Dead cell in the battery itself which requires you replace the battery. Can also be any light on the vehicle that is on pulling power from the battery. It can also be a relay that is stuck closed.
Bad batterry or defect in electronic or electrical circuits causing current drain from battery.
The key being in the "on" position itself. As for rapid battery drain, you could have a bad battery or there could be a loose connector. Source: Personal experience
something is causing a drain on the battery when switch is turned off
from me constantly raping your cars exhaust while you sleep.
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Bad battery or a parasitic drain
Yes, that would definitely drain the battery.
If the alternator is installed incorrectly, yes, it can drain the battery.
If the defect is causing a parasitic drain, yes.
A dead cell within the battery itself will drain the battery. There is no fix for this except replacing the battery. You may also have something that is pulling power from the battery. Examples are under hood light, dome light, glove box, trunk, courtesy light, and any numbers of relays.
If the battery is worn out it may not hold a charge for 8 hours. There could also be a short somewhere causing the battery to drain down.
most likely somehting is not turning off such as an interior light that has a broken switch or you could have an electrical short somehwere.