Nickel metal hydride (NiMH) batteries are an improvement on nickel cadmium (NiCad) batteries as they don't suffer from what is commonly referred to as the memory effect. The memory effect is when a battery appears to be fully charged, but goes dead quickly. However, NiMH batteries can have problems from voltage reduction. This is when the output voltage drops below what it's meant to be and you find that your device doesn't have the energy it used to. It is particularly noticeable when using power tools such as drills when speed and energy are important. The recovery and repair process for a NiMH battery is similar to a NiCad battery.
Check that your NiMH is fully charged. Put it in the charger and leave it to charge for an hour to be sure. Even dead NiMH batteries hold a charge for a while.
Put your NiMH battery into the device it powers. Turn on your device. If it's a power tool or similar device, set it to full power and let it run. If it's an application type device, such as an MP3 player, cell phone or similar device that doesn't use moving parts, open as many applications as possible. The best way to recover and repair the battery is to use as much energy in the shortest time possible.
Leave your device operating until it automatically turns off due to low voltage. The time it takes varies, depending on the condition of the NiMH battery. You find mechanical devices stop quicker than application-type devices.
Turn off your device once it has stopped operating. Put your NiMH battery back in the charger and charge until it's full; an hour is usually enough.
Repeat the complete process; put your battery in the device, turn on the device and use as much energy as possible. Let it run until it stops, then turn off the device.
Let the device stand for about an hour. Turn on your device and surprisingly it operates. If the battery is operating a mechanical device, it runs slowly and stops again quickly. Leave it for about 10 to 15 minutes then turn it on again. Repeat the process until it doesn't operate. If it's an application-type device, turn it on and open some applications. The more you open the quicker the device turns off. Repeat until it doesn't turn on.
Put your NiMH battery in the battery charger. Turn on the charger. You can tell how well your battery is recovered and repaired by the time it takes; the longer the better the repair process.
Turn off the charger, once the battery is full. Insert your battery into the device it powers. You find it operates for longer and, if it's a power tool, it operates faster and has more energy.