Rechargeable battery packs contain multiple cells. The cells are wired together in a series to give the battery greater voltage. If you find your battery goes dead quickly, or doesn't power your device properly, don't dispose of the battery pack: check the battery for a dead cell. One dead cell reduces the battery's energy capacity. It's considerably cheaper to replace a dead cell than to purchase a new battery pack.
Charge your battery using the battery charger until it's full. Once it's full, unplug the charger and leave the battery for several hours; overnight is best. Good cells retain their charge while bad cells go dead.
Remove the battery cover. Either remove the small screws using a screwdriver, or undo the clips that hold the cover in place. They are usually hinged clips, so insert a screwdriver under the clip and lever the hinge open. The battery cells are visible.
Read the voltage output on the cells. Battery cells produce 1.2 volts or 3.7 volts.
Place your voltmeter near the battery. Use the two wires from the voltmeter to touch the negative and positive terminal of a battery cell. The wires are coloured red and black, indicating positive and negative. The battery terminal are clearly labelled with "+" and "-" markings.
Read the cell output on the voltmeter. It reads 1.2 volts or 3.7 volts, depending on the type of cell. If it reads less than 1 volt for a 1.2-volt cell or less than 3.4 volts for a 3.7-volt cell, the cell is dead and needs replacing.
Repeat the process on all the battery cells. Mark bad cells with a marker so you know the ones to replace.