dwl
Posts: 24
Joined: Tue Jan 05, 2016 5:06 pm
Delivery Date: 08 Jan 2016
Leaf Number: 112097
Location: New Zealand

Balancing Pack With One Low Cell Pair

Wed Jul 19, 2017 2:10 am

A recent 30 kWh import from Japan is showing one cell pair quite low - for example at full charge: 95 pairs - average = 4.077V, max = 4.086V;
1 pair - 3.823V; 263mV max variation. All 95 higher pairs were closely balanced.

As the SoC reduces the low cell causes the car reported SoC to drop suddenly - e.g. at 67% SoC, dash says 25% - this is probably expected when the low cell drops is around the voltage knee - the low cell was at 3.617V.

There seems to be an issue if the cars are left unused, especially cold. It is reportedly usually a problem of balance, not bad cells, and has been seen only on the 30 kWh packs. One solution has been to open the packs and manually balance the low cell pair but this is quite intrusive.

Today we started with a slow process of leaving the car on with a light load (approx 5 to 6A), all cells except the lowest shunting, and noticed this pattern of voltage reduction over a period of 8 hours:
Min Avg Diff
3.701 3.968 0.267
3.689 3.946 0.257
3.617 3.853 0.236
3.568 3.816 0.248
3.519 3.766 0.247
3.436 3.71 0.274
3.234 3.678 0.444

Note that the low cell voltage is reducing at a slower rate than the rest above 3.6V which implies reasonable capacity but it depends on the effective shunt current. I had understood it was around 1A - is this correct?

Over the next few days some 100% charges and further discharge cycles will be tried. The top end balancing process does not last that long and I am hoping that using the shunts over an extended period may help balance the pack.

Has anyone successfully recovered this amount of poor balance in a 30 kWh pack? Any suggestions welcomed. Thanks for reading.
2014 S - 6000 km Jan 2016; 23000 km Sept 2016; 45000 km May 2017; 95% SOH

GetOffYourGas
Posts: 1500
Joined: Tue Dec 20, 2011 6:56 pm
Delivery Date: 09 Mar 2012
Location: Syracuse, NY

Re: Balancing Pack With One Low Cell Pair

Wed Jul 19, 2017 5:02 am

dwl wrote:The top end balancing process does not last that long and I am hoping that using the shunts over an extended period may help balance the pack.


One thing that seems to help, at least with the older 24kWh Leafs, is topping off the car on L1. This lengthens the process, and allows more time for balancing.

Good luck with your efforts, and keep us posted. I'm interested to know whether the cell pair can be brought back to life or ultimately needs to be replaced. Hopefully the former!
~Brian

EV Fleet:
2011 Torqeedo Travel 1003 electric outboard on a 22' sailboat
2012 Leaf SV
2015 C-Max Energi (302A package)

dwl
Posts: 24
Joined: Tue Jan 05, 2016 5:06 pm
Delivery Date: 08 Jan 2016
Leaf Number: 112097
Location: New Zealand

Re: Balancing Pack With One Low Cell Pair

Fri Jul 21, 2017 3:14 am

After a couple of slow discharge cycles there Is no evidence that there is any effective balancing occuring, despite Leaf Spy showing continuous shunting. I have now heard that there are several other examples of low cell pairs found in Japan (only in 30 kWh) which have seen full packs replaced under warranty. Unfortunately once the cars arrive in New Zealand with problems we are left on our own to find solutions. Nissan does not support the Leaf here for battery issues.

There is a theory that there is a design flaw where any significant out of balance cannot be recovered by the BMS in the 30 kWh packs. The available solutions seem to be a replacement module or manually balancing the low cell - both methods need the pack opened and we lack a supply of 30 kWh modules. For those that have been manually balanced they seem to retain the balance over time. I will update if any further info.
2014 S - 6000 km Jan 2016; 23000 km Sept 2016; 45000 km May 2017; 95% SOH

rmay635703
Posts: 371
Joined: Wed Jan 19, 2011 7:43 pm

Re: Balancing Pack With One Low Cell Pair

Fri Jul 21, 2017 5:14 am

Or possibly the pair is actually weaker than the others in some way and balancing them won't give you much

dwl
Posts: 24
Joined: Tue Jan 05, 2016 5:06 pm
Delivery Date: 08 Jan 2016
Leaf Number: 112097
Location: New Zealand

Re: Balancing Pack With One Low Cell Pair

Fri Jul 21, 2017 1:50 pm

rmay635703 wrote:Or possibly the pair is actually weaker than the others in some way and balancing them won't give you much

The indications are that the cell has similar capacity based on the way the voltage is tracking down compared to the average when under light load Another data point is 3.823V lowest and 4.077V prior to the start of the discharge test. When charging, the low cell doesn't rise much higher and I understand a weak cell would gain more voltage.

A cell capacity test when the pack is opened would be needed to confirm. I understand the low cells have shown full capacity when load tested and when manually balanced the indications are that balance is then retained with normal use. If only there was a more effective way to balance with an assembled pack we could avoid having to open up but nothing obvious yet.
2014 S - 6000 km Jan 2016; 23000 km Sept 2016; 45000 km May 2017; 95% SOH

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