voltage drop from battery to obd port?

Try measuring the voltage at your power port with the same DVM you used to measure the battery voltage at the battery posts/terminals.

 

The only way you get voltage at the cigarette light is by having the car on, if I'm not missing something. Maybe that accounts for the voltage drop? Have you tried checking voltage at the underhood jump start point, with a multimeter?

FWIW, I've got one of these, has obdII with 12 volt pins at one end, 12 volt plug at the other, could check it out myself in a bit.

 

All wiring has some resistance and thus some voltage drop when current is flowing.
The more current, the more drop.
Then there is potentially more voltage drop across connection points; more if corroded, less if clean and tight.
The difference might actually be on the ground side, IE not a really good ground.

 

Ok, got this:

At battery posts: 12.39 (volts)
At OBDII: 12.34 (measured at 12 volt plug on cable shown above)
At JumpStart point, underhood: 12.49

That last one kinda weird, not sure why. Good quality RMS meter too. Ambient temp 4C.

 

Yes.

 

Anyone have a good rule-of-thumb for what ambient temps do to voltage readings? All my readings are kinda low, wondering if at least some it is due to 4C ambient temp?

 

As Easy Rider stated above, a good ground point is really essential to getting an accurate reading. Don't use any old bolt on the sheet metal.

 

I rechecked my measurement at the battery. I got 12.39, both at a bolt adjacent to the battery, and directly to the negative post of the battery.

 

It was a figure of speech. SMH

 

Car and Deep Cycle Battery FAQ, Battery Manufacturers and Brand Names List, and
go down to Temperature Compensated Battery State-of-Charge (SoC) Tables
you can also find the same document at
Battery-State-of-Charge

Use the far-right AGM data.
According to the chart 12.39V @ 4°C puts your battery at around 60% charged.

Keep in mind the accuracy of the voltmeter doing the measuring & what sort of temp compensation the meter may have.

The chart should be accurate - but it may not match your specific battery. Still, it should be pretty close.

 

For more thoroughness, measure the 12V outlet (provided the probes have insulated sides so they don't short together) and the OBDII connector port with the same DVM you used at the battery and hood locations.

Measure all four location with the ignition in the same operating condition. Because the 12V outlet isn't powered when the ignition is Off, put the car in ACC mode before measuring.

 

Batteries are less efficient when cold.
4C isn't THAT cold and there should be very little difference due to that.