Question

why should we not keep the switch 'ON' for a long time?​

Answer 1

Do not keep the switch in the 'ON' position for a long time, otherwise the cell may become weak very quickly. seconds touch the wire..!

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Answer 2

The only thing that, over time, reduces a battery's lifespan is charging cycles, really. Letting the battery drain completely, and especially leaving it that way, is still detrimental to the health of the battery. So it's better to leave it on the dock, especially if you may not be using it for a long time.