11 volts with a lead acid battery is effectively flat. If you only run your car for 30 minutes after managing to start with a fairly low battery you will most certainly not fully charge it. Indeed with headlights and other equipment working and if the engine isnt turning over that fast, you might not charge it at all.
Best course of action is to take the battery off the car and use a battery charger overnight or longer to re-charge it.
