My batteries been giving me problems for the past few days.
The batteries only a month old, terminals are clean and tight, and the voltage meter in-dash says the battery has a 80% charge when it doesn't start.

Now, when first attempting to jump start it, the solenoid would open/close but not start when the Negative cable was attached to the car ground.
But! When I put the Negative cable onto the Stang's battery terminal, it starts up with EASE.(circuit completed)

I did the screwdriver magnet test with the back of the alternator, and it's a magnet so I assume it's working.

I'm thinking: Negative Battery Terminal? Negative Battery Cable?

I'm getting annoyed with having to Jump-Start the 'Stang, especially with the danger of frying my electronics...

 

take it to an advance auto parts.. they'll check to see if its charging for free..

 

Well, I'm pretty sure it's getting charged just from looking at the voltage meter and the fact that once the circuit was completed it started right up with a single turn-over.

 

Starter? Cold Weather? any ideas anyone?

 

Sounds like a bad cell to me. It could have sufficient voltage and gets sufficiently charged while the car is running from the alternator. But if there is a bad cell in it, it won't hold a charge and it's cold cranking amps will go low. Take it to Advance Auto parts of Autozone or something, they'll hook up a little tester and it'll tell you if it's a good battery or not.

 

Ran a handheld voltage meter on it, battery ran a 12.54 engine off, 12.54 engine off/lights on, and a 13.86 engine on.

 

sounds like a bad ground. any corrosion on the cable or terminaL?

 

Not that I can see, no calcification or rust.

 

+1 on taking it to an autoparts store. Pep Boys, Kragen, Autozone, Napa etc. Just take it there and have them test it. It's really the only option you have unless you have a battery test unit in your garage.