Battery Care Pays Off

Among the more neglected pieces of gear aboard most boats are the primary storage batteries, which faithfully crank the engine and light the lights. In the case of the typical lead-acid battery, even benign neglect is tantamount to abuse.

It’s small wonder that many battery manufacturers refuse to recognize the warranty on batteries used aboard boats. The normal marine battery cycling - relatively high discharge rates with no charging, followed by rapid charging at a high rate by high-output alternators - is murder on conventional automotive batteries, and not exactly a piece of cake even for deep cycle batteries, constructed to absorb this type of abuse.

You can, however, increase the life of your existing batteries, and dramatically extend the life of new batteries if you will take the time to give them the care they deserve both on and off your boat. This is particularly true when it comes to off season care of boat batteries. If you’re putting the boat away for more than a month, you should take several steps to guarantee that you’ll get maximum service from your batteries.

  • Check electrolyte level in each battery cell. Top up cells only to the appropriate level - check the battery caps to see what the full level indicator is for the particular battery.
  • Make sure batteries are fully charged. This may mean running the engine, plugging into shore power to operate the boat’s converter, or using an automotive-type 110 volt charger.
  • Thoroughly clean and dry the top of the battery.


When bringing up the electrolyte level in a low battery, use distilled water. Even though your local tap water may be relatively free of chemicals that might damage the battery, why take the chance? You can buy distilled water at any drug store.

Only clean the top of the battery when the caps are in place on the cell. This keeps foreign matter out of the battery. Use a hydrometer to check each cell in the battery.

Use the largest hydrometer available, rather than a 6” long miniature one. The scale will be more accurate.

Remember that hydrometers are usually calibrated for an electrolyte temperature of 77” F. A battery whose electrolyte is warmer than 77” - which it is likely to be while the battery is charging, or immediately thereafter - will give a specific gravity reading lower than the true specific gravity of the electrolyte. Conversely, a cold battery - one left on the boat over the winter and read on a 40” day, for example - will have an indicated specific gravity, as shown by the hydrometer, which is higher than its correct specific gravity.

Keeping the batteries charged while a boat is either out of commission or simply not being used for a while is vital to battery health. In cold climates, if the boat is laid up ashore, the best practice is to remove the batteries from the boat, storing them in a basement, shop or garage that isn’t allowed to get below freezing. Once a month, the batteries can be checked, and brought up to full charge as necessary using an automotive-type battery charger of about six amp output.

Don’t just leave the charger on the batteries all the time. Overcharging will cause the battery to gas, and the electrolyte level will drop. This will shorten battery life by damaging the plates.

If you are religious about checking and charging your batteries, there’s nothing wrong with leaving them in the boat when laid up, even in a cold climate. A fully-charged battery won’t freeze in temperatures well below 0” F. Remember, however, that an idle battery will discharge slowly, even with no load placed on it, so you can’t charge them once in the fall and forget them. Monthly checking and charging are still called for.

If your boat is equipped with a 110 volt inlet and a marine voltage converter/battery charger, the simple solution for off season battery maintenance might seem to be to plug in the shore power cord and let the charger do the work. This can be risky. If for some reason the charger fails to shut off when the batteries are charged, extreme gassing and virtual destruction of the batteries could result, particularly with a high output charger - one charging at 10 amps or more.

In addition, many boatyards won’t allow you to leave 110 volt power running to your boat 24 hours a day unless the boat is checked daily. They’re not being picky, they’re just being cautious. In addition to battery chargers, boat owners have the habit of leaving things like electric heaters operating which can quickly overload circuits and power cords. A 15 amp battery charger/converter and a 1500 watt heater can put a lot of load on the typical 30 amp 110 volt system, especially when you add a few 110 volt lights.

Even with a fully automatic charger, the safest practice is to only have the charger operating when you’re around to keep an eye on it. Besides, this gives you an excuse to hang around the boatyard for a day once a month during the winter. If you don’t have enough small projects to keep you busy one day a month, or don’t enjoy just sitting in your boat dreaming about warm weather, perhaps you should take up chicken farming.

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