Use MLVSS as simple pack voltage sensor

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MikeB
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Use MLVSS as simple pack voltage sensor

Post by MikeB » Fri Apr 05, 2019 3:40 pm

A question was asked on RCG:
"Can the MLVSS be used to just measure the pack voltage by wiring it to the ESC power leads."
Clearly this is not looking for correct or accurate cell voltages.

I've done some measurements and tests.
All 6 inputs to the MLVSS measure exactly 109.4k to ground. This leads me to suspect the way it is working is to measure the 6 voltages, and calculate the individual voltages by the difference between the measured values.
During my testing I notices that the 1S and 2S voltages never returned a value higher than 4.19, even though the actual voltage at the pin was higher than 4.19. However (testing with a 3S pack), the third cell returned a value so the pack voltage was correct.

What I have found is that a resistor chain of 6, 3K3 resistors, taping off at each point to connect to the MLVSS cell inputs does work, and the total pack voltage returned is correct, even though the individual cell voltages vary significantly from each other. I tested this with a 3-cell pack (3 resistors), a 4-cell pack (4 resistors) and a 6-cell pack (6 resistors).

Some cell voltages are returned as over 4.2V, but these are tolerated by ersky9x on the radio and simply used to calculate the pack voltage.

I'm about to build a small board with the resistors on and actually install this in a model for real use!

So, the wiring, for 6 cells, is:
Wire a resistor between between 6S and 5S.
Wire a resistor between between 5S and 4S.
Wire a resistor between between 4S and 3S.
Wire a resistor between between 3S and 2S.
Wire a resistor between between 2S and 1S.
Wire a resistor between between 1S and GND.
The connect the pack voltage to 6S and GND.

For fewer cells omit those resistors not needed and wire the pack to the appropriate input (it won't work properly if you use 6 resistors but a pack with fewer cells).

Mike
erskyTx/er9x developer
The difficult we do immediately,
The impossible takes a little longer!


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