NiMH Batteries Charging Strategies
Most of what we said above for NiCd batteries goes equally well for their NiMH successors. NiMH batteries can also be charged using the delta-V method, although the voltage drop is less significant, especially at lower charging currents. For this reason higher charge currents are used, and this works well because the internal resistance of the NiMH battery is lower and hence it can be charged more quickly: up to 1 C is possible. In the interests of safety fast chargers also contain temperature monitors in case the fall in voltage is not detected. Since NiMH batteries do not suffer from the memory effect, there is no need to start each charge cycle with a discharge phase. NiMH-compatible chargers therefore allow the user to choose whether the discharge phase should be included in the cycle. Also, with the newer NiMH cells that have a very low self-discharge, it is possible to dispense completely with trickle charging at the end of the cycle. Looking at Figure 9, only the part of the cycle between the two vertical dotted lines is required. Figure 10 shows a universal NiCd and NiMH quick charger for four AAA or AA cells.