I have an 8 volt battery in my 51 chevy pickup. The system is still 6 volt. If I drive it 5 or 6 miles everything is fine until I shut it off. Then it wont restrat. The battery is dead. I checked the generator and it appears to be charging. It is at 7.75 vdc. i am sure this is enough to keep a 6 volt up, but is it dicharging the 8 volt. What do I need to do?