12v reading in ready mode help

I have a device that plugs into the 12V socket in the center console which provides two USB sockets for charging other devices and a digital readout of voltage. When charging another device the display alternates between charging current and voltage. I regularly watch the display when making my Prius ready to determine 12V SOC and when I should connect up my battery maintainer. During the boot cycle there is usually one reading that shows battery under load and DC-DC converter not connected yet. That value is typically 12.1 to 12.4V and will drop as low as 11.8V when it's maintainer time. When made ready, the voltage reads 14.1V, which is a charging cycle by the DC-DC converter. After some time driving, the "charged" voltage drops down and is usually 12.9V. My driving voltage doesn't drop below 12.4V. You have confirmed that your ECU gauge is displaying the correct voltage. It shouldn't matter where in the vehicle a 12V measurement is made, but there may be additional load or other constraints when measuring from the OBD II port, where I assume your ECU gauge is connected. Have you confirmed with your DVOM, from another test point, these lower voltages as displayed by your ECU gauge?

 

In my experience, scan tools that show you the 12-volt system voltage are usually not asking an ECU in the car, but just measuring the voltage at pin 16 of the diagnostic connector where it plugs in.

My gen 3 varies the converter output voltage between about 13.5 and 14.7 depending on driving conditions and how much charging it thinks the battery needs. I think it also makes occasional, brief, lower dropouts in order to sample the voltage the battery holds up. I've never seen a sustained reading as low as 12.4 in my gen 3. But gen 4 has a more sophisticated battery state sensor and maybe it makes more aggressive choices. Also, the voltage you might measure at that pin could be reduced by voltage drops caused by other heavy electrical loads at the time.

 

WoW; I didn't realize indefinite has come and gone... Just kidding.
Try disconnecting your current sensor on the negative post of your aux. battery and run your test again. Let us know what your results are.

I'm assuming the car's charging ECU is pushing more electrons into the traction pack to optimize mpg and only ramping up 12V current when necessary. Also assuming, unplugging the current sensor will blind the charging ECU, so it would always pump 14V into the 12V system. I've unplugged that current sensor on occasions and it doesn't throw a CEL.

 

perhaps why so many gen 4 owners have had 12v troubles

 

I don't; but it was a lot of extra electronic junk in the gen5 along with a bad batch of batteries that started all this. IMHO.
Still on my original OEM battery...

 

this was a popular thread back in the day:12v-battery-is-discharging-re-charge-now.215528