New battery cranks great but won't start . I had saved old battery because it still held a good charge. Put old battery in for the hell of it same situation good crank no start . . Jumped car with the other battery started right up. When weather got a little warmer car would start on either battery. Old battery is 10 years old and always started the car without failure I replaced because of age and being cautious. Any suggestions would be appreciated. Tia

 

I was not clear on the other post. I used both batteries at the same time (parallel) and the car started right up .thanks

 

One/both batteries are not fully charged. Is your alternator up to snuff?

 

Alternator measured 14.55 iirc. Tia