I'm not sure canehdian, I think it might be the quality of the line coming in. When I plugged my quickchargepro into a 30amp 120volt, I am getting power numbers very close to the volt/amperage. It may be an indication that the amperage level is fluctuating up and down, but the CAN is not reporting variations in amperage only power. Actually, it reports 0 a lot for power which makes data aggregation troublesome.
OBD-II is connected to the car and I am sending CAN messages to communicate to the other connected devices in the car. I forget the difference, I think CAN is a subset of OBD or maybe vice versa?
True I am oversimplifying the equations by assuming constant power. In theory, this should be fairly true for 0-80% but may not be the case. In order to provide instant feedback, I do need to make some assumptions and the dashboard does the same.
For a full charge from 0miles (car reported 0.20 SOC) to full, the dashboard was off by +30mins and my system was off by +1hour. Honestly, I was very pleased with this. It took me milliseconds to get my calculation and 3mins for the OnStar calculation. I think this longer than reported charge time estimates might be because the factor I'm using 0.5625 isn't static across the entire charge cycle. It may also be because the power is being drawn much more efficiently from this 30amp plug.
The goal isn't for 100% accuracy, but really just to free myself from an OnStar subscription. This 1hour drift also aligns with the 30min drift from a 50% recharge on 120volts. I could continue fudging the numbers to bring those inline. Ironically, OnStar has been reporting 0:00/0:00 for recharge times probably b/c I'm over-saturating the CAN bus with queries for Car information.