April 8

Understanding Marine Electrics: How to Recover Lost Capacity in a Sulfated Battery

Welcome to another blogpost on batteries! If you have a led-acid battery that seems to be discharging faster than it used to and recharging faster than it used to, it may be sulfated. This is a common issue with batteries that are not regularly fully recharged. Sulfate crystals build up in the battery plates, reducing capacity and performance. 

This can happen with all lead-acid batteries. The sulfates are often recoverable in both wet-cell batteries and AGM (Absorbent Glass Mat), but unfortunately not with gel-cell batteries. Wet-cell batteries are the traditional type of battery with liquid electrolyte, while AGM batteries use a fiberglass mat soaked in electrolyte.

Recovering Wet Cell Batteries with an Equalization Cycle

Historically, wet cell batteries have been recovered from sulfation with an equalization cycle. This involves putting a continuous low level charging current (3% to 5% of the battery’s rated Ah capacity) into a battery which has already been nominally fully recharged. The voltage side of any voltage regulator is disabled, allowing the battery voltage to go wherever it wants to. This can be as high as 16.2 volts for a 12-volt battery. The battery must be disconnected from the boat to avoid damaging electronics.The low level continuous current charge is continued for 6-8 hours and occasionally longer on a seriously sulfated battery. 

The battery case should be periodically felt all over. It will get warm but should not have hot spots. If hot spots are felt, the test should be aborted and the battery replaced. The electrolyte will gas, so the test must be conducted in a well ventilated space.. After the equalization cycle, the electrolyte may need to be topped up. 

Conditioning AGM Batteries for Recovery

Sulfated AGM batteries are treated to a similar recovery regime, often called a conditioning cycle, except that in this case the voltage on a 12-volt battery may go as high as 17.8 volts. Although AGM batteries are sealed batteries, the deliberate overcharge will cause a build up of gasses inside the battery which will open its spring loaded vent valves, releasing hydrogen and oxygen, and drying out the electrolyte. There will be some loss of electrolyte which is not replaceable. Most AGM batteries have a small surplus of electrolyte when manufactured, which allows for this intentional overcharging once or twice in the life of the battery but if repeated more often the battery will fail from a loss of electrolyte. The conditioning cycle must be conducted in a well ventilated space and the case periodically felt for hot spots. 


Sulfated battery capacity in wet-cell and AGM batteries can often be recovered through an equalization or conditioning cycle, extending the lifespan of the battery and saving money on replacement costs. However, this requires a deliberate overcharge of the battery which in turn requires attention to detail and shutting down the equalization or conditioning cycle at the first sign of other issues. If the equalization or conditioning is continued in an unmonitored fashion with inadequate temperature control there is a risk of driving the battery into thermal runaway with potentially dangerous results.If in doubt, get help from someone with experience of the processes.

If you want to learn everything you need to know about batteries and how to size and treat them to make them life up to their maximum life expectancy, your should definetely have a look at our Marine Electrics 101 course!

For more content on recent developments in the field of batteries and solar, listen to Nigels talk on this topic.

About the author 

Nigel Calder

Nigel is often referred to as THE guru when it comes to technical systems on boats.

He is a long-time member of the American Boat and Yacht Council (ABYC) electrical Project Technical Committee (PTC) which writes the standards for recreational boat systems in the USA, and has also been involved in European standards development.

Nigel is best known for his Boatowner’s Mechanical and Electrical Manual (now in its 4th edition), and his Marine Diesel Engines (in its 3rd edition), both considered the definitive English-language works in their field.

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