[Dynamical]

The fact that your voltage was reading only 11.4V to 12.0V after a drive and 13.7V to 13.9V after started tells me your alternator is working but your battery is not holding a charge anymore. If it was my car, I would feel very confident that a new battery would fix it. Your battery should never be that low. If you don’t have a battery tester, bring it right to an auto parts store they will test it for free. I wouldn’t let them test your charging system, though, they are not qualified to diagnose today’s fluctuating charging systems. Battery testers are cheap. You can pick one of those up also if you want to start messing around with it.