Battery voltage readings

Obviously it has the 12 volt battery in the engine compartment but it also has a smaller battery in the trunk. I'd say a 6 volt battery. I'm under the impression that it's purpose is for the " automatic starting"feature. Not sure if that's exactly what it's called but a lot of times when you pull up to a stop light the engine will shut off. When the light turns green and you let off the brake the engine automatically starts again.

Yes

My battery saver started and the message read battery voltage system need service. What is the problem my battery or alternator