The alt does not take control!!! Your battery still powers everything, the alt charges the battery!I did run a test with Volt meter though and my battery read 12.26V while the car was off and Alternator was at 14.4V. My battery is old though. Haven't changed since I bought the car. ( 4 years ) If the battery is weak does that effect the system? I have heard the ALT takes control after the car is started
If you have a dead battery, your car won’t run worth a shit at idle, if it even runs. When your driving, it runs better, Because your alt is doing its job. But because your battery is dead, your alt is under abnormal strain, trying to keep up.
Change your battery!