It’s literal dead weight. It’s especially bad because the bikes already don’t have great ranges (and the estimated ranges are often pretty optimistic), and extra dead weight cuts into your limited range even further.
I'd be surprised if it was literally dead weight. It's much easier to design a software control system that uses a common set of cells and caps the full charge than one that switches different cell blocks in depending on what's enabled.
It is not as simple as it seems. If you connect the "unused" cell in a series, it could alter the battery's output voltage by as much as 4.2 volts, which has the potential to damage the voltage regulator.
On the other hand, if you connect the "unused" cell in parallel, it's absolutely crucial that it holds the same charge as the other cells at the time of connection. This is because the parallel pack's voltage will level out, leading to a substantial flow of current (essentially causing rapid charging/discharging of the cell). This can result in a fire or even a battery explosion if the cell voltages are too different.
All in all, physically detaching a battery might not be the best approach. The cells will experience different levels of wear, and variations in internal resistance will lead to problems with cell balancing and longer charging times.
No that would be more complex. You’d have to add a relay and something to control it. Much easier to just map 80% to “100%” on the display and in the BMS.
Every cells have to hold roughly same charge for the concept of battery pack to work. Else the weaker cell becomes overcharged as others take in the charge, and (in the absolute worst case) the entire thing goes kaboom. Not worth doing.
