We are seeing an increasing number of individual cell failures in 30kWh Leaf battery packs. As of early 2018, cell failures that we’re aware of have been extremely rare for all of the 24kWh models of the Leaf. We would naturally expect that at some point in the future, cells from heavily aged 24kWh packs will start to fail – their lifetime is still unknown, but it is certainly not infinite. However, the failures in 30kWh cars that we already know about have started appearing when the vehicles were only a year or two old and often with very low odometer readings. The number of cases that we’re aware of alone is concerning, but we suspect that there are other cases out there that we haven’t heard anything about. This problem is not isolated to cars built in any one of the three Leaf factories in the US, UK or Japan, but it does seem to be most prevalent in early 30kWh cars – built late 2015 or early 2016.
The exact cause of these failures is not yet known, but is suspected to be a defect created at some time during the cell’s manufacturing process. At first thought it seemed odd to think that with these cars being built in 3 factories around the world, that all 3 would have the same manufacturing problem. However, our further investigation revealed that all of the early 30 kWh cells & modules were actually manufactured in the US and shipped to the UK and Japanese factories to be assembled into packs.
Early signs of 30kWh Leaf cell failure – state of charge imbalance problems
We have seen many examples of 30kWh Leaf battery packs where one or sometimes more of the 96 cell pairs are at a significantly lower state of charge than the rest of the cells in the pack.
The out-of-balance cells not only cause a loss in the usable range, but they also cause the instrument cluster’s display of estimated range remaining and SOC to be very misleading – the dash goes from showing significant range and remaining SOC to very little in a short space of time. When the imbalance problem eventually becomes severe enough, the battery control module in the battery pack (officially referred to by Nissan as a Li-ion Battery Controller or LBC) will display fault code P33E6 (Cell Controller) and go into limp/turtle mode even when at a high overall state of charge.
We have come to the conclusion that the cause of this imbalance is excessive self-discharge of the failing cell(s) which over time causes them to end up at a lower state of charge. While the cells have high self-discharge, it isn’t appropriate to call these ‘weak’ cells, as the cell capacity is typically normal.
The LBC in the 30kWh battery packs does have some cell balancing capability; however it is far too weak to be able to counter most of these high self-discharge cases. It is also entirely passive balancing meaning that it can’t transfer energy from high SOC cells to the low SOC cell. It can only waste energy (dissipated as heat) in all of the high SOC cells until their SOC is sufficiently lowered. But with a maximum of 39 mW of power dissipation per parallel cell pair, the balancing effect is far too weak to have any meaningful effect that could correct the imbalance. However, upon reflection, that is probably a good thing as a more powerful balancing circuit could have masked what is effectively an early indication of a failing cell that should be replaced.
Opening the battery pack to manually balance the low SOC cell(s) will hide the problem for a while, but the issue will re-develop over time on the same cell(s). Furthermore, the failing cell is only likely to get worse over time and may eventually develop into a complete cell failure. Therefore, we consider the best option to be replacement of the module containing the faulty cell.
Complete Cell Failure
We have evidence that suggests that successive DC fast charges leading to a highly elevated battery temperature can push a cell that is showing early signs of failure to a point where it fails completely, bringing with it associated safety risks. Fortunately, in the complete cell failure cases that we are aware of so far, the safety systems in the Leaf’s LBC have detected the problem as loss of isolation. In turn, the LBC triggers the vehicle control module (VCM or sometimes refereed to as EV/HEV on scan tools) to trigger a fault code (DTC) P0AA6 and another DTC P31E7 which is associated with it. It is important to note that there are many other issues in an EV or Hybrid that can trigger a P0AA6 DTC, so this doesn’t always relate to problems inside the battery pack. However in the 30kWh complete cell failures that we’ve seen, these are the codes that cause warnings to appear on the instrument cluster which alerts the driver of a problem. The car can actually still continue be driven at full speed with these warnings on (which is NOT recommended), but fortunately in all of the cases that we are aware of, the driver has stopped and turned off their car. The car will not go back into ready mode with these DTCs, which seems entirely sensible to us.
The most practical solution for repairing failing or failed cells in a 30kWh variant of the Nissan Leaf is to replace the module containing that cell. There are 24 modules in a 30kWh battery pack, but the 12 in the rear stack are slightly different than the 12 in either for the two front-side stacks. In the 2015 and early 2016 30kWh packs, the original module part numbers are:
295B9-4NP0A (module from either side stack)
295B9-4NP1A (module from rear stack)
Both of these part numbers have now been discontinued and superseded, but we have been able to secure new replacement modules in limited numbers.
In countries where these vehicles were sold new, these cars will still be under Nissan’s manufacturer warranty, so the repair is likely to be completed by a Nissan dealer and at no cost to the customer. As a result, neither the customer or independent repair shops are likely to be involved in the diagnostics or solution. We suspect that in many other countries, Nissan’s support there is the main reason why we haven’t seen more reports of these failures worldwide. However, in our home country of New Zealand the opposite is true. All 30kWh Leafs arrived in NZ as used imports which Nissan NZ have been unable or unwilling to offer battery support for. Without any local support, these problems are finding their way to us only because we have a small supply of replacement 30kWh modules. We obtained these by sourcing a complete relatively-high-SOH 30kWh battery pack which we are robbing good modules from. With 24 modules per pack, this has already allowed us to correctly fix several 30kWh cars suffering from these cell failure issues which we would have otherwise had no solution for. In addition to supplying modules, we can also assist with correctly balancing the replacement module.
Whether you need a replacement module from us or not, we would appreciate contact from any vehicle owners or repairer around the world that encounters this problem. Providing details on the car (age, VIN, odo, etc) and information on usage habits leading up to the time of failure would also be very helpful. Please contact firstname.lastname@example.org to discuss.