NiMH is an acronym for Nickel Metal Hydride batteries. NiMH battery is similar to the rechargeable NiCd (Nickel Cadmium) batteries with the difference that the positive conductor (anode) is made up of less toxic hydride alloy as compared to cadmium. The battery of the type NiMH offers 3 times more energy storage in comparison to NiCd batteries but, at the same time, the discharge rate of NiMH battery is comparatively higher under high loads. The overall performance of this battery is better than NiCds.
NiMH and NiCd batteries are the hardest batteries amongst others in the charging process. The overcharge in lithium ion and lead acid batteries can be controlled by setting a maximum charge voltage. In the nickel based batteries, there is no float charge voltage so; the charging depends on the forcing of current through the battery. The voltage in doing this is not fixed. The charging efficiency of NiMH batteries is 66% which means, you need to put 150 amp hours into the battery for every 100 amp hours that you get out.
One of the cheapest ways to charge NiMH batteries is the Overnight charging in which the battery is charged at C/10 (10% of the rated capacity per hour) or below. This means a 100 mAh battery can be charged at 10 mA for 15 hours. This method ensures full charging and does not require a sensor of an end-of-charge.
In order to charge faster, use a timer to charge the NiMH battery at C/3.33 for 5 hours. This is Faster charging method. This method may be risky sometimes because it requires the battery to be fully discharged before charging. One way to prevent this is by using the charger that automatically discharges the battery to 1 volt per cell and then the charge is on for 5 hours.