Bestech BMS not balancing 1 cell

ESK8 Electronics
1

Hi, Recently I had a problem with my battery pack (10s3p 30q pack with an 80A bestech BMS) with the BMS cutting out while accelerating. This was due to one of the cells being much lower than the others and cutting out. I thought that it was a dodgy group of cells and replaced them today.

However after charging the battery pack all the cells were balanced except the first cell that was at 3.6V. before I wired the new batteries they were 3.6v meaning that the BMS did not charge them.

I’m not sure if I have a problem with my BMS or my charging brick (cheap eBay charger).

Any help would be much appreciated, thanks

2

Take a look at the resistors inline w/ the corresponding P group on the bms. Sometimes those go bad, sometimes visibly so.

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Thanks. Could it also be due to the BMS not being able to balance groups that are to big of a different voltage?

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4

I had similar things happen, replacing p groups back to back. Blamed the bms.

I don’t know your bms’ thresholds. You could try taking a voltmeter to the groups during charging.

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Why do I need to check voltages while charging? What am I looking for? Thanks

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Haha, I suppose there are better ways. Thinking out loud.

Ive been using lipos lately and I frequently have a cell checker in balance leads.

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I’ve got a cell checker and a the cells are at 4.2v while just one is at 3.6V. I’ve been reading that a BMS cannot balance when there is such a big difference in voltages. Could i charge the individual cell with a lipo charger while it is wired in the pack so that it brings it closer to the others to be balanced?

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You do know that the bam does not charge the cell right?

I don’t understand how that one cell stayed at 3.6v though, as even when you charge straight through the power leads every p group should still be charged up

9

Yes.

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You can charge individual cell groups through the jst balance connector with a TP4056 USB charging circuit like in the photo. 20180722_083335

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Be extra careful not to short out the P-group doing this.

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Yeah I guess if he has a balance charger on hand it’s easier and faster. Took two days of charging to get the pack in to 0.05V accuracy. After that I figured that the cells where dead and threw this peace of Chinese fire hazard out.

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You can test the resistors by setting a volt meter to test resistance. If memory serves me right bestech uses 30ohm resistors.

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The bms should be disconnected in that case, I guess.

As min the balance adapter.

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The BMS won’t balance if the difference between pgroups voltage is too high.

Take off your battery and balance manually.

If you get cutouts during braking on fully charge battery your problem is not an unbalanced pgroup but a BMS protecting the battery from overcharge buy cutting all connection between ESCs and battery

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Great will give that a try😀. Thanks

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I’ve got a very similar liion module, might do it that way as I don’t think my lipo charger has a Liion setting. Cheers :+1:

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IMG_20190418_081813

I’ve just put that cell on charge. does this looks ok to you?

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I can not say for sure. Your module is different, I connected the two balance leads neighboring each other and worked my way up from the black minus lead.
Did you located the low group? Sometimes this small modules charge cut off is not accurate, check the unbalanced group sorroughly for its charging status.

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Ok. Thanks. I located the lower cells as the first group. I’ll keep checking on the voltage :+1:

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