Switched off device is kept charging for a long time? Is this okay? Is it harmful for a switched off device to be kept charging for a long time? If yes, I would like to know why. (Not a very deep explanation but just a plain explanation as I am not an expert).
It is not altogether harmful, because once the battery is fully charged, it stops receiving more current automatically. This is true because most laptops can be use plugged in while fully charged to save the juice.
But however, its good practice to unplug your device once its full because some non-li-ion batteries tend to decrease in efficiency due to contactly being plugged in while full.
Hope this answered your question...