Battery Cell Manual Balancing with Welded Cells

[Frenchie90]

Hey everyone,

I'm hoping someone can advise on a solution to my issue.

I have a battery which was shipped to me without being top balanced in Cells 1,2,3 , these 3 cells have approx 25 percent more energy in them then the rest of the 1 cells. I have been monitoring these cells for 4 months and the issue remains just as it was the first time it ever charged to 100 percent.

The first 3 cells hit 3.62v and the other 13 never go above 3.4v as over voltage protection kicks in at 100 percent.

The BMS is a PaceEx 200A 16s 1P and the cells at EVE Grade A cells with QR codes to match, the issue is they are welded and are very difficult to access (3cm gap between a metal top plate that cannot be removed without completely disassembling the battery. The Pace bms only has a small passive balancer and could never deal with this inbalance and the supplier refuses to acknowledge an issue saying that it's normal at these high voltages and just to lower the charge threshold to get around it.

I had a 5A active balancer installed at the factory and it appears to do nothing when switched on by the button they provided, I left it on for 20 hrs at 100 percent and both Charge and Discharge mosfets turned off in the BMS and nothing changed other then the small amount of power drained naturally by the BMS being turned on so I could monitor the cells movements. I also tried this without the BMS on.

My only solutions that I can see, is to use some sort of long insulated probe to connect to each cell one by one to the positive and negative terminals in question and then draw down power using a 3.2v battery load capacity tester like the ZKE-Tech EBC-A20 until the cells voltages match those of the other cells or the other way around charge up all the cells to 3.65v ) this would take days as it would require 35/40AH per cell at 5A :-(

If someone has ever achieved this or done a similar method, can they advise on the device used to drain/charge the cells.

Can you also confirm that regardless of the cells being welded together that if I turn off the BMS and breaker, that if I connect the positive and negative of the discharger/charger that it will only charge/discharge the cell that it's connect to and not the other cells?

 

[Texas-Mark]

Yes you can discharge a single cell even if it’s welded. However it does not should like your active balancer is working, or that cell is really bad.

 

[GXMnow]

Can you unplug the balance wire harness from the active balancer or the BMS? What are the wire gauges?

You should be able to pull 2 to 5 amps at the balance wires. But be careful and make sure they don't get too hot. A car headlight makes a decent single cell load for drawing them down a little.

I also agree, it sounds like the active balancer is not working at all. I have a JK-BMS with 2 amp active balancing, and it works great. I also bought a 100Balance stand along 1 amp smart balancer. It also works great and balances my e-bike batteries in an hour or 2. It may take 2 days for the large 280 AH and larger LFP cells, and you do want to "top balance" with LFP cells as the voltage from 20% to 80% SoC changes very little. You need to balance the voltage as the cells exceed 3.45 volts.

 

[Frenchie90]

@Texas-Mark Yes you can discharge a single cell even if it’s welded. However it does not should like your active balancer is working, or that cell is really bad.

Thanks for the clarity on that, I'm also under the impression that the active balancer is doing nothing but it's very difficult to see how it is wire with such little space to get in there.

@GXMnow It may take 2 days for the large 280 AH and larger LFP cells

Yes, these cells at 320AH and 3 of them are equally sitting at 3.62v when the battery reports 100% charge, while all the reset will only reach 3.39v. Based on my calculations of an identical unit that I have that is top balanced, 3.39v equates to around 80 percent capacity. So with a 5A active balancer it should have only taken 64AH / 5 = 13 hrs to balance a cell and I left it for 20hrs and no change at all.

@GXMnow Can you unplug the balance wire harness from the active balancer or the BMS? What are the wire gauges?

So my current plane is:

1. Charge the battery pack to the reported 100 percent, this will leave 3 cells high and 13 low
2. Turn off the charging and discharging mosfets and let the battery naturally settle, this seems to result in the 3 high cells sitting at 3.51v and the reset at 3.35v
3. Turn off the BMS and battery breaker
4. Turn off the balancer
5. Unplug the balancer leads from the balancer board
6. Voltage test each connect to find the 3 cells that are too high
7. Connect the EBC-A20 unit and discharge at 5A until the voltage reaches 3.35v, rinse and repeat for all 3 cells
8. Try to figure out why the balancer wasn't working, might be simple wiring issue with power switch
9. Turn on breaker and BMS
10. Turn on charging mosfet and in theory it should charge up all balanced within 50mv

Can you confirm if I need to only connect to the positive of the cell in question, and the negative can be connected to any negative of any cell?

If I cannot use the balance leads for what ever reason, I'll make up a plastic rod with a 90- degree bend at the end that is the exact height of the gap between the cell and the roof of the battery, it will then have a cable tapped to it and exposed lead at the end that will make contact with the cell in question.

Thoughts on this execution.

 

[GXMnow]

Can you confirm if I need to only connect to the positive of the cell in question, and the negative can be connected to any negative of any cell?

You do want to connect the balancing load across the positive and negative of just the cell you are trying to pull down. If the 3 high cells are all in a row, you could connect across all 3 with a 12 volt load like a car headlight. But once they start to come down, do each cell alone to get them all matched up. Since the cells are all connected together, each positive balance lead is also the negative of the next higher cell. There should be one more balance lead than there are cells. A total pack negative, and then each cell positive from there. On some Pace BMS units, there may also be an extra lead for cell 8 positive and cell 9 negative. They go to opposite ends, but on the same bus bar, so they are basically the same connection.

 

[Frenchie90]

@GXMnow - I'm not sure I'm following, are you saying that I shouldn't use the balance leads to draw power down from each cell due to the way the active balancers are wired, and that I should connect directly to each cell instead with my home made contraption to ensure I'm draining the correct one to the same level as the other 13?

 

[GXMnow]

@GXMnow - I'm not sure I'm following, are you saying that I shouldn't use the balance leads to draw power down from each cell due to the way the active balancers are wired, and that I should connect directly to each cell instead with my home made contraption to ensure I'm draining the correct one to the same level as the other 13?

You can certainly use the existing balance leads. Just don't exceed a safe current for them. Up to 5 amps should be fine.

The main thing I was trying to say is that you have to make sure you connect the negative to the same cell as you have the positive to drain down just that one cell. You can't use any negative.

If you want to drain cell #3, you connect the positive of your load to Cell 3+ and the negative of your load will go to Cell 2+ which is the same as Cell 3-

Does that make sense?

TO drain Cell #2, the positive load is on Cell 2+ and the negative load wire moves down to Cell 1+.

 

[Frenchie90]

@GXMnow thanks for clarifying, I've given it a go with just a bench charger set to constant voltage with a maximum of 5A, I then read the BMS screen to see which cell increased and it did indeed match the wiring diagram of the active balancer.

So I think I'm good to go on that front, I checked the wire used for the active balancer and it's 18AWG silicone cable, quick Google said max 20A, maybe I'll play it safe at 15A max discharge when pulling down these high cells.

Attached is a picture, not the cable I'm using is much smaller as it's just the one I had for my bench charger.

 

[GXMnow]

That looks great, it should work just fine.

 

[mikefitz]

3.39v equates to around 80 percent capacity.

No , 3.39v , resting, is over 98% SOC. Measure the battery capacity and if near specification, just use the battery and let the built in balancers work.

 

[Frenchie90]

@mikefitz sorry, I should clarify that was the max voltage the other 13 cells reached when the other 3 hit OCP, below is a photo of the cells after 30 mins rest with the charge and discharge MOSFETs off.

I use the identical battery that is charging in parallel to this one to determine that figure of 80% as that is what all the cells in the balanced pack are at at that percentage. That battery doesn't don't hit any protection and happily sits at the 56.2v I've got it set to for 30 minutes before the amperage finally drops below 5A charging.

These cells were also heavily tested by OffGridGarage and below is a spreadsheet that I also use to match the charging voltages to and if I marry this up to mine at similar charging amps it would appear that 80 percent is roughly right based on the charging curve.

EVE MB Series - Google Drive

drive.google.com

Based on him charging at 40A constant current to 3.65v , from the point those cells hit 3.38 till they got 3.65v took 1hr. Which means 40AH is roughly missing from all 13 of those cells right? That's very far from 98 percent which I would be perfectly happy with of that was the case.

Don't get me wrong, I'd love to not to sit there for days getting this thing balanced like the other pack, but it has been left to it's own devices with the passive balance only for half a year and nothing has changed.

I even tried taking it down to 5 percent a few times to confirm it wasn't the SEI layer in those cells that needed to form and was causing the same issue Andy saw when testing these exact cells.

 

[Rednecktek]

Simple fix might be to disconnect the active balancer, turn the "Balance on Charge" setting in the BMS to OFF, and let it sit for a day. The BMS will balance the cells and they should be pretty even in a while. Check the voltages after a day and make sure they're changing. It'll be slow but once you get everything balanced up the first time you shouldn't have to ever do it again.

 

[Frenchie90]

Simple fix might be to disconnect the active balancer, turn the "Balance on Charge" setting in the BMS to OFF, and let it sit for a day. The BMS will balance the cells and they should be pretty even in a while. Check the voltages after a day and make sure they're changing. It'll be slow but once you get everything balanced up the first time you shouldn't have to ever do it again.

Any chance you can send me a screenshot of this setting, as I haven't seen it in any of the configuration screens.

I assume this would use the passive balancer, which from memory is only capable of 0.1A an hour dissipation, I imagine this would take weeks of being disconnect from our system?

 

[Rednecktek]

Sadly I don't have your make & model of BMS (in fact, my closest BMS is about 9000 miles away right now) but all the brands I've used have a similar sounding setting so you may have to dig around in the owners manual to figure out what they call it.

0.1a is really low, even the cheap passive balancers out there are doing 2a. Yes, it'll take time but if you can get it started it can run in the background while you're doing life stuff and at least make headway. There's probably only a couple dozen amps between the cells with the curve as flat as it is on LFP, I've never seen it take more than a couple days to even out my 8s packs. Charging it up will reduce the amperage difference too, like filling an ice tray, letting all the pockets even out, then topping it up at the end.

 

[Frenchie90]

Hey everyone,

Just an update, the issue was caused by the active balancer not being plugged in properly, with half the bank not connected and the active balance was soldered to always be on.

This has been resolved and I tested the active balancer and it's working as per normal.

I then disconnected it, and did indeed find that the 3 high cells had over 30AH of additional change compared to the other 13 cells. I topped up all cells to 3.65v and reconnected the Active balancer on a switch as it was supposed to be.

The rest of the build was great, just the quality control was not done on this unit.

Battery has been performing perfectly since resolving this.

Thanks for your help, as the charging the cells individually using the active balancer cables worked perfectly and meant I didn't have to open the batter pack completely up