One cell voltage always higher than the others

[thesillym]

Catl 280ah 16s, JBD 200a bms, the cell closest to the negative terminal alway has a higher voltage compared to the others, especially when near fully charged. I bought a 20s module from battery hookup, supposed to be new old stock. I removed 4 cells. All cells had identical voltage when it was delivered. I have top balanced a couple times. I have my inverter set to stop charging at 55v, absorb and float set to 56v. Any ideas of why this is happening?

 

[Skypower]

Bad connection between cells 1&2 or heat at negative on cell 1. It could also be a weak cell.

 

[Skypower]

That’s also too high a setting for cell over voltage protection

 

[ArthurEld]

I suggest you change your float to about 54 and absorb about 54.3 at least until you get that figured out.

 

[Pierre]

Catl 280ah 16s, JBD 200a bms, the cell closest to the negative terminal alway has a higher voltage compared to the others, especially when near fully charged. I bought a 20s module from battery hookup, supposed to be new old stock. I removed 4 cells. All cells had identical voltage when it was delivered. I have top balanced a couple times. I have my inverter set to stop charging at 55v, absorb and float set to 56v. Any ideas of why this is happening?

Float should be lower than absorb. Try 55 / 54

 

[thesillym]

Bad connection between cells 1&2 or heat at negative on cell 1. It could also be a weak cell.

The connections are factory welded aluminum strips except for between #8 and #9. I hope it’s not a weak cell, will be a pain to swap out.

 

[Skypower]

Catl 280ah 16s, JBD 200a bms, the cell closest to the negative terminal alway has a higher voltage compared to the others, especially when near fully charged. I bought a 20s module from battery hookup, supposed to be new old stock. I removed 4 cells. All cells had identical voltage when it was delivered. I have top balanced a couple times. I have my inverter set to stop charging at 55v, absorb and float set to 56v. Any ideas of why this is happening?

Go into settings and down to protections. See what your cell over voltage protection is. It should be something like 3650 mv.

 

[thesillym]

That’s also too high a setting for cell over voltage protection

I have it set to 3.65v, not sure how or why the voltage is able to get higher than 3.65v

 

[Skypower]

Some kind of voltage leak around. Bms issue or something other than a bms also connected to the cell?

 

[Skypower]

I wouldn’t be charging anymore until you find the problem. I’d be thinking about way to discharge that high cell a tad.

 

[thesillym]

Some kind of voltage leak around. Bms issue or something other than a bms also connected to the cell?

I’ve ordered a new JK bms. Nothing else is hooked to it besides the inverter and bms. The bms will eventually lower the voltage of the high cell after a couple of hours. When it discharges, the high cell quickly lowers to the voltage of the other cells. I’m really hoping this new bms will resolve the issue.

 

[Skypower]

I’ve ordered a new JK bms. Nothing else is hooked to it besides the inverter and bms. The bms will eventually lower the voltage of the high cell after a couple of hours. When it discharges, the high cell quickly lowers to the voltage of the other cells. I’m really hoping this new bms will resolve the issue.

I can’t think of a reason why that cell went way beyond the protection voltage. Charge off is charge off and voltage shouldn’t climb higher on any cells or transfer to make things worse. Another bms is the move I would take, however that cell may still run and cell high voltage protection may still trigger, but it shouldn’t ever get that high again. Hopefully a few trips to absorb with balance time will correct this problem. I give a 75% chance.

 

[OffGridForGood]

Keep us posted, if the new JK makes a difference.
If you set a top voltage of 3.65, the BMS should stop charging if any cell reaches 3.65v period.
I had a pack with a 'runner' in it, always first to hit high voltage. I took the pack aside and played with each cell and a bench source and a couple automotive lights, pulling the high cell down while charging lower cells very slowly (my bench source is max 600W) but I noticed that a cell even while being charged (or discharged) to some target voltage doesn't actually stay there once you remove the source/load, it moves part way back again closer to where it started. It can take a lot of patience to get all the cells to a single voltage and 'stay' there. It paid off, and that pack does a lot better since that couple days of tinkering.

 

[Skypower]

I’ve done a marathon manual assist to a JK to commission a new battery. Even with 2amps it can take a while with out of whack cells. I’ve found 500’ spool of #10 works well for a momentary cell energy dump and a bench top power supply is good for a cell boost. Cell always look nice in the flat but get weird at the knee when you have to intervene if you want to get it done fast. Can’t leave it alone for a second with any assist connections and always be sure that you’re looking at live data, not a frozen screen view.

 

[Skypower]

Make sure you precharge the charger/inverter just before you connect the batteries to it. FETs in the bms can fail closed if damaged.

 

[thesillym]

Make sure you precharge the charger/inverter just before you connect the batteries to it. FETs in the bms can fail closed if damaged.

How should I precharge the inverter? Will a lab power supply set at 48v work?

 

[Skypower]

I just use a 25 watt 30 ohm resistor for a few moments across the switch junction before I close the connection. The resistor value doesn’t have to be exact.

 

[thesillym]

So I swapped the JBD bms for another one of the same model. No change. One cell voltage still goes way high even though over voltage is set at 3.65v (see pic). I just received and installed a new JK bms with 2a balancing (JBD was 1a). Hopefully this helps ?

 

[HLDMYBR]

bad crimp on that balance lead?

 

[sunshine_eggo]

Have the BMS reported readings been confirmed with a separate meter?