Over just a few months, a pile of 9V batteries accumulated. Nothing was known about their status, except "They Don't Work" and that "This is getting expensive".

Some were new, some were old. There were Ni-Cad, NiMH, and rechargeable Akaline.

The LED on the device's charger said the batteries were charged, but reality said different.

I decided to learn to do by doing and ended up building 5 different battery chargers.

Ver 1.0 uses a resistor to limit current.

Ver 2.0 uses a constant current source to limit current.

Ver 3.0 improves on Ver 2.0. (Recommended)

Ver 4.0 uses an op amp and rheostat to provide 0- 16mA of constant current.

Ver 5.0 adds a transistor to Ver 4.0 to increase the current range to 0 - 160mA

Charge Rate

Wall Warts

Simple 1 Resistor Battery Charger

Constant Current Battery Charger from 2 LED's and a NPN Transistor

9V Battery Charger from 2 NPN Transistors

Adjustable Charge Rate Battery Charger from Op Amp

0 - 160 mA Adjustable Charge Rate Battery Charger

Test Results

741C Cheat Sheet