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
Radio Use when engine is not runningYes, use of the radio when the engine is not running will drain the battery. Also, IF the use of the radio requires the ignition switch to be in the ON position, then any other devices [such as on-board computers, electric fuel pumps, etc.] that are turned on when the key is on will also add drain to the battery.IF your vehicle ignition switch has an "accessory" position [usually one "click" counterclockwise from the OFF position, then only the accessories that you turn on, will drain the battery.
something is causing a drain on the battery when switch is turned off
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Yes, that would definitely drain the battery.
Yes
If no accessories have been added that are draining the battery it is probably the voltage regulator . The voltage regulator will short out creating an open circuit , and the battery will drain back through it .
A starter does drain a battery. The battery holds a charge so that you can use your starter to start the car. After the car is started the alternator recharges your battery and runs your car and all it's accessories. If your car won't start or is having a hard time starting you can drain your battery by repeated unsuccessful attempts to start it.
If the defect is causing a parasitic drain, yes.
You do not need an isolater, but it is reccomended as without one you can drain your battery connected to your starter (your vehicle won't start). I do not run an isolater, just don't leave your accessories running and the battery won't drain.
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.
YES!My version:A bad relay switch that goes bad can "stick" in the on position and leave what ever it powers on and drain the battery You should be able to see the item that is on.
If you use the radio without the engine running you will eventually discharge the power from the battery. It should be fine for a couple of hours, If not, your battery may need replacing.