Replace battery and red battery light on

The "Battery" warning light means the battery is not being recharged while driving. You need to have the charging system tested. You can start the diagnosis yourself. Use an inexpensive digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, the first suspect will be the generator.

The next step requires a professional load tester to see how much current the generator can deliver. If one of the six internal diodes is defective, all it will be able to produce is exactly one third of its design value. 30 amps from a common 90 amps generator is not sufficient to run the electrical system under all conditions. The battery will have to make up the difference until it slowly runs down.