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Overcharging SLA Batteries
What happened to my battery?!

This battery has been catastrophically overcharged. We recommend unplugging the charger from the wall before unhooking the battery’s terminals. The battery may contain volatile gasses that could react badly to a spark near the battery’s vent. When a battery is charged it creates gasses that re-combine into solution; however, when the charge creates gasses faster than they can re-combine, that gas creates pressure inside the battery.

So what happened to the battery in the picture? Typically, a battery’s vents will expel any gas pressure that builds up faster than the gas can re-combine. The battery pictured above, however, collected gasses faster than the vents could remove them, allowing pressure to build up internally. Luckily for the customer, the additional safety features in the battery limited the damage to the battery only—sparing the charger and the charging environment. The malleable plastic design of this sealed lead acid battery allowed it to balloon without breaking, and an internal shorting design ceased the collection of more gasses. Furthermore, these batteries are designed with the electrolyte, an acid, to be absorbed in a glass mat, preventing the spilling of acid even in the instance of a broken casing. Our best guess is that a large 12V charger was used on this relatively small 6V battery.

How can you prevent overcharging your battery?

The most common overcharging error we see is matching a battery to a charger that is not designed for use with that battery’s capacity even if the voltage is the same. For instance, our 12 volt 3 Amp charger should not, in general, be used on 12 volt batteries that have a capacity below 10Ah. A capacity miss-match will result in a charge that may be harder on a battery than it should be, shortening the battery’s life. As batteries are used, their chemical properties degrade. They will hold less and less energy as time goes by, meaning their capacity decreases over time. If a battery degrades to a level below the range a charger was designed for, the charger may begin to overcharge that battery. In that case, the battery will wear out faster and faster each time it is charged. Of course, if a charger was designed for higher voltage batteries, hooking up a lower voltage battery will overcharge that battery.

Chargers are often engineered with built-in overcharge protection; they charge in stages, stopping or reducing the energy going into the battery when it is full. Yet there are some models that are not designed to stop charging after a battery is full which will shorten the life of the battery, sometimes severely. This will often cause customers to believe they have a defective battery rather than a defective charger and they end up overcharging battery after battery.

These instances will rarely result in the kind of swelling you see in the photo, but it will shorten the life of your battery or render the battery unusable.

So pay attention to the charger you use on a battery. Don’t use a car charger with small sealed lead acid batteries (or, in general, any batteries that use a glass mat to absorb the electrolyte). If you aren’t sure a charger is slowing down its charge after a battery is full, take the battery off the charger when it’s fully charged. A rule of thumb is to not leave a battery on a charger that you are sure will charge the battery in about 10 hours. And last, but not least, pay attention to the voltage the charger was designed for and the voltage of the battery you are charging.

See all of our Battery Articles.