30kWh Leaf - Weak Cell - How Bad Is This?

[TCA]

Thanks Mike.

Yes, similar situation here. SOC falls off a cliff at around 65%.

One thing your post highlighted which I hadn't considered, was that you're still able to run the car on reduced power.

I've always associated turtle with running out of charge, although never done it myself.

So the car doesn't shut down completely when a weak cell hits 3V, or were you getting turtle before then and just recharging frequently?

 

[MikeinPA]

For clarity, let us say there are two types of turtle. Type 1 is the what happens below VLBW and at around 3 volts for the lowest cell. Type 2 is a permanent turtle accompanied by the yellow car-with-exclamation-point which you will get after "three driving cycles with (some kind of) 200 mv delta". After I got the P33E6 code, the type 2 was present at all states of charge until I cleared the P33E6 DTC. The P33E6 would come back after each driving cycle. I could pull up some old screen shots and probably get a lot more specific about millivolts and SOC.

I do not know if the driving behavior of a type 1 turtle is similar to a type 2. For the type 2, I would get about 3 bubbles of throttle and the car was fairly drivable. Felt like about 30 or 40 horsepower. For the back roads around here it was fine. I would not willingly operate a car with a type 1 turtle--this would be pushing the voltage too low for my comfort levels.

Something else to consider is that for some battery chemistries as the cell approaches full charge, internal resistance increases. This means that the highest voltage cells will start losing energy as heat, while the rest of the cells are charging normally. This will act to trim the highest cells somewhat, providing a balancing service that is separate from what the BMS is doing. I do not know if the LiMnO2 chemistry in the early Leafs is doing this--but it may be worth grabbing a log during charging and look for it.

 

[TCA]

Thanks again Mike. I understand now.

So the "type 2" turtle is triggered by the approximately 200mV cell difference (for around 3 driving cycles), whereas the very low charge "type 1" turtle is based on the 3V cell parameter. Got it.

I'm getting close to a "type 2" in that case.

Currently 90% SOC, 165mV, weak cell is still 3.9V. Two weeks ago at 63% SOC I had 143mV, weak cell 3.7V (which I think is very close to LBW) and haven't dared run the charge down so much since. Shortly after that I coincidentally recorded 143mV again, but at 90% SOC.

Now 165mV so it appears to be deteriorating quickly.

 

[MikeinPA]

All correct. I would add that there may also be a SOC level in conjunction with the "three cycIes at 200 mv delta" and other refinements (see note). IIRC, lots of people are seeing a 200 mv spread at very low SOC.

Is the pack still under warranty?

Note: I believe the BMS has moderately complex behaviors that are poorly understood. I have asked questions here and elsewhere but haven't gotten much information, I suspect that BMS is looking at several statistics it derives from all the cell voltages, and then makes decisions about balancing, error codes, SOC, etc. There is possibly a decision tree it is working from.

 

[TCA]

Although the 8-year battery warranty hasn't expired, here in England Nissan will only cover degradation not failing cells. Sticking strictly to the specific wording of the warranty.

There been successful warranty claims for weak cells but this was several years ago under the 5-year EV components warranty which has now lapsed for 30kWh Leafs. Really bad from Nissan.

 

[MikeinPA]

Sorry to hear that. Nissan Leaf, a good EV with not-so-great support.

My experience of rebuilding my pack was a positive one. The manual is excellent, once you get past the weird hierarchy, and the components are all very well made. It is clear to me that the Nissan engineers spent a lot of time working with prototypes and ended up with something that is designed to be pulled apart, adjusted and bolted back up. Repeatedly. I suspect they have been having module problems from the start.

 

[LeftieBiker]

If you want to reduce the likelihood of Turtling, you want to keep the loads on the battery as low as possible. This means Eco Mode, heat off when possible, and accelerating gently - especially up hills. It being Winter, I'd run the heat on even ground, and turn it off to climb those hills.