[bri3d]

It's quite likely that lower discharge rate requirements are a large part of what makes this system function. Batteries with different internal resistance can work reasonably well even in a naive series system at low discharge rates but absolutely will not work at high discharge rates.

[lstodd]

exactly.

state-of-charge / depth-of-discharge vs lifetime accumulated "discharge stress" so to say also matters a lot.

batteries aren't simple, even lifepo4 ones.