car will not turn over. good battery and alternator. on 1999 Ford Escort

this is strange. the car will not turn over at all when battery is connected to the car. however when I disconnect the battery cables and connect them to jumper cables the car fires right up. battery is showing 12.6 amps and alternator is putting 14.5 into the battery when running. I have double checked the terminal connections many times and they are tight and no corrosion. had battery checked at auto parts store and its fine (only 4 months old) car will run fine after jumped with battery terminals not connected to battery. really confused with this.. any help would be great.

Asked by for the 1999 Ford Escort
Replace the battery!! .. Yeah, I saw that it has been tested but not load tested .. Suspect battery is internally shorted.
1 more answer
Check the battery cables where they attach to the terminals. You might be touching the cable off the terminal with your jumper cables and supplying power to the car but if there isn't a good connection from the cable to the terminal you won't get any power from the battery.
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