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ESP32 logging individual Cell Voltage to SD card and displaying Minimum Voltage on OLED


KenL39
(@kenl39)
New Member
Joined: 3 weeks ago
Posts: 1
Topic starter  

As mentioned I would like to build a cell voltage logger for 3-6S lipos - to record individual cell voltage during flights. Using an ESP32 with OLED and SD card I think this could be done by building a voltage divider to plug into the balance lead.

I think iforce2d and Pawel Spychalski may have dabbled with this but not come up with this complete solution.

Displaying current and minimum cell voltage +/- total battery voltage on the OLED display would be the icing on the cake


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DroneBot Workshop
(@dronebot-workshop)
Workshop Guru Admin
Joined: 2 years ago
Posts: 1052
 

I like this idea, the one aspect I would need to try and figure out is how to measure the cell voltages when they are not all referenced to ground.

I'm curious to see if anyone else has any suggestions.

😎

Bill

 

"Never trust a computer you can’t throw out a window." — Steve Wozniak


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DaveE
(@davee)
Estimable Member
Joined: 7 months ago
Posts: 162
 

Hi Bill (@dronebot-workshop ),

  I am suggesting a couple of 'random' thoughts without leaving time for my brain to catch up, so please accept my apology if they are flawed. Hopefully, they will at least provoke a solution from someone.

I am assuming you are imagining an ADC referenced to ground, probably with a multiplexer front end to select each cell in turn, some of the cells whose individual voltage you wish to measure, have both of their terminals up to 10-20V above ground potential.

An 'approximate' method is a simple resistive divider for each cell terminal, measure the potential on each of the terminals of the cell with respect to ground, and subtract the appropriate values to obtain the difference. Obviously this has the disadvantage that you are reducing the effective resolution of your ADC, since any one cell will only use a limited part of the ADC measurement range. However, with say a 12-bit ADC, this may give an acceptable result. Careful matching of resistors, or allowing for mismatch by individual calibration may be more important than the ADC performance.

To overcome this approximation disadvantage, you could consider using a differential amplifier circuit for each cell, with the inputs being connected to the terminals of each cell, so that the output voltage reflected the voltage across the cell, and could be measured with respect to ground. The classical approach would be to use an 'Instrumentation Amplifier' based on three differential amplifiers, but you may be able to derive a 'cut down' variation for your purpose.

Best wishes

Dave


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