old 01-16-2002
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Winter Battery Storage

I am wet-storing my boat for winter, but I am concerned that the batteries will freeze during the few weeks when the temperature gets in the low teens. Do you have any suggestions on how to keep the bilge pump supplied with power?

Tom Wood responds:
A few weeks in the lower teens? Not to worry. Here are the facts:

Acid will freeze, of course, as will water. The temperature at which the electrolyte in a fully charged battery freezes is about 75 degrees below zero Fahrenheit—which is why cars still start in the arctic in winter. On the other hand, the electrolyte in a mostly discharged battery is close to the specific gravity of water, and it freezes just below 30 degrees. So the first imperative is to get the battery fully charged and to keep it that way for the duration of the winter.

The main threat to the health of a battery comes from it standing idle. When batteries are used frequently, being discharged and fully recharged on a daily basis like they would be in the family automobile, even a cheap one will last a long time. This is because the chemical process "stirs" the electrolyte and cleanses the lead plates. When batteries sit for long periods of time between use, some of the sulfuric acid forms sulfate crystals that coats the lead plates. This coating gradually gets thicker, interfering with the chemical process between the lead and the acid. Some of this cooking can be "blown off" the plates by an equalization process—applying an unusually high voltage to the battery for six or eight hours. But the best way to take care of it is to prevent it by keeping the battery charged.

All batteries lose power over time and need to be recharged, even if they aren't used. The rate of this "self discharge" is about 1/100 of a volt per day for wet batteries, less for AGM or gel batteries. This is about 25 percent of battery capacity per month, so it's a good idea to recharge idle batteries every two weeks or so.

If your marina does not have electricity over the winter to operate your battery charger, I would recommend that you invest in a small solar panel to do this job. A charge rate of about two percent of the battery capacity is a good rule of thumb because this amount would be incapable of overcharging the battery and would not require the addition of a voltage regulator. This translates into roughly 25 watts of solar panel for every 100 amp-hours of battery capacity—a little more for northern boats in the winter because the sunlight is weak, there are many cloudy days, and daylight hours are short.

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