MAX712 MAX713 gel cells , NiMH , NiCd charger circuit schematic

Using the MAX712 MAX713 we can make a charger for Nickel Metal Hydride (NiMH) and Nickel Cadmium (NiCd) batteries .
This charger based on MAX712 MAX713 chip require few external components and is easy to build .
For the MAX712 MAX713 charger is needed a power supply with a output voltage with at least 1.5V higher than the maximum battery voltage. Charge completion is determinate by a voltage-slope detecting analog-to-digital converter , timer, and temperature window comparator .
The MAX712 terminates fast charge by detecting zero voltage slope, while the MAX713 uses a negative voltage-slope detection scheme .

The MAX712 MAX713 are powered by the DC source via an on-board +5V shunt regulator.
Main features of the MAX712 MAX713 charger are : Fast-Charge NiMH or NiCd Batteries , Voltage Slope, Temperature, and Timer Fast-Charge Cutoff ; Charge 1 to 16 Series Cells , Supply Battery’s Load While Charging , Automatically Switch from Fast to Trickle Charge , Linear Mode Power Control , 5μA (max) Drain on Battery when Not Charging , 5V Shunt Regulator Powers External Logic . This charger can be used even for gel cells batteries . This circuit charger can be used for charging 12 volts gel cells batteries .
The value of the Rsense is around 0.62 ohms and 1 watt resistor and the power transistor used can be TIP42 , 2N6109 or other similar type .

Power dissipation ( worst case ) of the PNP power transistor and diode D1 can be calculated using this formula : PDPNP = (maximum wall-cube voltage under load - minimum battery voltage) x (charge current in amps) .

Limit current into V+ to between 5mA and 20mA. For a fixed or narrow-range input voltage, choose R1 in the Typical Operation Circuit using the following formula: R1 = (minimum wall-cube voltage - 5V)/5mA Rsense can be calculated using this formula :
RSENSE = 0.25V/(IFAST) .
Using this two tables presented bellow you can program the number of charging cells and the maximum charge time .