I suggest posting this same inquiry in the "Bullet with UCE engine" section, where it will get a lot more eyes, and responses. This forum section is not really for asking advice.
That said, I'd say you have either a parasitic draw constantly on the battery, or the battery has an internal short. I'd disconnect one battery cable and use my digital multimeter to see if any substantial current draw is occurring with the ignition off.
If not, I'd disconnect the battery, charge it fully ( voltage at least 12.8 volts three hours after charging is complete), and let it sit on the bench, checking the voltage twice a day. I would think it would take at least 6 weeks to self discharge down to 12.3 V. If it did this in a matter of days, I'd be looking for a refund.
Topic: 2020 Classic 500 battery loosing power after 3 days, (Read 1000 times)