ScanGauge II worth picking up to monitor battery voltage?

+1 to Mendel's reply.
The battery voltage is best read with the car off for >15 minutes. This means no ScanGauge or bluetooth module - and no cigarette lighter meter either, since the power ports are disabled when the car is off. Additionally, I wouldn't trust the voltage that it reads anyway - it's certainly better than what's shown on the display in maintenance mode, but I'd still bet it has at least a few tenths of inaccuracy.

Far better to get a digital multimeter. These are about $10-$100 depending on quality and features. They're useful for other things too - measure the voltage on that random battery you found lying around, or make sure that the power is out before you start some DIY electrical work. Mine also does temperature when attaching a thermocouple - so I can calibrate my oven temperature, for example. When one of my landscaping lights went out, I used it to help diagnose and find the loose connection.

The ScanGauge is a fun toy - I have one of those too. But it's more useful for watching engine RPMs, coolant temperature, and traction battery current, if you're interested in that sort of thing.

 

I guess the point is to consider what data you want to monitor, how often and what you're will to pay to do it, then go with the option that fits those parameters. I have little doubt that the SGII is probably the most robust and most accurate, and it fits nicely on the steering wheel shaft. I prefer the Bluetooth & phone/tablet option, but then I don't continuously monitor any data. I've bought into things like the SGII in the past only to find I didn't use them, so I'm cautious when it comes to spending money on nice-to-have stuff. I only bought the ODB module after I paid $70 to have an error code read/reset on our van when there was nothing wrong and I never got the code again. Now I can check the code, reset it and if it comes back, then take the car in for service. I had 2 more codes on the van on a trip, both valid, but I saw that servicing could wait until we got home. The $20 module easily paid for itself and an SGII would have too. I'm actually a little surprised that Toyota, with the Prius multifunction display, doesn't integrate the ODB port with the display, at least as an option.

 

Reading 12V battery voltage on the Scangauge while the car is in READY will only display the voltage that the DC/DC converter is supplying to the 12V battery - typically ~14.7 (or sometimes ~13.8) VDC. It's not an indication of actual battery voltage.

See attached...