Here’s a project that we sadly let slip through the cracks a couple of years ago. Luckily [Brian] dusted it off and added an Easter Egg to the firmware in order to include it in the Fubarino Contest. The device is a rechargeable battery capacity tester. It discharges NiMH or NiCad batteries through a load resistor at about 1 Watt. [Brian] includes a discussion in his write-up about the hardware’s inability to work with 14500 Li-Ion cells. He includes enough info for you to figure out how to make changes to the circuit if you want to enable this option.
There is a MOSFET for switching each of the three battery positions. The ATmega168 takes readings from the cells once per second. It displays status information on a Nokia 5510 cellphone screen. This is where he chose to inject the Hackaday URL. When a cell’s discharge is complete, the image above scrolls onto the screen and remains there for a short time. See for yourself after the break.
http://www.youtube.com/watch?v=7HhEzlcrpjY
This is an entry in the Fubarino Contest. Submit your entry before 12/19/13 for a chance at one of the 20 Fubarino SD boards which Microchip has put up as prizes!
It seems counterintuitive to test battery capacity by discharging them. Surely the charge level will be different once they have been charged again?
The best way to test capacity is to charge them to full first and then do a discharge test, like the Technoline BL-700 AA/AAA chargers I use do:
http://www.batterylogic.co.uk/technoline/technoline-BL700.asp
They’ll also do an automated charge/discharge/charge/discharge etc. cycle to get batteries that have been left unused for a long time working to their full potential.
Yes… the idea is that the battery must be fully charged before the discharge test begins. Perhaps I should have explicitly mentioned that.
I built one of these enclosure and all last spring for a senior project. The code is clean and the circuit well designed, still use it today to gauge how much life a rechargeable has before it is useless.
“Here’s a project that we sadly let slip through the cracks a couple of years ago.”
http://hackaday.com/2011/02/10/rechargeable-battery-capacity-tester/