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Project Help and Ideas » Checking voltage on individual cells in a balance tap.

July 13, 2010
by akschu
akschu's Avatar

Fellow Nerds,

I want to program my mcu to watch the voltage on a battery pack using the balance tap. The tap has a lead to each cell as shown here:

http://www.all-battery.com/productimages/cr123a/pack_wiring_taps_conv.jpg

The problem is that for cell 1 the positive is the negative for cell 2. I'm trying to figure out how to plug this into my MCU but it's not obvious to me because I can't put each lead to ground as it will short out the battery, and I can't just put each tap lead on a ADC pin because it will give me the potential between GND and whatever battery I'm on and everything in-between.

I want to get accurate readings of each cell, and I would rather not play with voltage dividers.

To top it all off, I want to be able to read 8 cells, but because I'm going to run out of pins, I would rather using an external A to D converter. I'm thinking of this little guy:

http://ww1.microchip.com/downloads/en/DeviceDoc/21298D.pdf

It suggests using this reference voltage chip:

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en019719

But it's output voltage is 4.096 which makes sense when tied to a 12-bit converter, but wouldn't that prevent me from measuring voltages above 4.2?

So recap:

  1. How to I connect a balance tap to ADC pins on a ADC?
  2. How would I connect the linked ADC so that I can get more pins and accuracy?
  3. Can I use a voltage reference that is lower than what I want to measure?

Thanks, schu

July 14, 2010
by akschu
akschu's Avatar

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1242048945/4

This looks like What I want for question 1, still would like answers to the other questions :)

Nerdkits Guys, I have tons of ideas (and questions) so I may need more hand holding than others, but on the flip side I think these threads will be useful to other users, and perhaps even some new project ideas will come out of this.

schu

July 14, 2010
by mongo
mongo's Avatar

Reference all of them to the negative lead. First cell should read about 2 volts, the next would be about 4 and so on. If a cell is out of sorts, the voltage would not match up very well and the differences between the cells can be easily subtracted to get their true values.

If you are going beyond the power supply range, you are going to have to use some type of voltage divider circuit to keep the inputs from going beyond their limits and frying the MCU.

Since you are looking at multiplexing several inputs into one output, it may be easier to use a pair of CD4066 quad bilateral switches and direct the common output to a single analog input for reading the value. Only one voltage divider would be necessary and you only need three control lines to multiplex 8 inputs.

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