Hello, the f150 will have its battery gauge flicker when you use anything power like the power windows. The lighting on the gauge will flicker too. When you run the AC the truck bogs down bad and the battery gauge dips a little bit. Its a 5 speed manual with a 4.9 I6 motor. Any ideas?
First check the battery voltage with the engine off. It should be near 12.6 volts. If it's around 12.0 volts it's discharged. If it's around 11 volts it has a bad cell and must be replaced.
Next, check that voltage again with the engine running. It must be between 13.75 and 14.75 volts. If it is low or high, have a professional load test performed on the generator. You'll likely find all you can get is exactly one third of its rated current of 90 or more amps. If all you can get is around 30 - 40 amps, the generator has a defective internal diode. It's a lot of work to replace them so most people just replace the entire generator.