If you have a lead-acid battery that seems to be discharging faster than it used to and recharging faster than it used to, it may be sulfated. In fact, it probably is sulfated! This is a common issue with batteries that are not regularly fully recharged, which is also a common situation with boat batteries. Sulfate crystals, which are resistant to being recharged, build up in the battery plates, reducing capacity and performance.
This can happen with all lead-acid batteries, including sealed ‘no maintenance’ batteries. The sulfates are often recoverable in both wet-cell batteries and AGM (Absorbed Glass Mat), but unfortunately not with gel-cell batteries. Wet-cell batteries are the traditional type of battery with liquid electrolyte, also known as ‘flooded’, while AGM batteries use a fiberglass mat soaked in electrolyte.
Recovering Wet Cell Batteries with an Equalization Cycle
The longer a battery is left in a discharged state the more resistant the sulfate crystals come to being recharged. This charge resistance can be overcome by raising the charging voltage. The process is to first put the battery through a normal full-charge cycle, which will recharge the non-sulfated material, and then to continue charging at a low amp rate with no voltage limit. The voltage on the battery will climb, and as it does so the sulfated material will be driven back into active service. Critical to success is keeping the charging current (amps) low. Given high amps, the battery will be destroyed.
This process with wet cell batteries is known as an ‘equalization’ cycle. Once the battery has gone through the normal full-charge cycle, the voltage side of any voltage regulator is disabled, allowing the battery voltage to go to whatever it wants to, with the charging current limited to somewhere between 3% to 5% of the battery’s rated amp-hour (Ah) capacity. This will be found on a label on the battery. During the equalization cycle the voltage may go 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 maintained for 6-8 hours and occasionally longer on a seriously sulfated battery.
The battery case must be periodically felt all over. It will get warm but should not have hot spots. If hot spots are felt, there are problems other than sulfation: the test must be aborted, and the battery replaced. The equalization cycle must be monitored. If the equalization or conditioning is continued in an unmonitored fashion with inadequate temperature control, there is a risk of driving the battery into a condition known as ‘thermal runaway’ with potentially dangerous results. If in doubt, get help from someone with experience of the process.
During equalization the electrolyte will gas, emitting hydrogen and oxygen, 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 buildup of gasses inside the battery which will open its spring-loaded vent valves, releasing hydrogen and oxygen, and drying out the electrolyte. Once again, the conditioning cycle must be conducted in a well ventilated space with monitoring, and with the case periodically felt for hot spots. If hot spots occur, the conditioning cycle must be stopped and the battery discarded.
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.
Over the years I have recovered numerous expensive AGM batteries, some with up to a 65% capacity loss which have been brought back to over 90% of rated capacity. For the 12-volt, 100 Ah, AGM batteries on my boat, I use a 4-amp, 24-volt battery charger which is not smart enough to know it is connected to a 12-volt battery and as such tries to drive the battery to 24 volts; it works great!
Conclusion
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, sometimes by years, 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 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.
