Feng wrote:I'm curious to know: Any harm in swapping only a few of the modules out rather than the whole pack? Eg, suppose I don't want to spend so much but want a range boost, could I go halves on a salvage pack with a fellow LEAF owner?
That is why LeafSpy is so useful.
Look at the cell balance when the pack is full (SoC near 90%) and again when the pack is empty (SoC near 20%)
You will see that the average cell voltage changes from about 4.1V down to about 3.7V and the difference between modules is typically around 20mV, should be less than 100mV anyway.
But if you find a cell that is high when the pack is full and that is (extremely) low when the pack is empty,
that is a cell with lower capacity than all the others. If you find that situation, it makes sense to replace just the module containing that cell to increase the capacity of the whole pack, because the capacity of the whole pack depends on the lowest capacity cell, because all cells are wired in series.
Most packs that I see have very similar degradation between cells, they stay pretty good "together" at low and high State of Charge (SoC) so there is no point in replacing half the pack.
To compare it with another similar example:
if you need to hoist something with a rope and you have two short ropes, each rated for 1000 pounds and you need to tie the two ropes after each other to get the length you need, it does not make sense to replace one rope with a stronger one as the second rope will still limit at 1000 pounds, only if you replace both with a stronger rope will you be able to hoist more weight, because the ropes are in series so the weaker one determines the strength of the total.
If however you find one rope deteriorated more than the other (like finding a weak cell in the pack) then it makes sense to replace that only.
Initial problems that were quoted where a single module was replaced were due to one failing cell in the pack, so replacement brought the entire pack back to original capacity. The complaint was that the usable range was suddenly half the original range. This matches the situation of a single cell failing, because a module for a 24kWh pack actually consists of 4 pouches (cells) which are 2-by-2 in parallel and the two pairs are in series. Each pouch has 32Ah capacity at 4V so the capacity is doubled from the parallel connection to give 64Ah capacity and the two cell pairs in series add the two cell voltages, so 4+4= 8V for a module.
Losing a single pouch in a module, reduces the capacity to 32Ah so the total pack capacity (range) is suddenly half the normal.
This would be similar to half the strands of one rope breaking for the example of hoisting: the remaining half of the strands can only carry half the original weight. The weakest link determines the strength of the total.