get your alternator tested first use a test light to see if there is any power going to compoents in the car while the car is off
something is causing a drain on the battery when switch is turned off
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Yes, that would definitely drain the 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
If the defect is causing a parasitic drain, yes.
Yes, a loose battery cable will prevent the alternator from charging the battery. The drain on the battery is not caused by the engine not running unless an accessory is left on. Most likely, the drain is caused by accessories (radio, lights, etc) while the engine is running due to the battery not receiving its charge from the alternator and then not having enough charge left to start the engine.
The battery gauge in a vehicle will move when you use your power mirrors as your causing a small drain on the battery & lowering the voltage slightly. The effect is normally only really visible when the engine is not running.
If it stays on all the time it will drain the battery
If the engine is running, then no. The alternator constantly keeps the battery charged.
The Legacy subwoofer will really drain your battery. It's not a good idea to run it while you're not running the engine.
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.
A stuck relay in the car can drain the battery. A dead cell in the battery itself will drain the battery overnight. If this is the case the battery must be replaced as there is no way to fix it. To find out disconnect the negative battery cable for around 8 hours. If the battery goes dead it has a dead cell. If not then something is draining the battery. Anyplace that sells batteries can test the battery for you.