I have a f-350 it will not start in exst. cold battery is good and charging system is good. what do you thank it is

What size engine?

If it is a v10 or diesel the batteries have to be 1000 cca for cold weather no less.

If you battery has a dead cell it doesn't matter how long you put a charge on it for it won't make the battery any stronger. You have enough power to start the engine when it is above freezing but it takes twice to 3 times the charge of the battery to start it below 32 degrees. A bad cell in your battery will give you these symptoms. Take the truck over to Autozone or similar Auto parts store and have them do a load test on the battery. They don't charge for this service. They can also check your cable connections too at the same time to see if there is any corrosion on the terminals. It sounds like you need a new battery. Good Luck, Kenny/ MrBlueOval

My point exactly blue oval.

I have had auto zone test it and the test showed battery to be good as well as charging system.

Did you ever look to see what the cca's are on your battery?

Then it's got to be your starter. The starter is drawing too many amps and your battery can't handle it when it's real cold outside. That's pretty much all that is left to check. Kenny/MrBlueOval

Is this a diesel engine? Does your vehicle have an engine block heater? If so, you will need to plug into the electricity in order to warm your fluids. In extreme cold temps, the fluids tend to thicken.