Battery only charging halfway

[PV1]

I'll get the screenshots tonight. Not completely sure, but seem to have lost a few miles while at work. The warning light is now on solid, and I had to stop at the Nissan dealer and get a charge (level 2) before going home. Probably could make it without it, but it's tight. We'll see if I can still make it to the dealer for my appointment on the 27th.

I'll also try to sort out the data and post some graphs.

 

[PV1]

Here's some screenshots and graphs.

This one is full throttle climbing a hill:
https://www.dropbox.com/s/4o3yyclfjg5pxzq/pict_2015-05-18_07-35-26.png?dl=0

This one is at 2 bars, 18% SoC:
https://www.dropbox.com/s/ndsjw7g5om952ug/pict_2015-05-18_17-36-50.png?dl=0

Here's a graph of the cell voltages, cell #10 vs. the average of the other 87 cells (which are all within .005 volts of each other):
https://www.dropbox.com/s/6jhbpur23ijmne9/Cell%20Voltages.png?dl=0

Here is a graph showing the difference in voltage between cell #10 and the average of the other 87 cells. I tried to get a SoC overlay, but I had trouble matching up the timestamps. The higher the voltage difference, the lower the SoC. The sharp rises are driving, and the slow decreases are charging.
https://www.dropbox.com/s/s5u5ljhpip82657/Cell%20Disparity.png?dl=0

Also, here is the spreadsheet with all cell voltage data. Row 2490 on sheet 1 is where the data starts. Everything before it is before the cell started acting up. Column K is cell #10. Sheet 2 is degraded cell data only with charts.
https://www.dropbox.com/s/6mwacgm15wcawop/cell%20voltages.xlsx?dl=0

 

[kiev]

That's some excellent data you have collected. i haven't parsed it yet to try to determine if the car is just using the lowest cell voltage or the pack average as it's source for the fuel gauge indicator, but i thought that might be interesting also.

The comparison graph of cell 10 with the rest of the pack average indicates to me that the cell is not damaged, but that something in the instrumentation is at fault.

Cell 10 charges and discharges with the same slope as the pack, it just appears biased at a different voltage--so is it real or memorex, what is the 'truth'? i.e. the cell is actually at its normal voltage the same as the rest of the pack, but the instrumentation is indicating an incorrect value; or the instrumentation is in error and causing the balancer to bleed off the cell, and the cell voltage is correct. But if the voltage reading is based off of the same sensing circuitry that is in error, then it is suspect also and there is no way to know the truth.

But the other truth is that you will likely get a new pack soon!

 

[PV1]

Just going by memory, the voltage of cell 10 falls in line with the SoC calculations I've seen in the past. So, it seems that SoC is calculated by the lowest cell voltage, plus standard coulomb counting. As the voltage of cell 10 continues to decline, so does the charge level.

I'd like to believe that it is a fault with the management board, but the voltage difference grows as SoC declines, which indicates some capacity loss in the cell. It also seems that the other cells have some damage, as they aren't as in balance as they used to be. They're still very close, but not exact like they were.

I am now at less than half a charge available. I sure hope it can last another week. I may have to park it at my friend's place about 5 miles from the dealer.

 

[kiev]

...
I'd like to believe that it is a fault with the management board, but the voltage difference grows as SoC declines, which indicates some capacity loss in the cell. It also seems that the other cells have some damage, as they aren't as in balance as they used to be. They're still very close, but not exact like they were.

The difference gets larger as the SOC declines because cell 10 never received the same amount of charge as the rest of the pack, then as energy is drawn from the pack cell 10 reaches the slightly steeper slope of the discharge curve while the rest of the pack is up on the flat part of the curve-- hence the greater delta-V at lower SOC.

i would suggest that there may not be any 'damage' to the rest of the cells if the instrumentation/sensor components are out of spec for cell 10 and this has thrown the entire logic and control into a limp-home mode...the pack is never getting fully charged and allowed to go thru it's balancing phase due to the 'error' detected for cell 10.

i wonder if there is any sort of CAN error message being sent from the BMS to the VCU to notify the system of a mis-matched voltage in the pack and trigger some sort of limp-home mode?

The tesla pack can detect the loss of one cell in its 96s74p pack and has balancer boards with 39 Ohm bleed resistors to pull down the excess energy such that it becomes a 96s73p pack, quite clever.

 

[PV1]

I'm not sure when the limp-mode is enabled, but it is there. It would be enabled when the turtle light comes on, but I'd imagine that would be either when the cell develops high internal resistance (if it is the cell), or when the voltage falls into the range where SoC calculations are lower than 10%.

The cell hasn't increased in resistance, as full throttle definitely still has the get up and go.

Either way, it'll all be replaced most likely.

 

[tigger19687]

I would park it at your friends house, just to be on the safe side.

 

[PV1]

That's the plan once my brother's done borrowing Koorz. His car broke down.

 

[PV1]

Well, the poor ol' polar Bear is only charging to 4 bars now. I've managed to get it to my friend's place after charging at work. The seven miles to the dealer from there should be pretty easy. Hope the dealer takes good care of my i-MiEV.

 

[kiev]

Did you get a canion reading of cell 10 voltage compared to the rest? Is cell 10 just really very low or is the whole pack now lower overall?

It's all very curious that cell 10 can't be charged...? Sure will be interesting to someday get the source code to see what the controller is doing, or thinks it needs to be doing...

i wonder if they put the car in the kiddie pool for changeout and handling of a 'defective' pack?

 

[PV1]

Cell #10 charges and discharges with the rest, but just keeps slipping on the average voltage.

I'd like to see how the battery management is coded, too. It doesn't seem to want to balance like I've heard others doing. It'll slow down, then stop just like it does at the top.

I doubt it. Corporate probably wants to put the techs to work and find out what goes on with these packs, so I doubt they'd tell the dealer to kill the pack with water. With a wreck, though, that may be a different story. The other thing is, submerging the car enough to flood the pack would also flood the interior.

 

[JoeS]

My impression is that the 'system' state of charge (number of fuel bars) is based on the capacity of the weakest cell; thus, your system charges up to 360v, but your weakest cell never gets to its full charge voltage while the other ones are sitting totally full (87 cells sitting with a voltage just a tiny bit higher than normal and with their individual cell manager refusing to let the cells absorb any more current). The 360v is held steady and then the charging current decays and the system shuts off, leaving you with just a few bars showing because of that one weak cell, which never got to its full voltage. See my CaniOn screenshot back on Page 2. Like I said, that's my simpleminded impression. PV1, I hope you can make it to the dealer...

 

[PV1]

Thanks. I share your impression.

I have 3-4 bars to go 7 miles on main roads. I shouldn't have any issues. I'll grab a final screenshot of CaniOn when dropping the car off. The RR estimates seem to be very sensitive with a battery in this condition.

I guess this is also a good time to get my annual service and recalls done, and maybe even get my parking sensors fixed.

 

[Aerowhatt]

Cell #10 charges and discharges with the rest, but just keeps slipping on the average voltage.

I'd like to see how the battery management is coded, too. It doesn't seem to want to balance like I've heard others doing. It'll slow down, then stop just like it does at the top.

What does the temperature data look like close to cell #10 both on charge and discharge? I've seen symptoms like this before but there is always a heating issue with the failing cell. My guess is it is balancing but it is burning more than balancing can make up for. The BMS shunts power at a certain voltage set point to slow the charging of the highest cell, while others catch up. The shunts can't divert that much power though, so the highest cell is still charging but slower than others. When the high cell reaches the next voltage set point the BMS interrupts charging. Some BMS set ups will resume charging once the shunts bring any high cells below the first set point (in other words when all the cells have been disconnected from their shunts based on their decreased voltage). From the data and trends you have seen it looks like the #10 cell has an internal issue that is diverting some charging power from charging it. That power should show up as heat.

Aerowhatt

 

[PV1]

I made it to the dealer and dropped off Bear.

Screenshots:

Full charge at my friend's place:
https://www.dropbox.com/s/eor7ei59kve7aw8/pict_2015-05-27_08-22-27.png?dl=0

Remaining charge at dealer after 7 mile drive:
https://www.dropbox.com/s/bj5meisdcgev5n1/pict_2015-05-27_08-42-46.png?dl=0

Battery temperatures at the dealer:
https://www.dropbox.com/s/ylioz3zwj53q0z2/pict_2015-05-27_08-42-54.png?dl=0

On a side note, has anybody pulled the dome light fuse for any length of time? Did you lose your trip odometers and radio presets?

 

[JoeS]

What does the temperature data look like close to cell #10 both on charge and discharge? I've seen symptoms like this before but there is always a heating issue with the failing cell. My guess is it is balancing but it is burning more than balancing can make up for. The BMS shunts power at a certain voltage set point to slow the charging of the highest cell, while others catch up. The shunts can't divert that much power though, so the highest cell is still charging but slower than others. When the high cell reaches the next voltage set point the BMS interrupts charging. Some BMS set ups will resume charging once the shunts bring any high cells below the first set point (in other words when all the cells have been disconnected from their shunts based on their decreased voltage). From the data and trends you have seen it looks like the #10 cell has an internal issue that is diverting some charging power from charging it. That power should show up as heat.

Of course, and this parameter I/we forgot to look at while charging! Could someone provide a link to where we have a correlation between temperature sensor number location and cell numbers? Aerowhatt, thanks for pointing this out.

 

[siai47]

I don't think there is a "easy" way to correlate the sensors. I appears that there are 6 sensors in each of the 8 cell modules and 3 in the 4 cell modules. The sensors seem to be on a PC board attached to the top of the cells and only relate to an area near the cell---not the cells themselves. If you look at CANION temp data, a lot of times you will see temps going from lower to higher (or higher to lower) in groups of three. And then there's that one temp (#64 I think) that is always sky high during charging. I wouldn't bet the farm on temp data if you are trying to find a bad cell unless it's a really, really bad cell !

 

[Aerowhatt]

I don't think there is a "easy" way to correlate the sensors. I appears that there are 6 sensors in each of the 8 cell modules and 3 in the 4 cell modules. The sensors seem to be on a PC board attached to the top of the cells and only relate to an area near the cell---not the cells themselves. If you look at CANION temp data, a lot of times you will see temps going from lower to higher (or higher to lower) in groups of three. And then there's that one temp (#64 I think) that is always sky high during charging

could you quantify sky high for me? As in, how much above the others? Do you have any idea where it is in the pack!

Shame the temp sensors are not reading cell case temperatures

Aerowhatt

 

[tigger19687]

There is a couple posts about that one cell #64. But it is imbedded in other posts.

I think there was another Cell that was mentioned ... 84? or something.

Might be good to move those to it's own thread. I was searching for that the other day and couldn't find it.
Search would not bring anything up

 

[kiev]

edit: i think the cell layout shown on page 2 is not correct, the picture should be removed or deleted.

a correct version can be found here:

http://myimiev.com/forum/viewtopic.php?f=23&t=3943&p=38824#p38832