What happens after two weeks? Does the battery fail to crank the engine? Is it totally dead? To be properly tested at the auto parts store, it has to be slowly charged, then tested under load to see how much current it can deliver while maintaining a specified voltage.
The place to start is by testing the charging system. The first part you can do yourself with a digital voltmeter at the battery. It will read 12.6 volts if the battery is fully-charged. If you find it's around 12.2 volts, the battery is okay but it's discharged. If it's around 11 volts or less, it has a shorted cell. That would not be common on a newer battery.
Next, measure the battery voltage again while the engine is running. It must be between 13.75 and 14.75 volts. If that is okay, the rest of the tests must be done with a professional load tester to see how much current the generator can deliver. If it has one bad internal diode of the six, you will only get exactly one third of its rated output, and that's not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference until it runs down. That could take from a couple of days to a month or more, depending on how much you use the heater fan, light, wipers, etc. Besides the low output current, "ripple" voltage will be very high when the generator has one bad diode.
I have to go out of town for a day. I'll check back tomorrow night to see if you had the load test performed and what the results were.
Tuesday, July 21st, 2020 AT 5:45 PM