Cell spikes

LifeIsADeam

New Member
Individual cells spikes when put on charge for first time.

I have 200ah cells hooked up to jbd 120a bms. The cells arrived within 1-2 mv of each other. Grade A matched. I let them sit in parallel for a month while waiting for other things. I wanted to try a couple of cycles without top balancing to possibly avoid buying a bench top charger for one time use.

Everything looked good hooking the bms up. Settings added from overkills site. When then hooked to the victron mppt i got voltage spikes of around 0.2v difference on cell 1, the bms app showed red for that cell triggering the overvoltage cutoff. I checked the connections and when re connecting it was now cell 3 spiking. It seemed like a connection issue with the balance leads as when using a voltmeter on the terminal screws it showed a different reading than the bms by 0.2v.

I got better ring terminals and re crimped which seemed better until it spiked again. Ive attached screen shots of bms display and settings. Any advise. Perhaps i just get a bench top charger? I thought wouldn't be an issue until perhaps when reaching the top end of charge?
 

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Hedges

I See Electromagnetic Fields!
Not top balanced, poor contact due to aluminum oxide. Those were my thoughts as well.
Since you have one high cell, you could connect a resistive load and watch it with a volt meter until it drops to same as the others. Then disconnect load, see what it stabilizes to.
Then (after also cleaning the aluminum surfaces) you can series charge again with BMS, bringing all the cells closer to full so top balance takes less time.
 

LifeIsADeam

New Member
Not top balanced, poor contact due to aluminum oxide. Those were my thoughts as well.
Since you have one high cell, you could connect a resistive load and watch it with a volt meter until it drops to same as the others. Then disconnect load, see what it stabilizes to.
Then (after also cleaning the aluminum surfaces) you can series charge again with BMS, bringing all the cells closer to full so top balance takes less time.
Thanks. The cell shows a different voltage when taken from the screw terminals to what the bms is showing so contact issue makes sense. It seems strange for at least one cell to go from 3.35 to 3.60 while the other stay around 3.30
 

Hedges

I See Electromagnetic Fields!
Not strange if being charged. They're supposed to run away at some point if not already top balanced.
If poor contact, that will cause voltage rise but only while current is flowing.
Are you able to poke DMM probes into the cell terminal itself, under the busbar? That's the only reliable place to take voltage if there is a chance of poor contact.
The balance leads have exposed metal on their crimp terminals, so try measuring those and comparing to BMS reading. Negligible current flowing, so ought to read identical (except for accuracy of instruments.)
 
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