Measurement failures

A project log for LoRaWAN Battery Pack Monitor

Continuously checking on my Li-Ion battery packs from afar using LoRaWAN

tiefpunkttiefpunkt 01/25/2021 at 21:140 Comments

I did a bit of a hack in the circuit. I need to use voltage dividers to measure the voltages on the individual cells, but instead of building 7 separate voltage dividers for all 7 cells, I split the divider, and added a multiplexer in the middle to switch between the different cells. In addition to making for a fairly simple circuit, it would've also allowed me to "switch off" the voltage dividers completely, so there wouldn't be any drain on the batteries. It seemed like a good idea, but the results were questionable.

The voltage divider and multiplexer part or the schematic

Measurements with only a single cell connected
Turns out that doesn't work. Why? Well, the multiplexer doesn't just switch between channels, there are also some diodes and other protection stuff, and that does some weird things at times.

What's the short term fix? Having 7 "full blown" voltage dividers. I wanted to try it on this PCB, so I did some funny air circuit fun.


ed things. For now at least. However, now, it actually uses power all the time, from the batteries. As it's a prototype, it's not that bad, and I can also scale up the resistances quite a bit, to reduce the power drain. That's a quite simple fix hopefully, and then we have a working node.

Longterm, I could use a different method of measuring, but that's for the future