Bad connection, or bad cell...?

joemoco7

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Sep 9, 2021
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Okay, so, search for "bad cell," or "bad connection" on here, on you get a flood of results 😅 It seems like most people wind up needing to get in there and start trying to fix stuff before they figure out what's wrong; but before I go ripping apart my pack and testing things, I wondered if someone might be able to advise on where to start, just based on the BMS readings.

I'm running a 16s, 310Ah 3.2V LiFePo4 pack. With this, they're attached to a Daly 48V 16S 150A BMS (model R32U1, I guess), as well as a 5A active balancer.

I had a long adventure of top balancing the pack - spent days on a benchtop power supply in parallel, but could never quite get it up into the steep part of the charge curve. Finally opted to top balance it in series with the inverter, relying on the active balancer, where it got it within a few millivolts at ~3.45V before I stopped.

Anyway, after all that, I thought I was in business - but now I'm getting some problematic readings/behavior on the BMS while charging. It seems like all the cells have been discharged into somewhere along the middle of the charge curve (just from running some loads in the house), and they're all sitting at ~3.3V within a few millivolts of each other.

When the inverter goes to charge the pack, the first cell in the string quickly starts to spike up over 3.65V, cutting off the BMS (and, of course, the inverter freaks out... 🙄) - but then, the cell drops back down to within a few millivolts of the other cells within seconds of the charging cutting off. Seems too fast for the active balancer to be keeping up, and seems to be too big of a voltage difference (3.3V up to 3.65V, and back to 3.3V) for it to just be a runaway off-balance cell... I attached some pictures for reference.

The positive terminal on this cell did get messed up at some point while I was working on it--I needed to re-tap threads--but I thought I fixed it. Like, it feels like it's solidly connected... And I've also seen something similar happen when I had a different cell as the first one in the string (although, not so dramatically...). I have replaced the ring-terminals on the wires for the first few cells a couple of times too, just to make sure, but it seems to be acting more or less the same...

At any rate, any insight into what's happening would be super appreciated!
 

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sunshine_eggo

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Can the BMS report internal resistance of each cell? Some do and some don't. Not sure about yours.

If you have known issues, you should address those first.

Your symptom may be caused by:

Bad contact between cell terminal and busbar and/or cable. Insufficient clamping force and/or corrosion/debris.
Bad crimp or other BMS sensing lead issue.
Bad cell - is both the high voltage AND the low voltage cell, but eliminate all other issues first.

Connections don't just need to "feel solidly connected." They need to have sufficient clamping force between faces to ensure high conductivity.

Once you've eliminated the other causes, the best course of action is to slowly charge the battery at a rate the balancers can manage, i.e., something under 5A. Once it's in balance, conduct a discharge test to empty and see if #1 is the problem there too. If that's the case, you likely have a bad cell.
 

joemoco7

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Sep 9, 2021
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Super appreciate the reply! Good to know a recommended place to start investigating - and that the behavior might still be related to a bad connection (even if it's looking a bit fishy, in that regard).

Yeah... definitely contemplating a BMS upgrade - lots of people complain that the Daly's don't give you enough useful information, and by now, I'd tend to agree 😅
 

MisterSandals

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positive terminal on this cell did get messed up at some point while I was working on it--I needed to re-tap threads--but I thought I fixed it.
This event stands to to me. Is it possible that tapping this thread created a volcano like ring around the hole that limits contact area?
And/or maybe there is some cutting oil on the terminal surface?

And, happen to have a pic of your overall battery wiring? We find quite a few things with quite a few eyes...
 

joemoco7

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Sep 9, 2021
Messages
22
Thanks again, you guys! I can confirm that it was a bad connection 🤦‍♂️ While the new terminal post was good and tight, in my concern that I might strip the threads again, I must've gone too gingerly on the nut clamping down the bus bar. When I took it off to investigate, the bottom was blackened from arcing... 😅

I went ahead and did an overhaul on all the balance leads - made sure they were clean and connecting well. I found some potentially problematic ones, so I'm hoping I avoided some future problems regarding the connections, at least 😕

So far, the pack has been okay taking in a 0.2C charge, which it's never been able to do before without freaking out - so, so far so good!
 
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