The circuit below will trickle charge a four cell pack of AA or AAA NiMH batteries. The circuit draws current from the +5v available a USB connection and pumps about 70ma of current into the battery. This should be enough current to fully charge a pack of 2500ma-hour cells in about 36 hours. The circuit uses a single 74HC14 hex Schmitt trigger inverter in conjunction with a voltage doubler charge pump circuit.
Source: DiscoverCircuits
