To test a fuse you would "ohm it out", ala testing the resistance across the fuse itself. Should nil, none, 0, or at least CLOSE. if it is reading "XX MEGAohms", an infinity symbol (an 8 on its edge), or anything higher than about 2-3 ohms, then you have a bad mega fuse, which will cause:
ZERO charge to the battery
Constant charge indicator light on the dash
To also narrow down what it happening here, measure the DC voltage on the battery with the car running, across the terminals it SHOULD be 13.9ish-14.5-ish
Then shut the car off, turn on the headlights and leave them on for 60 seconds to remove the "surface charge" off the battery. Test across the terminals with the car off and you should have 12.0 volts or higher.
turn off you headlights and let us know what you found regarding the voltages and the resistance of that fuse.
