The battery on my 2017 Fiesta Titanium keeps losing charge. This causes the Stop-Start to stop working as the battery is constantly below 85% charge. After 2 weeks the battery is completely flat. After a long run of 300 miles the battery only tests at 50% charge 11.8volts. So far the dealer has replaced the battery twice. Is anyone else having the same issue and any solutions?

That's an Alternator that is the issue. Alti should be putting out 13ish volt while charging and, after 300 miles, you should have enough charge for the national grid

I would put a voltmeter on it with engine running and see what happens on idle and revving. also try with high electrical load (lights etc).

On my brother's Focus the problem was the one way clutch built into the alternator pulley had failed so the alternator itself was turning a lot slower then the pulley all of the time.

Not all cars have a clutch built into the pulley. I do not know if yours does, but I would expect them to last a few more years then the age of your car normally