Code B1317, high voltage?

Where I would start is by charging the battery at a slow rate for a good three or four hours, and unplug the three-wire plug at the rear / side of the generator. Given the two symptoms, a good suspect is a shorted voltage regulator. It's mounted to the back of the generator. That three-wire plug actually plugs into the regulator. If the battery no longer runs dead in a few days, that regulator can be replaced separately and is a lot less expensive than replacing the entire generator. I can put some photos together that will help. On some models, Tempos in particular, this can be done without removing the generator from the engine.

Let me know what you find.

The back of your generator will look similar to this photo. By around 2000, the engineers discovered they had made this system too easy to work on so they added another cover over it. The repair is the same but it takes a lot longer.

The blue arrow is pointing to a test terminal. This is similar to what GM used up through 1986, but it's easier to get to, unless you have that cover. The circuit in question is the yellow wire at the top, right of the photo. That wire has full battery voltage on it all the time and is the only one that can drain the battery if the regulator is shorted. The light green / red wire at the top comes from the dash warning light. That's the turn-on circuit that tells the voltage regulator when to wake up and start doing its thing. With the ignition switch in "run", engine off, the "Battery" light should be lit up. At that time you'll find very close to 2.0 volts on that light green / red wire. To be valid, the voltages have to be taken with that connector plugged in. Once the engine is running and the generator is working, the regulator puts system voltage back out on the light green / red wire to oppose the voltage on the other side of the warning bulb coming from the ignition switch. With the same voltage on both sides of the bulb, the difference is 0.0 volts, so it turns off. Besides a failure to charge, this voltage regulator can turn on the warning light for a low-charge condition, and an over-charge condition.

The yellow wire supplies the current to run the generator's "field" circuit. That's the spinning electromagnet. The maximum possible current that will ever flow through that circuit is 3.0 amps. Under normal driving conditions when the system does not have to run wide open, more like one amp is typical. If that part of the regulator is shorted, it will pass the full three amps all the time which makes the generator really strong and puts out much to high of a voltage. Being shorted, it also won't turn off when told to do so. That will run a good battery dead overnight.

If you need to replace the regulator, remove the four Torx screws in the four corners, not the other two where my blue arrow is pointing to one of them. Once you pull this off, it can't be reinstalled until you reload the two brushes into their holders. A new regulator will come with the new brushes already loaded, and they'll be held in with a toothpick or something similar in the small hole, (red arrow). Once the regulator is bolted into position, that toothpick is pulled out. You'll hear the two light clicks as the spring-loaded brushes pop into place. If you try to reuse the old regulator, you have to push those brushes back in first, then you can even use a stretched-out paper clip to hold them in until the regulator is installed.