Workshop: Reviving Dead Sealed Lead Acid (SLA) Batteries
SLA batteries can be a little bit cantankerous when they get old, and there are a couple of reasons for this. If the battery has been continually deep cycled (discharged until flat, then recharged), the plates will eventually corrode and the battery will hold less charge. This is an irreversible corrosion, and there is no option but to buy a new battery.
If your battery has been trickle charged with a low current over a long period of time, you may be able to restore the battery to (almost) it’s full potential by using some technical jiggery-pokery. Older SLA batteries suffer from sulfation – a crystaline coating that prevents the battery plates from reacting with the acid. The purpose of the technical jiggery-pokery is to reverse the effects of the sulfation.
The first step to take is to try cycling the battery through repeated charge and discharge cycles. Charge the battery as normal, then discharge and repeat. If this works, then you should be able to regain about 80% or 90% of the original battery capacity. If the battery is being particularly stubborn, you will probably need to increase the charging voltage to the battery for the first hour or so. Be very careful when you do this, because sealed lead acid batteries can boil, explode, vent gas, or implode if they are handled incorrectly. Do not increase the voltage beyond 2.5v per cell, and constantly monitor the temperature of the battery while it is being charged at the higher rate. Remember: You can buy a new battery, but you only get one body. If you break your body, you can’t get replacement parts.
If you are still having problems with the battery, you could consider using a desulfator. You can buy these ready made, or make your own. There are plenty of circuits available on the internet, so just google for desulfator circuit if you are interested. A desulfator circuit supplies high voltage pulses to the battery, and essentially shakes the sulfate crystals loose, forcing them back into the solution. The circuit for one desulfator can be found here.
If all of these methods have failed, then it is probably time to go and buy a new battery. I should point out that although I started this article talking about sealed lead acid batteries, the advice given here also applies to ordinary lead acid batteries. My advice about safety also stands.
A Final note about battery safety: People get complacent about batteries because they are so commonly available. A battery is essentially a portable acid bath that can fail to the point where it explodes, produces corrosive gas, melts the lead plates, and releases a hydrogen fireball. I have seen molten lead fly out of exploding batteries, and know of at least one person who is now partially deaf as the result of a battery explosion.