Leaf range after battery module replacement

My Nissan Leaf Forum

Help Support My Nissan Leaf Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

gary65536

New member
Joined
Sep 21, 2017
Messages
3
Hello all,

I recently had some battery cells replaced on my 2018 Leaf with 7000 miles under warranty. This was due to internal battery leaks in some of the cells.

Since I received the car back after the repair I believe my range is lower. Before the repair I found I was consistently getting the rated mileage of 150 miles / charge. After the repair I'm getting around 100 miles / charge even though the distance remaining shows 150 miles after a full charge. I have used the Leafspy app and have linked to a screenshot.

https://ibb.co/RzpxSYB

From this screenshot I can see some of the cells have a lower voltage difference than the others. Maybe these were the cells that were replaced. Does anyone know if this difference could be causing the issue I have? If yes, this will help me make a stronger claim to the dealership when I have them look at this new issue.

Thanks
 
The answer is "probably." Give it a few full charges, running it down to maybe 20% after each charge (don't let it sit for more than a couple of hours fully charged), and then take another LeafSpy reading. The new cells may equalize with no extra effort.
 
Looks like inexperienced technicians did the replacement.

When you do a module replacement, you need to do it at the extreme sides of the SoC. Either at 100% or at close to 0%. If you do it in the middle SoC range, you will often find yourself in this scenario.

Another thing I've noticed is that some places don't even balance the cells before installing them.

60mV diff at that high SoC is unacceptable. It will take months or even years for the puny battery balancing shunts inside the BMS to take care of that imbalance.

I'd take this piece of info and talk to the place that did the replacement. You are in a bad spot, since this will not trigger any check EV system lights, but it will cause range loss. Let us know if you can get them to re-do it.
 
Yes, the 60mV difference in cells will affect your range. The low cells hit the bottom first and you can't access the energy in the higher cells. On charging the higher cells hit the top first and limit charging of the low cells. Balancing happens all the time according to Wolftronix, so if you can drive and chargge it for a few days or a week, and capture the cell chart at the end of charging, you will be able to see the progress.

If you take it back in, they will likely just put it thru numerous cycles in the shop to affect the balance before doing anything, so you may as well drive it and cycle it as you use it. They might keep it a month to balance it in the shop, if cycling works then there is no reason for them to re-open the pack. It would be a complicated procedure to open the pack and selectively chargge and balance individual cells, plus the hazard of exposed HV in a working service area...i'm guessing that won't happen.
 
Try to discharge the battery as deeply as you can between charges. Deep discharge will get the other cell voltages closer to the low ones and also extend the charging time so it should help the balancing process.
 
GerryAZ said:
Try to discharge the battery as deeply as you can between charges. Deep discharge will get the other cell voltages closer to the low ones and also extend the charging time so it should help the balancing process.

How do you figure they get closer? That theory doesn't make sense with the data.

If you monitor the cell voltages with the phone app you will see that the difference gets greater as the SOC decreases--the lower cells drop quicker, they fall off the linear slope first and the voltage heads toward the bottom end at a faster rate, whereas the remaining cells are still up on the flat linear slope. So the difference grows greater, not lesser.

If you monitor the cells with the phone app with the car READY but not driving, you will see the balancers cycling on and off. It might be quicker to reach balance by charging and then just turning the car ON and let it sit and balance. Don't know if it balances when the car is OFF like Tesla does.
 
nlspace said:
GerryAZ said:
Try to discharge the battery as deeply as you can between charges. Deep discharge will get the other cell voltages closer to the low ones and also extend the charging time so it should help the balancing process.

How do you figure they get closer? That theory doesn't make sense with the data.

If you monitor the cell voltages with the phone app you will see that the difference gets greater as the SOC decreases--the lower cells drop quicker, they fall off the linear slope first and the voltage heads toward the bottom end at a faster rate, whereas the remaining cells are still up on the flat linear slope. So the difference grows greater, not lesser.

If you monitor the cells with the phone app with the car READY but not driving, you will see the balancers cycling on and off. It might be quicker to reach balance by charging and then just turning the car ON and let it sit and balance. Don't know if it balances when the car is OFF like Tesla does.

I think he meant that the deeper the discharge, more energy is added to the unbalanced cells on the subsequent charge.
 
SageBrush said:
...
I think he meant that the deeper the discharge, more energy is added to the unbalanced cells on the subsequent charge.

Huh? How does this explanation even make sense either? The cells are in series and all have the same current--where or how would this extra energy originate?
 
nlspace said:
SageBrush said:
...
I think he meant that the deeper the discharge, more energy is added to the unbalanced cells on the subsequent charge.

Huh? How does this explanation even make sense either? The cells are in series and all have the same current--where or how would this extra energy originate?
It is not difficult to understand if you set aside your attitude and think for moment: Charging is limited by the first cell to reach 'full,', just as discharging is limited by the first cell to reach 'empty.'
 
Excuse me, but you still haven't provided any information of how more energy is added to the lower cells than the higher cells during charging?

If you made a mistake or don't know, then at least be man enough to admit it. Don't imply that i have an attitude problem when i ask about the rationale for statements about charging and cell energy. Nobody wants to read or repeat bad information on the forum.

The same current flows thru each cell of a series string, so if the car is charging at 10 Amps for 3 hrs, then 30 A-hrs has been added to each cell. The only way to correct the OPs issue is for the balancer circuits to activate for a long time to bleed down the higher cells, or to open the pack and put charging current thru only the low cells to bring them up, or replace the low cells with some that are closer in capacity to the rest.

People come here looking for help to solve problems; let's give them solid answers based on facts and data, and not wild-axed guesses.
 
nlspace said:
Excuse me, but you still haven't provided any information of how more energy is added to the lower cells than the higher cells during charging?

If you made a mistake or don't know, then at least be man enough to admit it. Don't imply that i have an attitude problem when i ask about the rationale for statements about charging and cell energy. Nobody wants to read or repeat bad information on the forum.

The same current flows thru each cell of a series string, so if the car is charging at 10 Amps for 3 hrs, then 30 A-hrs has been added to each cell. The only way to correct the OPs issue is for the balancer circuits to activate for a long time to bleed down the higher cells, or to open the pack and put charging current thru only the low cells to bring them up, or replace the low cells with some that are closer in capacity to the rest.

People come here looking for help to solve problems; let's give them solid answers based on facts and data, and not wild-axed guesses.


The BMS has shunt resistors. These are controlled by the BMS and they open to allow a small amount of current to flow between the cells. The greater the voltage difference between the cells, the more current is able to flow. Greater voltage differential, as you stated, exists at lower %SOC.

It definitely makes sense that at low pack voltages, the greater voltage differential would result in a higher current flow across the shunts. Practically speaking, I don't know how much that will accelerate the balancing, but there will be an effect.

Personally, I'd take the car back and say that the new cells are either bad or the pack has not been properly balanced. The repair was simply not done correctly. It's possible that over a few months the problem may work itself out, but that's an awfully long time to wait.

If gary doesn't want to do that, it might make sense to leave the car running with no AC/Heat, etc from 50-5% SOC for hours in the garage, following it with an immediate L1 charge. This would allow for a prolonged period of low-amperage discharge with a prolonged period of charge--maximum balancing time.
 
Let's not get into a fight over cell balancing please. ;) Sagebrush means that charging from a low SOC adds more energy to all of the cells, and that the charging time is extended. This doesn't directly balance the lower cells, but it helps the process indirectly. How much more energy is added? How about "5"? I don't know of a specific number, especially as it will vary from car to car and probably from BMS version to BMS version.

Lothsahn slipped. I think he knows what is happening, but is frustrated by not seeing it quantified.
 
LeftieBiker said:
Let's not get into a fight over cell balancing please. ;)

Agreed.

LeftieBiker said:
Lothsahn slipped. I think he knows what is happening, but is frustrated by not seeing it quantified.

What do you mean Leftie? I hope I didn't sound frustrated--I'm not frustrated at all.. I was trying to provide some clarity on why it would cause faster balancing at lower SOC with higher voltage differentials. As you said, I have no idea how to quantify how fast the balancing will occur--the only out of balance pack I've seen or read about is Dala's, and he fixed it by opening it up and manually balancing the affected cells. :)


This is all so much easier on RC airplanes. The charger has a wire to each cell and can just directly apply a charge for each cell. :)
 
One other thing--I bet this car will now fail a CVLI test--because the new cells are so much lower than the rest of the pack, I bet the voltage difference at low SOC will be enough to trigger consult into flagging those cells as failed. Gary may be able to use that as additional ammunition to get his car fixed properly.
 
Can someone school me on how the BMS switches the shunt resistors on/off to balance the cells? With a cell difference of xx mV what type of device could be used to switch the resistors since the drop across the switch junction would be more the cell voltage difference? Mechanical switches seem like a non-starter so how does the BMS connect 2 cells with a 10mV cell voltage difference so they can be balanced? And why use resistors? Is the balancing current that high when 2 cells are just xx mV apart, given the internal resistance of the cells, etc?
 
goldbrick said:
Can someone school me on how the BMS switches the shunt resistors on/off to balance the cells? With a cell difference of xx mV what type of device could be used to switch the resistors since the drop across the switch junction would be more the cell voltage difference? Mechanical switches seem like a non-starter so how does the BMS connect 2 cells with a 10mV cell voltage difference so they can be balanced? And why use resistors? Is the balancing current that high when 2 cells are just xx mV apart, given the internal resistance of the cells, etc?

Can't say I fully understand the Leaf BMS behavior, but here's the electrical diagram that explains it:
https://www.mynissanleaf.com/viewtopic.php?t=17470

If you have specific questions and I have some time, I could study the diagram and try to figure out the details of how it all works.

If you connected a shunt with no resistor, I'm pretty sure that'd be bad. I'm quite tired and not thinking straight right now, but you've got a bunch of cells in series. If the shunt had no resistance, you'd be basically placing a wire between two of the cells in series, which would affect the output voltage of the pack and possibly create a lot of current on the shunt.
 
Thanks for the link. Apparently the balancing isn't as simple as I imagined which makes a lot of sense now that I think about it. I was envisioning some simple electronic switch to connect a higher voltage cell (group) to a lower voltage cell (group). I guessed the groups would be a pair of cells but from the schematics, a group appears to be 4 cells. Even then, as you mentioned, the cell groups are also in series so that complicates matters more. But the BMS does have plenty of voltage and current available to charge any cell group, the trick is just to determine where to pull the power from and where to send it. On closer thought, that's not a trivial problem so it makes a lot of sense that the BMS doesn't supply a trivial solution. Thanks again for the link.
 
You have an incorrect assumption--that the BMS balancer pulls current from one cell to charge another.

The bleed resistors (430 ohms) on the LBC board can only burn off excess energy of a higher cell to bring it down to the level of the lower cells. The ASIC chips have internal transistors that provide the switch path for the balancing control of each individual cell in the 4-cell groups that each chip monitors and controls.

There is no shuttling of charge around the board from high cells to low cells.

LjkIRuP.png
 
Back
Top