I experienced some success today putting together a circuit to amplify the small voltage developed across a heavy duty shunt resistor that can be attached to a storage battery. It worked on the third try. The first time, I decided that I should put some of the circuit resistors right on the shunt. The advantage of this is that the resistors could limit the current, and prevent the wires from burning up if there was a short at the Arduino end. The disadvantage, I discovered later, is that the higher impedance and the capacitance between the long run of wires parallel to one another, caused the op-amps to oscillate (similar to the acoustic squeal when a stage performer gets his microphone too close the the speakers).
The second time, it didn’t oscillate, but the voltage that I thought should be .2V was only .02. I had carefully calculated the gain needed, but then I made a mistake in the math, and the gain turned out to be 10 rather than 100.
Third time, I was able to see the current going into the battery when it was connected to the charger, and see it going into an XO on the charging sled. I still need to calibrate the current readings at two points on the amps vs voltage curve to get it more accurate, but a real classroom situation will make that easier.
The most recent frustration was in not being able to find the paper copy of the circuit diagram at right. Cleaning up and packing for the trip was a little chaotic. So now the picture is all that I’ve got left. I can redraw it if I need to. Maybe I should enter it into a open source computer aided design package (see http://store.curiousinventor.com/guides/kicad/).
I don’t think OLPC deployments are going to do breadboard circuits. But it wouldn’t take much to develop a simple silk screened board that could be pretty cheap. The Arduino was about $24 with shipping, the shunt $11, and the op-amp was $4. The whole thing could be under $60. I think it would be a good investment until a pattern of usage, (charge, and discharge), is understood at a school, so as to extend the life of the expensive battery(ies).
I expect the the batteries and charger will be $400 or so.