Circuit Project Electronic

This design is a 3A battery charger intended for use with 5-cell Ni-Cd or Ni-MH battery packs (but can be modified to suit other numbers of cells). The circuit includes automatic shutoff that occurs when the battery temperature rises 10°C above ambient.
This battery charger shown in Figure 1 was developed specifically for applications using either Nickel-Cadmium (Ni-Cd) or Nickel-Metal Hydride (Ni-MH) batteries that will accept a 3A fast-charge rate, and provides automatic shutoff of the high-current charge when the battery is full. After shutoff, a continuous (low level) charge current is used to “trickle charge” the battery which keeps it topped off and prevents charge loss due to any internal leakage. The trickle charge rate used must always be low enough that the amount of gas developed within the cell is small enough that it can recombine, thus preventing pressure build-up and venting (opening of the cell’s internal vent to release pressure). The maximum safe trickle charging rate is determined by the size and type of battery (this is covered later in the paper).

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Read More Source:
http://www.national.com/an/AN/AN-946.pdf