Charging NiMH batteries
To properly charge a Nickel Metal Hydride (or NiMH) battery, the charger must be able to measure the battery's temperature, and it must also be able to determine the amount of current flowing in or out of the battery. When it approaches full charge, it will begin to heat up very rapidly.
Never let the temperature exceed 60 degrees C, because the battery could leak or explode!
To reduce wear on the batteries, it helps to replace only the amount of charge which has been spent. If this value is unknown, try to replace battery's rated capacity instead, but monitor the temperature to know when the battery is nearing full charge.
Charging might proceed in four phases:
- For the first phase, charge the battery at 1 Amp for as long as it takes to put in what was taken out. For example, if 2.1 Amp-Hours were spent, then phase one should last for 2.1 hours. Keep track of how long this phase has gone on. If the battery temperature exceeds 45 degrees C, proceed to phase 2.
- In phase two, charge at 0.5A for as long as it took to complete phase 1. Start checking temperature 20 minutes into this phase. If the temperature exceeds 45, move on to the next phase.
- In phase three, charge at 0.2A for as long as it took to complete phase 1. As in phase 2, start checking temperature 20 minutes into this phase. If the temperature exceeds 45, move on to the last phase.
- In this last phase, the battery is considered to be fully charged. NiMH batteries can be damaged by trickle charging (charging at low currents) for longer than 10 hours. Instead, keep the charging current just high enough to stop charge from leaking back out of the battery.