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BMS flipping out

Darce

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Dec 4, 2020
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Overkill bms with a piggy back Heltec active balancer. Pack is 48v EVE 304’s. The BMS is reading some cells spiking to 6 volts while the cell next to it drops to .6. Reversed between 3 cells. Reading with a voltmeter showed one cell spiking as high as 3.8 but I adjusted the probe and it dropped back to 3.34. Maybe that was a fluke... All the other cells read normal. I also noticed that it now reads 380 amp/hrs where it used to read 304. Then it would read 0 amp/hrs. I have included a couple screen shots. The only thing that changed on my end was going from using Apple app to Android. I had to shut it all down and thankfully I can go back to line voltage while I get this sorted. Did the BMS take a dump on me? I’m trying to find out how to reset the BMS to factory settings to see if that helps. Kinda freaked me out.
 

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BMS's shut down their balancing current for a moment when they make cell voltage measurements to avoid voltage drop on the sensing wires to battery.

If you have an independent high current balancer sharing sense lines to battery with BMS you can get a lot of voltage drop and wire inductance spikes on sense lines. The balancer is operating asynchronously to BMS voltage sampling so there is going to be collisions between BMS cell voltage sampling and balancer current pump.

You will need to have the active balancer have its own leads directly to cell terminals and keep the proximity parallel coupling between BMS leads and active balancer leads to a minimum.
 
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I think the battery has developed a BMS connection issue near the #2 and #3 cell. Verify with your voltmeter.
 
BMS's shut down their balancing current for a moment when they make cell voltage measurements to avoid voltage drop on the sensing wires to battery.

If you have an independent high current balancer sharing sense lines to battery with BMS you can get a lot of voltage drop and wire inductance spikes on sense lines. The balancer is operating asynchronously to BMS voltage sampling so there is going to be collisions between BMS cell voltage sampling and balancer current pump.

You will need to have the active balancer have its own leads directly to cell terminals and keep the proximity parallel coupling between BMS leads and active balancer leads to a minimum.
Yes the bms’s each have their own leads and harness. I dig what your saying about the one bms reading cell voltage and the other bms attempting to balance at the same time...wouldn’t this happen to any cell within balance range? Or maybe these 3 cells entered into some type of balancing death loop lol. I’m building up a new harness now and will post up the results. I will verify everything with a meter before plugging in the bms again. Anybody seen this before?
 
If you are only having trouble on three cells it is more likely a connection problem. You should deactivate balancing on Overkill BMS.
 
My Heltec and Overkill balance together just fine. No issues with the cell voltages.
 
So I did a factory reset on the BMS and removed the Heltec. Checked all connections and now the random spike is across multiple random cells. I have another Overkill BMS I’m going to swap everything, harness and all and see what happens.
 
Finished swapping in a new Overkill BMS and all is calm.
Just had the same problem with a Daly BMS. Removed it and went bare back. Everything working fine. Except my balanced cells are no longer balanced.
 
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