Because your alternator is shot.
If working on the electrics of a car then it is a sensible precaution to disconnect the battery.
Disconnect one cable of your battery and turn the car key to ignition position two times, connects your battery and turn the car key to ignition.
Don't turn it on. If you mean how to prevent it starting when someone tries to, you can disconnect the battery.
No, you can charge the battery while it's in the car.
it should because car is running off alternator and if you do this it will not charge battery back up
It's probably a stuck relay.
no
Check the stop light switch for alignment or perhaps defect
Disconnect the negative cable first when disconnecting a car battery
Disconnect the negative battery cable.
Disconnect negative cable first from old battery and connect it last when installing new battery.
You may have to disconnect and recharge your car battery for your car stereo to work again.