In the majority of cases that we have seen, the Nissan LEAF’s lithium ion battery cells tend to naturally degrade fairly evenly. As a result, most cells in an aged pack tend to have similar capacity and in these cases just swapping a few modules in the pack is a pointless exercise.

For LEAF owners with a heavily degraded battery pack wanting a significant increase in overall capacity (range), we feel that a complete battery replacement or upgrade is often the best option.  We have solutions to allow battery swaps and upgrades to be performed quickly and safely without opening either the new or old battery pack. The old battery pack often still has life and value for a stationary storage project.

Of course, there are some valid cases where opening the battery pack to complete a repair is still justified as detailed below:

Failed cell causing an isolation fault

A ruptured cell with leaking electrolyte is extremely rare on Japanese-built 24kWh LEAF models, but we have encountered several failures of this type on UK-built 24kWh battery packs as well as 30kWh, 40kWh and 62kWh packs.  In these cases replacing the module that contains the leaking cell is required.

Successive DC fast charges leading to highly elevated battery temperatures certainly increases the risk of this occurring. At an elevated temperature, the cells will swell. Excessive temperatures can push the cell’s foil pouch to it’s breaking point at which point it will leak electrolyte.  Fortunately, in these cases the safety systems in the LEAF’s Lithium Battery Controller (LBC) detect this problem as loss of isolation.  In turn, the LBC triggers the vehicle control module (VCM or sometimes referred to as EV/HEV on scan tools) to trigger DTCs (fault codes). The primary code is P0AA6 which in turn also triggers another codes P31E7.  It is important to note that there are many other issues that can trigger P0AA6, so this doesn’t always relate to problems inside the battery pack. These fault codes 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 once the car is turned off it will not go back into ready mode.

Cell discharge faults causing imbalance problems

Cells with a high self-discharge problem result from a manufacturing defect and can cause an imbalance  problem to develop over time. The excessive self-discharge of the faulty cell(s) causes
them to end up at a lower state of charge compared to the other cells in the battery pack.  While these cells do have a fault, it isn’t appropriate to call these ‘weak’ cells, as the
cell capacity is often the same as the other cells in the battery pack. This type of issue tends to develop to the point of being noticeable within a few years after the vehicle’s manufacture date. Assuming that the issues is correctly resolved through through a replacement module, it is therefore less common to see this issue on vehicles later in their lifespan.

Cases of a prematurely degraded cell

Occasionally an individual cell can develop high internal resistance or low capacity (or both) prematurely before the rest of the cells in the battery pack. However, this difference needs to be significant and limited to a small number of cells to justify a battery pack repair. On some high odometer vehicles with battery packs that have had a hard life (frequent overheating etc) there may be a cell that is obviously worse than the others, but if there are several others that aren’t far behind it then a successful repair would involve a large number of replacement battery modules. Repairs that require more than a couple of replacement modules are rarely viable compared to fitting a complete good replacement battery pack.

A suitable replacement module with correct balancing is important

When fitting a replacement battery module to repair a battery pack, the replacement module must contain cells with a capacity that is as good or better than the rest of the cells in the battery pack. The cells in the replacement module must also be carefully balanced or the result will be significantly reduced range, improper range calculation and in extreme cases the vehicle entering limp (turtle) mode.