NiMh Cell Voltage
0.9 to 1.5V (cell voltage typically specified as 1.2V nominal)
High self-discharge rate of over 30% per month in common nickel metal hydride batteries
There tends to be 2 grades of battery:- consumer where capacity may be higher but number of discharge cycles will be lower (big numbers are best), and industrial where capacity will be a bit less but discharge cycles of say 500 are common specs.
What kind of protections are needed for Ni-MH batteries, respectively?
In addition to the self-protections of the battery cells, a well designed charger and protection circuitry are also very important. For the protection circuitry, Ni-MH is much easier than li-ion. Often chargers include a polyswitch connected in series with the batteries and a thermoresistor with one-end connected to the ground power line and the other end to charger control. When short circuit happens, the polyswitch will be heated up and the resistance will increase rapidly to stop the current. When the temperature cools down, it will resume the supply of the power. The thermoresistor is used by the charger. When the charger find that the battery temperature increases (through the increase of the thermoresistor's resistance), it will shut down the charging process. These two parts cost very little and are much cheaper than the li-ion protection circuit board.
What's the maximum instantaneous discharge current of Ni-MH battery cells?
Typically the Ni-MH battery cell can be discharged at 5C~10C condition for several seconds. For example, an ED2000mA battery cell (capacity: 2Ah) can output 10A~20A current (2Ax5 ~ 2Ax10) if the discharge does not last for too long. This makes Ni-MH good for many high current applications, such as power hand tools and UPS.
What's the maximum fast discharge current of Ni-MH battery cells?
Typically the Ni-MH battery can be discharged at 3C~5C condition for 12~20 minutes. For example, an ED1600AA battery cell (capacity: 1.6Ah) can continuously output 4.8A~8A current (1.6Ax3 ~ 1.6Ax5) for 12~20 minutes