Ni-Cd and Ni-MH batteries

Another chemistry used in battery manufacturing is Ni-Cd. This offers good power density and can be used very efficiently in multi-cell batteries. The major disadvantages are low energy density and a memory effect that decreases usable capacity if the battery is partially discharged or continuously overcharged. Given the very high toxicity of cadmium, there is a growing environmental concern about replacing it in batteries.

Although they cost 2 to 3 times more than lead-acid batteries, nickel-cadmium batteries are widely used in portable power applications, for example, power tools, radio transceiver batteries, or TV cameras. The main advantages are a higher energy density, especially at high discharge currents, and a longer lifespan. Typically approx. 500-1000 cycles, as opposed to approx. 200-500 cycles for sealed lead-acid batteries. Unlike lead-acid batteries, Ni-Cd batteries can be stored for long periods without maintenance, without affecting their operating capacity.

Unfortunately, Ni-Cd cells self-discharge quite quickly, especially above 35 degrees C. In addition, if we regularly partially recharge (do not completely discharge the battery) the cells suffer a temporary loss of capacity called the memory effect. This effect is reversible and maximum capacity can be restored through several deep discharge/charge cycles at very low currents (approx. 5% of the battery capacity).

Another operational problem with NiCd batteries is that the voltage varies very little during discharge, making it difficult to estimate the state of charge.

Due to environmental concerns, NiCd batteries have been replaced in many applications by Nickel-MH batteries. Unfortunately, they do not retain their capacity as well as Ni-Cd batteries and cannot be used at low temperatures. Compared to lead-acid batteries, charging efficiency is poor, typically requiring 140% to 160% of the Ah consumed to restore capacity, compared to 120% for lead-acid.

In the event of overcharging, the nickel hydride cathode becomes fully charged and begins to generate oxygen, which recombines with hydrogen at the anode to form water and heat. At low charging rates, the NiMH battery can keep up with the oxygen generation and recombination cycle. However, at high charging currents, oxygen can be produced faster than the anode can recombine, resulting in internal pressure in the cell. Once the pressure reaches a certain value, the valves open and remove the oxygen. Advanced charging systems monitor the battery voltage to prevent gas generation during battery charging.

There is continuous improvement of this system, although it may not be able to compete with lithium-ion in the future.