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. Usually it includes a polyswitch connected in serial 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 about US$0.2 together. So, it is much cheaper than the li-ion protection circuit board.
