I’ve had a variety of Li-ion batteries for some time that I’ve wanted to use in projects, but I hadn’t because I had no safe way to automate the charging process. To solve this problem, I built a smart charger powered by an Arduino Duemilanove. With a couple transistors and a few resistors, I built a circuit that could charge and monitor a cell. In addition, I added an LCD display from a hacked caller ID. The original caller ID’s microcontroller still controls the screen, but I send repeated incoming caller id signals containing the information I want displayed. The unit is programmed on the safe side; it aborts at even minor signs of unusual charging. False alarms have been dealt with by simply disconnecting and reconnecting the cell. I prefer this inconvenience over the risk of fire. With easy access to the USB port on the back, I can always upgrade its software with new functionality such a safety checks and charging patterns.
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