David A. Johnson

The circuit below will trickle charge a four cell pack of AA or AAA NiMH batteries. The circuit draws current from the +5 v available a USB connection and pumps about 70 mA of current into the battery. This should be enough current to fully charge a pack of 2500ma-hour...

I designed and built this thing many years ago. I published the hobby circuit in the now defunct “Radio Elecronics” magazine back in 1992. The electronic circuit injects a precise 1 Amp of current into any unknown resistor, up to about two ohms. Two...