A 7.2V 1C NiCad/NiMh battery charger.
Date: August 2003
License: Creative Commons Attribution 3.0 Unported License/
It charges a battery to approximately 80% charge in 22 or 33 minutes at a current of 2.4 Amps. Once the fast charging has ended, it trickle charges the battery. Power is from a 13.6V, 3 Amp, switchmode power supply. There is also provision to use power from a 12V accessory (cigarette lighter) socket. As there are no transformers inside the case it is lightweight.
Note: You will require a power supply of greater than 13V to operate this circuit. Failure to do so will cuase the fast charge termination to operate incorrectly. I used a car accessory adaptor from Halfords, it provides 13.6V @ 5 Amps, more than enough for this circuit.
The design is based upon the example circuit from the MAX 713 datasheet. Due to the high current, there is a large heatsink on the power transistor, T1 and I also added a small CPU cooler to cool everything down. The cooling fan runs from the 12V supply to the PCB.
It was used as my travel charger for my 'Fun' R/C cars as it is lightweight and works great with the 2000-3000mA NiCAD and NiMH cells I use
PCB and schematic in Eagle CAD format
NICAD.Zip 94 Kb
Prototype unit pictures
The ventilated PSU case is available from Maplin, part number YU32K.
Next inside the case showing the wiring and the placement of the cooling fan.
Finally, detail of the power and LED connections
Updated 14 May 2018