I used the test program in this post from @rickkas7 to produce the data listed below: Argon check power source?
| Scenario | PWR | CHG | vBATT |
|---|---|---|---|
| USB Power & Full Battery | 1 = USB Power | 1 = Not Charging | 4.0 to 4.1 |
| USB Power & No Battery | 1 = USB Power | 1 = Not Charging | 4.1 to 4.2 |
| No USB Power & Battery | 0 = No USB Power | 0 = Charging | 4.0 to 4.1 … 3.8 to 3.9 |
| USB Power & Weak Battery | 1 = USB Power | 0 = Charging | 4.0 to 4.2 |
Based on these data points, I don’t see a definitive way to detect the presence or absence of a battery when the PWR = 1 (top two scenarios).