Battery light

You need to have the charging system tested professionally, but that has to be done while the warning light is on. You can start the testing by measuring the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If that is okay, it only means it is okay to do the rest of the tests. Of particular importance is the "full-load output current" test, and "ripple voltage".

The output current will be very close to what your generator is rated for, 0 amps, or one third of its maximum rating. With one failed internal diode of the six, you will only be able to get one third of the generator's rated maximum current. That is not sufficient to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down. The voltage regulator turns on the dash warning light when it sees system voltage dropping, and that can occur intermittently.

Worn brushes in the generator start out by causing intermittent loss of charging. That can go on for weeks or months before it becomes a permanent failure. The clue here, unlike with a failed diode, is you'll see the sudden change in voltage as dimming head lights or slowing heater fan or wipers.

I need test results to analyze. Failed diodes are rarely intermittent, but worn brushes and corroded wiring connections are. When a diode shorts, it stays that way and will show up as very high ripple" voltage and an inability to get the full rated current during load testing.