poorjohn,
The following extract from
www.mpoweruk.com may provide more insight into the shortcomings of NiMH cells.
- High self discharge rate.
- Can be stored indefinitely either fully charged or fully discharged, but
- Suffers from memory effect though not as pronounced as with NiCad batteries
- Battery deteriorates during long time storage. This problem can be solved by charging and discharging the battery several times before reuse. This reconditioning also serves to overcome the problems of the "memory" effect.
- High rate discharge not as good as NiCads
- Less tolerant of overcharging than NiCads
- As with NiCads the cells must incorporate safety vents to protect the cell in case of gas generation.
- The coulombic efficiency of nickel metal hydride batteries could be up to 85% but is typically only around 65% and diminishes the faster the charge although this is projected to improve.
- While the battery may have a high capacity it is not necessarily all available since it may only deliver full power down to 50% DOD depending on the application.
- Cell voltage is only 1.2 Volts which means that many cells are required to make up high voltage batteries. The competing Lithium cells typically have 3 times the cell voltage (3.2 Volts to 3.7 Volts) and a much higher energy density.
- Lower capacity and cell voltage than alkaline primary cells.
- Limited supplies of rare earth element Lanthanum. Mostly in China.