What Causes a Runner?

[alferz]

Since installing my 8s2p 498ah LiFePO4 setup, I've had recurring issues with so called "runners". By that I mean, 1 or 2 cells in the pack will go to a much higher voltage than the rest of the cells whilst under charge voltage. For instance, when charging my pack to 28.00v, at 27.9v or higher the runner will get to 3.62 or 3.63 volts while the rest of the cells are at 3.4 to 3.5 volts.

This can cause problems with the charge cycle due to BMS shutdown for overvoltage. I have had success "bleeding" off the high cell using a 100 watt 1 ohm power resistor connected to two multimeter needle probes. Bridging the resistor across 1 (or 2) cells will bring down the voltage to get it in-line with the rest of the pack. That will work for 1 or 2 charge cycles but inevitably, that same cell will drift and become a runner again. It always seems to be cells 7 or 8 in the pack, and both of my 8s packs have the same behavior. All cells are well balanced, within .003 volt of each other through their entire cycle, they only become runner's at the end of the charge cycle.

Things I have made sure of:

Top balanced both packs to 3.65v in parallel for several hours prior to wiring in series
Bus bars and terminals scotch-brited and NoOx applied

I have been able to manage by dropping the charge voltage to 27.6v (3.45/cell) but I would like to understand why top-balanced cells that seem to perform so well in every other situation become runners like this, and what if anything I can do about it.

 

[sunshine_eggo]

Imbalance refers exclusively to state of charge.

Voltage is only an indicator of imbalance above 3.40 and below 3.10 (give or take a little on each). The 3mV deviation you see in the working range is completely normal and in no way represents a state of balance. Even notably imbalanced cells will exhibit this behavior.

Cell imbalance:

Causes:

• Poor sense wire connection/crimping/pin contact with connector.
• Poorly matched cells.
• Poor top balance.
• top balancing too soon before commissioning (with immediate use and near daily cycling).
• Storage for 2+ weeks.
• Failure to regularly charge to voltages that permit balancing.

Set BMS to:

• balance at all times (not just during charge)
• above 3.40V
• when voltage deviates more than 0.02V.

Charging to 3.45V/cell with a 2 hour absorption period should attain 98%+ SoC. Additionally, there is evidence that charging to this lower voltage and subsequently lower current for longer periods extends cycle life.

Floating at 3.40V/cell should give additional balancing time.

Float at 3.375V/cell is preferred if balance is not an issue.

EDIT: My preference is to use the same power supply you used to top balance to bring the low cells UP. Simply set to 3.65V and start charging the lowest cell. No guessing how long to leave a resistor applied. You do not need to break down the battery to do this. Given that all your cells are above 3.40, I would expect a total of about 2 hours of charging to bring all cells up to 3.65V

 

[Craig]

This is just a guess but it's possible those cells have less an internal resistance is much different than the others. If you put your resistor across the terminals what is the instant voltage drop? (Assuming the resistor is large enough to draw down) compare the drop of the bad cells with the other cells. Or those cells have a slightly different capacity. This is why pack balance is so important.

 

[alferz]

EDIT: My preference is to use the same power supply you used to top balance to bring the low cells UP. Simply set to 3.65V and start charging the lowest cell. No guessing how long to leave a resistor applied. You do not need to break down the battery to do this. Given that all your cells are above 3.40, I would expect a total of about 2 hours of charging to bring all cells up to 3.65V

@sunshine_eggo this seems like a good idea -- when doing this I assume I have to charge them all individually (16 times), or is there a way you do them in parallel to speed up the process? And where should I stop the charge, under 1a or even lower than that?

Are there any reasons you could think of where the runner would be the last couple cells in a pack, like cell 7 or 8 versus the cells earlier in the series?

 

[sunshine_eggo]

@sunshine_eggo this seems like a good idea -- when doing this I assume I have to charge them all individually (16 times),

Yes.

or is there a way you do them in parallel to speed up the process?

No.

And where should I stop the charge, under 1a or even lower than that?

Doesn't matter as long as you're consistent. Ensure you're reading CELL voltage, not what the power supply says.

Are there any reasons you could think of where the runner would be the last couple cells in a pack, like cell 7 or 8 versus the cells earlier in the series?

No.

 

[DIYrich]

What BMS are you using? Does it have active balancing?

 

[alferz]

What BMS are you using? Does it have active balancing?

They are JBD Smart BMS, 2 x 8s. I believe these are passive balancing the packs, and do so at a pretty low current. I'm going to try manually balancing as sunshine_eggo suggests and then I'm hoping the passive balancer will maintain the balance with daily charge/absorprtion cycles.

 

[BMGD]

Check your busbar connections, higher resistance can cause some cells to charge slower. I had a similar issue and now that I cleaned all my busbars and ensured good connection between them and the cell terminals my problem has gone away.

 

[Daddy Tanuki]

What is the relationship between the cells internal resistance and its health? If one cell has a higher resistance is it worse off then one with lower resistance?

 

[alferz]

After struggling with the passive 150ma balancer on my JBD BMS, I decided to try the JK 8s 2a active balancing BMS (JK_B2A8S20P). Wow, what a difference. In only 40 minutes the JK active balancing BMS equalized a 125mV cell delta down to 15mV, something that would have taken many hours with the JBD. Using the slow bulk method I described earlier I'm able to balance during bulk charging which usually ends below 20mV, allowing me to stop the charge with no absorption time and get my packs back into service sooner. I highly recommend an active balancer or active balancing BMS like the JK for anyone with this problem.

I also realized the core of the problem - I had bought my 16 CALB 230ah cells in tranches, first ten, then 2 more, then 4 more all from Docan power but separated by a few months. The last batch of 4 have a slightly lower capacity than the first 12 which is the root cause for the cell delta - those 4 get full faster and go higher voltage as a result. With active balancing I'm able to keep all the cells at a similar voltage level. I probably lose about 2-3% of capacity at the low end but the overall pack is oversized for my use case and this doesnt impact anything really. So my overall takeaway is, if you have one or more runners, the root cause is most likely a) the pack wasn't top balanced to 3.65v to begin with or b) some of the cells have a capacity mismatch and need more frequent (preferably, active) balancing with a balancer that is size appropriate to the overall AH capacity of your cells/packs.

 

[Supervstech]

After struggling with the passive 150ma balancer on my JBD BMS, I decided to try the JK 8s 2a active balancing BMS (JK_B2A8S20P). Wow, what a difference. In only 40 minutes the JK active balancing BMS equalized a 125mV cell delta down to 15mV, something that would have taken many hours with the JBD. Using the slow bulk method I described earlier I'm able to balance during bulk charging which usually ends below 20mV, allowing me to stop the charge with no absorption time and get my packs back into service sooner. I highly recommend an active balancer or active balancing BMS like the JK for anyone with this problem.

I also realized the core of the problem - I had bought my 16 CALB 230ah cells in tranches, first ten, then 2 more, then 4 more all from Docan power but separated by a few months. The last batch of 4 have a slightly lower capacity than the first 12 which is the root cause for the cell delta - those 4 get full faster and go higher voltage as a result. With active balancing I'm able to keep all the cells at a similar voltage level. I probably lose about 2-3% of capacity at the low end but the overall pack is oversized for my use case and this doesnt impact anything really. So my overall takeaway is, if you have one or more runners, the root cause is most likely a) the pack wasn't top balanced to 3.65v to begin with or b) some of the cells have a capacity mismatch and need more frequent (preferably, active) balancing with a balancer that is size appropriate to the overall AH capacity of your cells/packs.

Ah.
This makes sense.
Mismatched cells will indeed perform differently.