Nickel Metal Hydride batteries are very similar to NiCds, and have been accepted with open arms to the radio control community. A NiMH cell has the same nominal voltage of 1.2V as a NiCd so they can form packs in the same way. Where the NiMH polymer lithium battery really shines and has the advantage over the NiCd is in its capacity. For the same size package, the NiMH cell carries double the energy capacity. For example a typical AA cell NiCd has a capacity of 700mAh whereas the NiMH AA has about 1400mAh of capacity. The energy to weight ratio for NiMH is much better, almost double.

There are tradeoffs, however. The NiMH cell has a higher internal resistance, almost double that of the NiCd rechargeable battery and as a result is limited to the speed at which energy can be charged or discharged with the cell, lower charge rates and lower discharge rates. This is particularly true of the smaller NiMH cells such as AAA and AA, however, some of the larger capacity cells, currently in the 3000 to 3300 mAH range, are designed for higher current rates. In some applications, such as powering electric model aircraft, the higher internal resistance can be noticeable on the power output when compared to NiCds. To help offset the drop, an extra cell is often added to the pack. For instance, where a 6-cell 7.2V NiCd pack is used, select a 7-cell 8.4V NiMH pack. Internal resistance increases with drop in temperature and decreases with increased temperature.

Another disadvantage to the NiMH is the useable working life. Lifespan in terms of age for NiCds and NiMH cells is similar, around 4 or 5 years, however, NiCds are capable of a greater number of usage cycles. Typically NiCds are capable of 1000 or more discharge/charge cycles whereas NiMH batteries are limited to around 300 to 400. This may or may not be limiting for the user, depending upon one’s application.

Self-discharge also applies to NiMH cells. In fact, one may experience a higher self-discharge rate with NiMHs than with NiCds.