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 th

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.