65 C10 will not turn over.

[mrdave03]

I parked the truck last week. Today I went out to start it and nada. Wouldn't do anything. I tested the battery it was reading only 7 volts so I figured 4 yrs old time to replace. New battery in still nothing. I put my booster/jumpstart unit on it and it turns over fine and starts right up. I notice the alternator gauge moves dead left to D and will not come up to charge. I'm confused because it starts right up with the charger on "jumpstart" the new battery reads just over 12 volts so it seems to be fully charged. Could I have gotten a defective battery? It is odd tho that the alternator gauge wont move to charge but why does it start by jumping. I'd think it would start with the new battery even if its not being charged IE: alternator gone bad... It also only reads 12 volts while the truck is running not 14 as it should if it were charging..so I think bad alternator for one thing..hmmm... help?