On charging batteries
03. Jan 2009 
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I have a bluetooth mouse that runs off a single AA battery. This is annoying since the charger I have can only charge doubles at a time and I seem to have lost all but two of my AA rechargables. So in my infinitely perfect logic, rather than simply buying a few more rechargeables, I spent the evening building a single AA battery charger using a constant current source inspired from wikipedia and an Arduino.
My design kind of sucks for a number of reasons but mostly because it pushes 1.25 A through a 2 Ohm resistor while charging the battery at about 1.5V @ 1.25 A. That is less than 50% efficiency not even including the voltage drop across the regulator. It also tries to sense end of charge through sensing “negative delta V” across the battery. According to a few sources, that is not the best way to do it; a better way is to sense temperature. I figured it would work ok but my first battery has yet to finish charging (and it hasn’t gotten that hot yet).
It seems to be common to use the LM317 IC in constant current applications. This guy built something similar to mine except he uses the cheap, nasty PIC micros with an LM317. His circuit achieves better efficiency, on the order of 50% with a single AA cell, but still pretty shitty. (He uses his design to charge a bank of cells, so his efficiency is actually much better I think.)