As some people may have heard of or unfortunately experienced, there have been quite a number of cases of 2^nd generation Leaf batteries failing. I was planning and trying to make this into a video format, but as I’m still recovering from a cold my voice isn’t quite up for narrating an essay, so text format it is.

Hopefully this can raise some awareness and provide a central source of info as to what’s going on, why it’s happening, what to do about it, and things to consider. There are two separate issues currently causing outright failures of 2018+ Leaf battery packs, and so will be split up into two sections.

Cold-weather SOC Freefall

• What’s going on?

Under load in cold temperatures, especially below freezing and below 50% battery, the charge level and range estimate will suddenly begin to plummet and continue to drop rapidly until the load is removed, at which case the charge level starts to bounce back and quickly increase again.

The car may go into turtle mode, or in severe cases even cut off the high voltage battery completely, in which case the car will stall and may not be able to be restarted.

Here is a video example of what this sudden drop of charge level looks like.

This issue affects both the 40 kWh and 62 kWh packs. The 40 kWh packs seem to be much more susceptible, but as the 62 kWh packs use the same cells and chemistry, they are not immune. This also includes 40 packs that were installed as retrofits under warranty in 2016-17 Leafs.

• Why is this happening?

This is occurring due to one or more weak cells within the battery pack sagging excessively in voltage under load, which is greatly exacerbated when the battery is cold, due to increased internal resistance.

How the battery works is that the shown and usable state of charge is defined by that of the lowest cell. This is to prevent over-discharge of any weaker cells at lower charge levels, as that would cause even further damage to them.

Often this can be witnessed in the LeafSpy cell chart as one or more cells that clearly drop lower than the rest of the pack under load, but not always. There have been a number of cases of SOC freefall with seemingly no weak cells, indicating that there’s other data/calculations within the BMS that the user cannot see.

Getting a weak cell is essentially luck of the draw. There have been packs with very low mileage and seemingly great numbers that still failed (such as in the video above), while there are other packs with lots of miles and abuse that are still working just fine. How the pack is treated/babied does not seem to affect your chances in the roulette.

• What to do about it?

If you are encountering this issue as an owner, and the car is still under warranty, document everything as much as possible. Record video(s) of the issue happening, noting the charge level and outside temperature the battery is at. Submit this proof to the dealer as a copy, and demand that it be used in their case to Nissan corporate.

Bring the car to the dealership for diagnosis with as cold of upcoming weather as possible. Some dealers may try poorly to replicate the issue, often in the interest of charging the customer the diagnostic fee. You want to make the issue as obvious and easily to replicate as you can.

If you have to drive the car around, ensure you either have LeafSpy Pro, or keep an adjustable or 10mm wrench with you in the car. If the cell voltages ever drop low enough for the car to shut down, it is likely to set a high voltage fault code and won’t be able to restart until the fault is cleared. Clearing the fault via the LeafSpy service menu, or disconnecting and reconnecting the 12V battery, will at least allow you to limp the car to a charger or safe location off the road.

UPDATE: For 2019-2022 Leafs, the "fast charging recall" software update now adds programming to the BMS to look for and flag SOC variations, setting a fault code (P33ED) if it does. This has the potential to "brick" the car in the same way as a high voltage isolation fault (read section below) would. Be cautious of this so that it does not potentially leave you stranded.

• Things to consider

If you own a 2^nd generation Leaf (especially 40 kWh) that is still under warranty, still on the original battery, and live in a climate that gets winter, check for signs of this issue BEFORE warranty ends!

During cold weather, park the car outside overnight with 50% charge or less, then take the car out for a drive and see if there are any strange fluctuations in the state of charge during acceleration.

If you are considering purchasing one and there is no proof of the high voltage battery being replaced, ensure there is at least one year worth of warranty remaining on the car so that you can spend a winter with it and will still be covered in the case of this failure.

LeafSpy data can sometimes be a pre-indicator that you will likely run into this issue if there is a noticeably low Hx value (much lower than SOH), but not always. Low Hx is a strong sign of possible weak cells, but weak cells will not always cause low Hx.

Cell Expansion / High Voltage Isolation

• What’s going on?

The car will one day show the message “Service EV System – Unable to restart after power off”, and indeed will not restart after it is powered off.

The vast majority of cars affected by this issue are 62 kWh Leaf Plus/e+ models. It is not unheard of on 40 kWh cars, but it is quite rare unless the pack was heavily used and abused.

There is a Nissan TSB on this issue which confirms they are well aware of this issue, and there are revised battery packs and modules that are issued in replacement if the car is still under high voltage battery warranty.

• Why is this happening?

Thermal expansion of the battery cells causes them to eventually expand to the point of pushing up against each other, and in some cases even leaking electrolyte out of the cells into the battery casing, eventually causing the car to trip a high voltage isolation fault.

This is most commonly happening in specifically the rear stack of cells within the 62 kWh pack, because they are so tightly packed together, that they don’t get much cooling nor much room to expand before they begin pushing up against each other.

High voltage isolation is important, because obviously you do not want any high voltage potential to be contacting the metal battery casing that is bolted to the metal body of the car. If the BMS detects that there is even just a little bit of voltage leaking from one cell to another, or the cells to the battery chassis, an isolation fault is tripped.

• What to do about it?

If you encounter specifically the “unable to restart after power off” message, and the car is still within high voltage battery warranty, DO NOT REPLACE THE 12V BATTERY.

This specific message is ONLY generated due to a high voltage isolation fault, and although a weak 12V battery can cause many weird issues, it will NEVER generate a high voltage isolation fault. Unless there is an extremely urgent need to continue driving the car, have it towed in the “broken” state to a Nissan dealership.

It is a much more objective issue than weak cells, and basically the only diagnosis they would need to approve a battery replacement is to see the stored isolation fault code. Disconnecting the 12V battery however clears this fault code, making it not possible for them to verify the issue unless it happens to fault again in only one drive cycle.

Replacing the 12V battery gives a false sense of success because it allows the car to start and drive again, but only until the fault eventually comes back. It's just a waste of money and a perfectly good battery to not actually address the underlying issue.

If you urgently need the car to work again, clearing the fault code in LeafSpy Pro or by disconnecting and reconnecting the 12V battery will allow it to start. Just be aware that by doing this, you won’t be able to have the issue verifiable by Nissan until it pops up again. It may only take 1 drive cycle, it may take 100.

• Things to consider

Unfortunately there isn’t really any way to foretell that this issue will happen soon or to check for it, as you can’t just force an isolation fault to appear like you can with weak cells.

Battery packs in hot climates and/or lots of quick charges are more susceptible to this issue due to it being a result of thermal expansion. Packs torn apart after diagnosis of this issue often exhibit very obvious “spicy pillows” within.

If you are a current owner, especially of a Plus/e+ model, the best you can do is to avoid overheating/rapidgating the battery whenever possible. Doing so once every few months is not a big deal, but frequently doing so will likely lead to long-term damage.

If looking to purchase a Leaf Plus/e+, or a high mileage 40 kWh car in a hot climate, make sure to check Leaf Spy, and it may be wise to avoid any examples that have lots of QCs shown.

There is a great in-depth video by LEAF expert Dala explaining this issue more in detail for those who are interested.