Arduino as Battery Monitor

Voltage amp using lt1495 dual dip

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.

Breadboard sitting on top of Arduino

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.

Advertisements

About George Hunt

Retired electrical engineer and programmer, enthusiastic about OLPC as a vehicle for gathering together volunteerism, mine and so many others', for helping education in developing countries.
This entry was posted in Uncategorized. Bookmark the permalink.

2 Responses to Arduino as Battery Monitor

  1. kevix says:

    is this to monitor a UPS batter?

    • George Hunt says:

      Well it could be UPS or charging sled. Any place where we want to make sure the battery is being treated in a way that it will survive for 3-5 years, rather than the one year it would last if over discharged, or over charged.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s