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First, I am new in this topic, so please excuse unnecessary questions.

I want to measure the (four) cells of my lipo battery. The connector of the battery has a plus pole and four minus poles. (Image)

battery cell connector

Now I want to measure the cells with the Raspberry Pi, which max. voltage is 3.3 Volt. I use the ADS1115 to add analog pins to the RPi (works well).

So I drawn a schematic for a voltage divider to bring the max. 16.8V to 3.3V (works well)

enter image description here

Now my problem:

How I make the schematic bigger, because if I connect every battery minus to ground I am not able to measure the difference.

I want a different measurement for every cell.

EDIT:

Solution [by Elliot Alderson]:

Solution

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    \$\begingroup\$ No you can’t. Draw all cells and the ground connection and you will see where the fire starts. \$\endgroup\$
    – winny
    Sep 2, 2019 at 19:25
  • \$\begingroup\$ Ok, what are the possible ways to measure the cells separately? \$\endgroup\$
    – Alwin
    Sep 2, 2019 at 19:28
  • \$\begingroup\$ I'd be interested to see the internal wiring for the cells in the battery. I don't see how having one positive and 4 negatives works to use the cells individually. Are you sure it isn't +/- and something else like a thermistor? \$\endgroup\$
    – Ron Beyer
    Sep 2, 2019 at 19:47
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    \$\begingroup\$ @Alwin07 Connect the red (+) lead of your voltmeter to the red lead of the connector shown. Connect the black (-) lead of your voltmeter to each of the individual black leads, one at a time, and report the figures you get from the voltmeter (and which of the black leads it applies to.) Some batteries are used in series, but provide individual connections for parallel charging. Some have thermistors. Etc. You've provided too little information for any certainty. Perhaps you could also provide the model/manufacturer of the battery? \$\endgroup\$
    – jonk
    Sep 2, 2019 at 19:49
  • \$\begingroup\$ The battery: amazon.com/TATTU-2300mAh-Battery-Racing-Drones/dp/B013I9SVD2 \$\endgroup\$
    – Alwin
    Sep 2, 2019 at 20:22

2 Answers 2

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You will need to connect the "bottom" of your voltage divider to ground, but do not make any additional connections between ground and your batteries.

So, you will need to use four voltage dividers. The first will measure the voltage from cell 1, the second will measure the total voltage of cell 1 + cell 2, and so on.

In your software, measure the voltage for cell 1. You will need to convert the ADC reading to the actual corresponding voltage. Now measure the voltage for cell 1 + cell 2. Convert the ADC reading to actual volts and subtract the previously calculated voltage for cell 1 alone. Now you have the voltage for cell 2. Repeat for all four dividers.

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  • \$\begingroup\$ I tried to draw a schematic with your answer. Schematic: i.imgur.com/7VawiKQ.png Sadly it is not working. \$\endgroup\$
    – Alwin
    Sep 3, 2019 at 12:53
  • \$\begingroup\$ Only the cell which is connected to GND shows the right voltage. E.g. If I connect GND to "All Cells" Only the fourth cell shows the right value. \$\endgroup\$
    – Alwin
    Sep 3, 2019 at 13:02
  • \$\begingroup\$ The connections to the ADC inputs are fine, don't change those. Now disconnect the 7.5k resistors from Cell1, Cell1+2, and Cell1+2+3 and connect them all to ground (the common Cell -). Disconnect R6, R7, and R8 from AllCells. Connect R6 to Cell1, R7 to Cell1+2, and R8 to Cell1+2+3. You may need to change the resistor values so that each divider gives about3V when the cells are fully charged. \$\endgroup\$
    – Elliot Alderson
    Sep 3, 2019 at 13:05
  • \$\begingroup\$ Thanks a lot. I have posted the new schematic in the question. \$\endgroup\$
    – Alwin
    Sep 3, 2019 at 14:12
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This is not directly connected to your question but it will surely help. I suggest you to put an opamp as buffer after voltage divider output. And RC network to filter some noises caused by voltage divider resistances and opamp.

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