Battery Logic

Slowly we have noticed our batteries losing their “punch.”  No longer do they hold as much power as they used to. But after 7 years, it is to be expected. The experts pronounce that anything over 4 years is better than expected, so we can not complain.

Life on a cruising sailboat away from regular shore power is very tough on batteries.  For almost all types of lead-acid batteries just about the worst thing you can do to them is not recharge them all the way.  Leaving them in a state of “partial charge” leads to an early death.


Batteries make electricity by converting lead and sulphuric acid to lead sulphate, when we recharge them, the lead sulphate is converted back to lead and sulphuric acid.  (To all my chemist friends:  Yes, I know that is an over-simplification, but it is a good enough model to work with here!)  There are a lot of reasons that this process is not perfectly reversible, and they all contribute to batteries slowly aging and losing capacity, but the biggest issue for batteries on cruising boats is called “sulphation.”

When batteries first make lead sulfate as they are discharged it forms in a crystal form that is easily redissolved.  This is because this crystal form is not the most stable form at room temperature, so getting it to come apart again is easy.  As those crystals sit in the battery they slowly convert–all by themselves–to a much more stable crystal form.  This stable crystal is almost impossible to convert back to lead and sulfuric acid by normal charging processes.

If the battery has short cycles–discharged quickly, and then very rapidly recharged to full, the crystal conversion does not have time to happen, and the batteries capacity all (almost!) comes back with each charge.  This is very much what happens with an automobiles starting battery.  We draw a lot of power out in a few seconds to run the starter motor, and then it is immediately and fully recharged as the engine runs.  Sulphation never gets a chance to happen, and the batteries in this service can live a very long time. Modern, high quality, automotive batteries can start your car thousands of time and last ten years.

On a boat, things are very different.  First, we very slowly draw power out of the battery.  We might discharge the battery for 12 or 24 hours, and then take an hour or three to recharge the battery.  What is worse, it is rare for many boat batteries to get fully recharged to 100% full.  Lead sulfate sits there for hours, and days. Thermodynamics eventually wins, and the crystal form slowly changes.  More and more of the battery’s working surface becomes clogged with stable crystals of lead sulfate and its capacity declines.

Making Up is Hard to Do.

Why are boat batteries not fully charged?  Bringing a battery from 20% or 50% charged up to about 80% is easy, fast and efficient.  The battery takes power almost as fast as we can feed it.  Once we get to about 80% full, things start to change.  Other processes start happening.  Some of the electricity we push in goes to make Hydrogen gas, some gets lost as heat, and the whole charging process starts to slow down.

By way of example, when we run our generator, we can bring our batteries from 65% charged to 85% charged in about 60 minutes of running.  To bring them up to 95% would take another 2 hours of generator time.

Kicking them Hard

Some kinds of batteries can have some sulphation  reversed with what is called an “Equalization Charge.”  This is basically a controlled overcharge at higher voltages than the battery would normally see, and higher than it could tolerate for long periods.  Some sulfate is redissolved, and some is simply pushed out of the way as the battery generates gas as a result of the high charge voltage. The effectiveness of this charge varies from one battery type to another.  It is also a process that can seriously damage some types of battery, so consult your  manufacturer.

Different Strokes

These days large capacity batteries come in a few different flavors.  Traditional Flooded Lead Acid (FLA), Absorbed Glass Mat (AGM), Gel, carbon foam, and probably a few others I haven’t had much contact with.  There are much more exotic lithium batteries, but they haven’t really made the reliability and cost breakthrough to the cruising boat market (yet!).

The FLA batteries are the cheapest, they survive the fewest charge/discharge cycles (about 300), and the are moderately tolerant of abuse.  They require regular filling with distilled water.

AGM batteries are significantly more expensive, when treated well they can last much longer, (600discharge cycles) and the can be charged much quicker, but they are more sensitive to sulphation if not brought to full charge frequently.

Gel batteries are even more expensive.  They are the least common on the market, and seem to have the most variable performance.  Some people report that they last a very long time, even when abused, others have experiences that do not justify the costs.

Carbon Foam is the technology developed by Caterpillar (the construction equipment maker) that claims to avoid sulfation under partial state of charge.  If true, and the preliminary independent testing data would indicate that it is, then these batteries would have a major advantage in cruising boat service.

You can see results so far of our installation of the Firefly batteries here.