Last fall I noticed a considerable loss of range, but did nothing about it during the winter. This spring it became worse, even to the point that the system would shut down due to voltage drop under heavier loads like a long hill. It was time for new battery, but I thought maybe I could just replace a few cells. I charged the pack, took it apart and found 8 of the 40 cells were in such bad shape that they would barely register on a voltmeter. Seems like almost one whole series string was bad. Makes me suspect that there may have been an issue with the BMS rather than the cells as the remaining 32 all seemed fairly uniform.
So instead of rebuilding, I just replaced the whole pack with a new 50 cell unit with 25% more capacity. That increase sure works well on the tough hills, and the range is considerably better than the previous pack ever was.
I wound up with 32 18650 cells that all measure over 4V at full charge, so I bought a small charger for them and replaced all my old D-Cell flashlights with units that use 18650 cells - so far they seem to be performing fine, so it will be a while before I ever buy flashlight batteries again! Too bad I can't do the same with all the various battery-eating remote controls in the house.