As a matter of fact, a battery can "appear" to be fully charged @ 12.6 volts yet not have enough amperage to spin the starter motor. The dealer of your car will tell you exactly the same thing.
Connections at the battery must be cleaned to a shine with a brush or knife. then installed tightly and tightened down. They must enclose uniformly encircling the post. Then you can seal them with spray or grease. Noco pads go on the top of the battery under the clamps. Loss of available voltage during attempted start is called voltage drop. The connection must be losing half your power if your tests were correct. You can prove it with a DVOM.
Just like they show you in an effective driver's ed class.
or auto shop class.
HI, it could be the cables or their conection with the battery. I'd start there. pull the cables off at the battery and clean the inside of the cable clamp and the post sticking out of the battery.. these parts need to be shiny clean to make good contact or you well have the problem you are having.
good luck
tim
Possibly, Terminals, the electrical fittings on the battery post.
If it started w/ jumpers then:
Disconnect the battery, wire brush brush everything, set (the battery) on a good charger, test the battery(not just looking for 12v, we need amp delivery, did you "load"test it.) connect the battery, start.
If a jump starts it... it's the battery or the alternator.
How did you check each component?
how did you "check" these items?
