13.4 Volts On Scanguage Running

I thought that I had read that 13.8 is the nominal voltage when the car is in "Ready". I haven't used the SG to read battery voltage. There is a diagnostic screen on the MFD that will also display 12V battery bus voltage.

 

I suggest that you check the battery cables to ensure that all connections are tight. When READY the voltage on the DC bus should hold constant at around 13.8VDC. When IG-OFF, a fully-charged 12V battery should produce 12.6V or better.

Therefore it is disconcerting to see your report that the voltage is fluctuating when the car is READY. If you don't find a loose connection, the implication is that the DC to DC converter is becoming flaky or that your measuring device is not working properly.

 

The voltage can also vary if you turn on lots of load, such as headlamps/foglamps along with rear window defroster. With only DRLs on Pearl runs at a constant 13.8V. Off for one or two days the Scangauge reports the voltage at 11.9-12.1, depending on temperature (you can turn on the Scangauge without turning on the car - just don't expect OBDII readings to mean anything, but the voltage on the 12V buss is measured by the Scangauge directly).

 

I have had 2 Prius with scangauges on them. My 2007 would nominally read 13.8. under extreme cold would go up to 14.1, and under extreme load would go down to 13.7

Totally different on my new 2008. It starts out at 13.7 to 13.8, but if I have very little load then it quickly drops to 13.2. Turn on the AC or lights and it comes back up to 13.8

I suspect there have been changes in firmware in the 2008 inverters Looks like it may be to extend the life of the 12v battery by lowering the charge voltage when the battery is charged. Makes sense to me and 13.2 v is still high enough to maintain the battery.