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How to diagnose; weak cell or faulty bms? (Bigbattery 100Ah 24V)

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whatismouse

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Oct 11, 2020
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76
One of my cells is not like the others...

But how do I determine if it's a fault with the BMS or simply a weak cell (actually pouch)?

I did a discharge until BMS cut-off due to cell 1 being at 2.5V. Cells 2-8 were +/- 3mV of 3.260V.

I called Bigbattery support and they had me do a BMS re-set: unplug for 30 seconds, plug back in, and charge to 100%.

Here were the voltages at 100% charge:

3.364
3.661
3.657
3.657
3.653
3.441
3.651
3.667

I let the battery rest 90 minutes and measured again:

3.344
3.483
3.490
3.492
3.496
3.367
3.494
3.514

I discharged halfway-ish and measured again:

3.248
3.306
3.306
3.306
3.307
3.306
3.306
3.306

I'm not quite sure how and when the BMS functions as a cell-balancer. It would appear that under discharge, cells 2-8 are being balanced. But at full-charge, there is greater discrepancy between cells.

How likely is it that the BMS is simply not balancing cell 1? Or is it more likely the case that it is weak? But even so shouldn't in some cases the weakness of a cell be overcome by the BMS' ability to balance voltages?

I will continue trying to reach out to BB but customer support is hit or miss, and I wonder whether they've flagged me as a hopeless complainer because I've had so many issues...
 
The only conclusion that can be made is your battery is out of balance. Balancing currents are tiny. They work simply by bleeding off excess charge of the high cells as heat until they are brought down to the level of the lower state of charge cells.

What was the tested capacity?
 
I believe about 70 Ah.

I'm currently charging the weak cell to match the voltage of the others. Then I'll cycle some more and monitor to see if it stays closer.
 
whatismouse said:
I believe about 70 Ah.

I'm currently charging the weak cell to match the voltage of the others. Then I'll cycle some more and monitor to see if it stays closer.
Click to expand...

It's important that you actually fully charge the cells. Just matching the voltage may not suffice if the higher cells' voltage have settled. Your cells are fully charged at 3.65V and 5A.

That being said, a battery that only delivers 70Ah on a rated 100Ah isn't acceptable. I would request a replacement. Your cells were poorly matched, and there is not sufficient testing at Big Battery to catch it.
 
snoobler said:
It's important that you actually fully charge the cells. Just matching the voltage may not suffice if the higher cells' voltage have settled. Your cells are fully charged at 3.65V and 5A.

That being said, a battery that only delivers 70Ah on a rated 100Ah isn't acceptable. I would request a replacement. Your cells were poorly matched, and there is not sufficient testing at Big Battery to catch it.
Click to expand...
I will try to top balance the cells using my cheap power supply and see if capacity increases.

I had to send back my other battery to BB and it wasn't cheap and the replacement also had a problem, and I was without a battery for a long time, so that will be my last resort....
 
If you haven't already, I would create a thread summarizing your experience. If this is your 3rd battery as it appears, that's grossly unacceptable and indicates Big Battery is delivering consistently underperforming product as the probability of you being the only one who happened to get the only 3 problematic packs is incredibly unlikely.

When you do, tag @MrGreen in the post.
 
Well its not *quite* as bad as it sounds. I started with two batteries, one of which had pretty much exactly the same issue as I'm having now, but worse. BMS tripped at 50% SOC due to cell 1 hitting 2.5V while all other cells were at 3.3V. At the time, BB had only a 30 day warranty, and as I was just outside of that when I finally got my system together, I paid return shipping but they did grant me a replacement battery.

The replacement battery tripped BMS pretty soon after putting it in service. It had a loose BMS connection that I discovered upon dissassembly. Not a huge deal (still shouldn't have happened) but it has been fine since.

The battery that is the subject of this thread is the *other* of the initial two and it has been working and in service for over six months. I never ran a capacity test, as I didn't own a benchtop power supply to be able to charge at 24V. I set my LVC on my inverter at 49V and hoped to not experience a BMS event. But now I've had the BMS trip multiple times, even after raising the LVC to 50V, so its clear that capacity is compromised.
 
I count 3 batteries with issues out of 4 in your possession. That's a poor record.

Even more concerning that this battery has been operating for 6 months and remains out of balance this much.
 
Yes it has not been fun. I laughed at the idea of creating a new sub-forum for Big Battery: Beginner friendly "Plug-n-Pray" Lithium Batteries
 
I’ve got two of those shunt/meters in operation but I’ve learned not to rely on them solely for capacity’s.
I use a 150w capacity tester that is common here. I run it at no more than 135-140w because they will make smoke if pushed higher. At 10.5a they take a while but are actually pretty accurate.

7DA70434-14A5-4757-8744-5F81B7582C93.png
 
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