When you buy a new battery there is always a doubt if rated capacity is correct. Especially you can’t trust highly rated batteries that usually come from China. The only way to find out is to measure battery capacity by yourself and use a thick marker to write real value. So this project is pretty simple it uses a Teensy USB development board based on AVR USB chip. So data can be easily sent to PC via USB connection. There can be several ways of measuring battery capacity but this one uses pretty simple solution – it discharges battery with constant current.
Circuit is built of LM317T adjustable voltage regulator. It is set to 2.5V which is lower that battery and stays like this during all discharge. Then output is loaded with 10 Ohm 10W resistor that draws constant 250mA of current. This way microcontroller simply reads voltage drop on battery during time and calculates battery capacity. There is also LCD screen attached where current status is displayed like current on load, voltage across battery, time and calculated capacity. So this solution may serve as quick and dirty battery checker.