Save Big Money on Boat Batteries

by Shane Granger
(Historic Vessel Vega)

Boat batteries are a major investment so doing all we can to extract the maximum use from them makes good sense.

Over the years, often through expensive lessons, I have discovered several important tricks to make your batteries last longer and give you better service.

There are basically 2 types of batteries used on boats, starting batteries and deep cycle batteries.

Starting batteries are made to give out high amperage for short periods of time and be quickly recharged.

Deep cycle batteries are designed to be drained deeper over longer periods of time and then recharge without damaging the life of the battery.

That said every time you discharge a battery below 50% of its capacity, or below about 12.4 volts, you reduce its useful life.

All of these tips are for normal liquid acid type batteries, but many apply to other types as well.

Always consult your battery manufacturers website for details or visit for loads of professional advice for all types of batteries.

Lets start with how to store your batteries when the boat is being laid up.

First thing to do is make sure the batteries are topped up with sufficient water then equalize them according to the manufacturers instructions.

Most modern marine 3 stage chargers have a special equalizing function that makes this an easy task.

Be sure to follow the instructions carefully and switch off all of your systems as you will be charging at between 15-16 volts, (30+ volts for a 24 volt system) with very low amperage.

To cut a long story short equalizing remixes the acid in your batteries and helps keep the plates clean.

When the boat sits on the charger for long periods in a marina the acid stratifies and deposits form on the battery plates.

Equalizing mixes up the acid again and helps clean the plates, so you get back more power.

The next step is to charge the batteries completely.

Now this is a tricky bit.

Many chargers have a built in amp counter and or only charge to a certain point before switching to float.

The battery is only fully charged when it is accepting no more than 1-1.5% of its rated amps.

A fully charged 12 volt battery should have between 12.85 and 13.2 volts when completely disconnected from the system.

Once fully charged disconnect both battery cables from the batteries and after a few hours read the voltage on each battery.

Make a note of that voltage and keep it handy for when you are ready to recommission the boat.

Be sure to disconnect the batteries from each other when you do this.

Recommissioning is more or less the opposite of the above.

Read each individual batteries voltage and compare it to the note you left yourself when you disconnected them.

Due to internal resistance the voltage will be somewhat less.

That voltage drop should be uniform across all of your batteries.

If one of them is noticeably lower than the others it may well be going bad.

Reconnect the batteries to each other and the system.

Check the battery water levels and charge the batteries until completely full.

If you had a questionable battery with lower than normal voltage disconnect your batteries from the system and from each other once they are fully charged.

Let them stand for a day then recheck the voltage for each battery separately.

The voltages should all be fairly close to each other.

If one or more are lower than normal consider replacing.

Most manufacturers suggest equalizing your batteries several times during the year depending on how often and how deeply you cycle them.

This is important advice that can save you a lot of money by extending the life of your batteries, so be sure you do it.

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I am perfectly aware that the majority of Wooden Boat aficionados are sensible folk.
However, I need to point out that I am an amateur wooden boat enthusiast simply writing in order to try to help other amateur wooden boat enthusiasts.
And while I take every care to ensure that the information in DIY Wood is correct, anyone acting on the information on this website does so at their own risk.