Cell spikes above 3.65v when charging with more than 10amp

[Thindra]

Hi, i have a problem with one cell spiking when charging and dropping much more than the others.

I have a 12v pack of 190ah. I did a top balance to 3.65 and did an "in real life" capacity test aboard my boat which I live in to see how many ah I could get from them. This means that amp out varied between 1a and 40a (brief moments). I set the BMS disconnect to 2.7v per cell and when I had gotten 188ah out of the pack the bms triggered when i used a water pump which does a sudden draw of 10amps, as cell no. 2 made a sudden drop below 2.7v. I could have gotten more out of the pack but decided to stop as cell 2 dipped below 2.7v every time I used more than 10 amps at that point.

I watched the cells closely during the test and noticed that cell no 2 dropped down to about 2.9v every time I used any more than 10 amps. The other 3 hardly noticed the sudden increase in power going out of the battery. Once usage settled back at around 2-3a, cell 2 stabilized but stayed .1-.2v under the others.

The real problem began once I connected the charger (victron ip22 30a) set to 30a, cell 2 spiked almost immediately to 3.4-3-5v while the others stayed together at 3.1-3.2. When I had gotten around 50ah into the pack, the bms disconnected as cell 2 reached 3.65v. I then lowered the charge to 15a (while using 5a = 10a going in) and I got a total of 120ah into the battery before it disconnected again....

So to sum up, cell no 2 spikes to 3.65v immediately when more than 10a is going into the battery, is this fixable or is the cell really bad? After all I did get 188ah out of the pack which is good.

It is worth noting that I used the battery for a while before balancing, and the cells behaved much better then. But after a while the cells started drifting so I figured a top balance would be good.

I appreciate any input here!

 

[Steve_S]

Have you rechecked all the connections and leads to ensure everything is right & tight ? just gotta ask.
You didn't mention how you top balanced the cells. A quick description could be helpful.
Did you shift cells around during your top balancing, or are the cells in the same position ?
If same position, did cell #2 show any odd behaviour previously ?
.

 

[Thindra]

All connections are checked, and I double checked cell #2 with my multimeter both when charging and discharging.
I top balanced by connecting the 4 cells in parallel and charged to 3.65v with a benchtop power supply, I let them charge until the power supply was "flickering" between 0.0a and 0.1a. All cells where in the same position when balancing - I marked it #2 when balancing and gave the cells the same numbered position when making the 12v pack.

I have not tried shifting positions around yet, should I do so just to triple check that it is all about that cell?

 

[Steve_S]

OK, Good info. A BMS lead can easily get thrown off and cause issues, that's pretty much the last thing that could be an outside issue. Swapping positions might help, or rather "verify" that the cell is out of kilt.

Now, it is quite normal for the voltage sag to occur when something like a fridge, pump etc starts and the cells will recover quite quickly. A runner may drop faster than the rest as well but that would have been the case from the first time you started using the battery pack. Same applies for charging, it would have appeared right away. Unless something "happened" to the specific cell not much should have changed.

A poor connection, dirty contacts, even things like ridges or burrs around the holes on busbars can cause issues.
Next, I would take the assembly apart, top charge each cell to 3.60/3.65 independently till Amps taken drops. When all of them are done, connect in parallel and finish top charging the "set" and once Amps taken drops, turn off charge and let them settle overnight. When reassembling the pack in Series, put #2 in another spot and note that obviously LOL. Be very sure that all surface points for connectors, busbars etc are cleaned, rubbing alcohol works great. Then test it all again and hopefully it should be resolved, well revealed anyways.

Tip: Connect the BMS harness to cells first, doublecheck the wires & sequence with dvom/dmm for a good connection, and then plug into the BMS. Takes very little to mess them up.

 

[Hedges]

All connections are checked, and I double checked cell #2 with my multimeter both when charging and discharging.

Although you double-checked,

The real problem began once I connected the charger (victron ip22 30a) set to 30a, cell 2 spiked almost immediately to 3.4-3-5v while the others stayed together at 3.1-3.2. When I had gotten around 50ah into the pack, the bms disconnected as cell 2 reached 3.65v. I then lowered the charge to 15a (while using 5a = 10a going in) and I got a total of 120ah into the battery before it disconnected again....

I think you're saying that after significantly discharging the pack, one cell went from being first to be fully discharged to being higher voltage than the rest as soon as (any?) recharging was applied. And then by the time it was recharged to barely over 25% (still close to 75% DoD), with 30A charge rate it hit high voltage disconnect.

That really sounds like contact resistance between cells.

When you checked between cells, what exactly did you measure?
Cell 1 to cell 2 (same terminal)?
Voltage across built-in positive and negative terminals of cell 2?
Voltage drop from terminals of cell 1 and cell 2 that are connected by a busbar?

Did you confirm cell #2 voltage really was high like 3.65V when BMS said so (and all others were lower)?

Another possibility, one guy found bms lead wasn't retained in its connector.

 

[nickwoz]

Thindra, did you ever figure out what was the problem with your cell? I have 2 packs and they each are experiencing a similar thing. I witnessed with my multimeter my cells jumping up and down as the there was a load and no load.

 

[Bob B]

Thindra, did you ever figure out what was the problem with your cell? I have 2 packs and they each are experiencing a similar thing. I witnessed with my multimeter my cells jumping up and down as the there was a load and no load.

You might want to start another thread to keep from confusing this one too much ...... but, in my experience, this kind of problem is almost always from a bad connection / connections.