Is it a short, the battery or the starter or none?

2001 GT
My car wont start, and its a mystery to me. It super cold when it all began...

Optima Red Top Battery, had it charged twice, they say its OK

I threw in my Jeep battery and it seemed to start fine, sometimes it would need to be unplugged and put back on, but usually works

With my Optima battery it reads 10v on my gauge, even after a full charge. It sometimes can be jumped, and sometimes not. It usually clicks when I try to start it, or the engine slowly turns over.

What do you guys think it is? Im thinking about replacing the starter, but I dont know anymore. Thanks.
 
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Have you tried a different battery in the mustang? Have you tried jump starting the mustang using a different vehicle/battery?

I'm thinking your battery is bad. Throw in a different battery or try to jump start the car using another battery and you will quickly find out. If it starts, then you found the problem.
 
Optima Red Top Battery, had it charged twice, they say its OK

Who is "they" and how did they make this determination? Did they put the battery on a load pile and check the terminal voltage while drawing a few hundred amps out of it? Do you have some sort of test sheet from them with the numbers on it?

I threw in my Jeep battery and it seemed to start fine, sometimes it would need to be unplugged and put back on, but usually works

Not sure what "..would need to be unpluged and put back on..." means. You mean the car would start fine and then not and just mucking about with the battery terminals would fix it again?

With my Optima battery it reads 10v on my gauge...

Which gauge?

, even after a full charge. It sometimes can be jumped, and sometimes not. It usually clicks when I try to start it, or the engine slowly turns over.

Do any of the battery leads get warm to the touch?

What do you guys think it is? Im thinking about replacing the starter, but I dont know anymore. Thanks.

I'm thinking the battery is kaput and/or the battery leads or chassis ground may be defective or corroded or loose or...
 
+1 on everything above.

I frequently help ppl that tell me their battery is in good shape. However, they think nothing of the green corrosion cauliflowers growing on/in/around the battery terminals.

Make sure that all terminals and cables are clean and tight.

Bottom line. Today's cars simply will not run right without a strong battery and electrical system.
 
An Optima battery (spiral cell battery) will fail if completely discharged. Did this happen? If so the battery is 95% likely yo be toast. Spiral cell batteries cannot be fully discharged, it ruins them.
 
An Optima battery (spiral cell battery) will fail if completely discharged. Did this happen? If so the battery is 95% likely yo be toast. Spiral cell batteries cannot be fully discharged, it ruins them.

At least not fully discharged (deep cycled) more than two or three times before it won't completely charge anymore.

Btw, you should be able to take the Optima back to the store for a new one at no cost.