Nickel-metal hydride cells are essentially an extension of the proven sealed nickel-cadmium cell technology with the substitution of a hydrogen-absorbing negative electrode for the cadmium-based electrode. While this substitution increases the cell electrical capacity ( measured in ampere-hours ) for a given weight and volume and eliminates the cadmium which raises toxicity concerns, the remainder of the nickel-metal hydride cell is quite similar to the nickel-cadmium product. Many application parameters are little changed between the two cell types, and replacement of nickel-cadmium cells in a battery with nickel-metal hydride cells usually involves few significant design issues. Table1: compares key design features between the two cell chemistries.

Application Feature	Comparison of Nickel-Metal Hydride to Nickel-Cadmium Batteries
Nominal Voltage	Same(1.20V)
Discharge Capacity	Ni-MH up to 40% greater than Ni-Cd
Discharge Profile	Equivalent
Discharge Cutoff Voltages	Equivalent
High Rate Discharge Capability	Effectively the same rates
High Temperature(>35 Celsius) Discharge Capability	Ni-MH slightly better than standard Ni-Cd cells
Charging Process	Generally similar; multiple-step constant current with overcharge control recommended for fast charging Ni-MH
Charge Termination Techniques	Generally similar but Ni-MH transitions are more subtle. Backup temperature termination recommended.
Operating Temperature Limits	Similar, but with Ni-MH, cold temperature charge limit is 15 Celsius.
Self-Discharge Rate	Ni-MH slightly higher than Ni-Cd
Cycle Life	Generally similar, but Ni-MH is more application dependent.
Mechanical Fit	Equivalent
Mechanical Properties	Equivalent
Selection of Sizes/Shapes/Capacities	Ni-MH product line more limited
Handling Issues	Similar
Environmental Issues	Reduced with Ni-MH because of elimination of cadmium toxicity concerns.