It's Still the Battery

disconnect it. you need to measure the quiescent drain when the car is off, and look at the service manual for normal.

 

I think there may be a couple of issues with the way you are measuring voltages on your battery. To get an accurate measurement, you will need to let the battery's surface charge dissipate after it has been charged, even if that charge is coming from the car's DC to DC converter, and you will need to ensure that none of the car's systems are drawing down the voltage when you are taking that measurement. That would usually mean you should leave the hood unlatched overnight, and then approach the car in the morning without the key in your pocket and measure the voltage at the front jump points. You should get a reading of 12.5V or better under those conditions. Unless you are getting trouble codes indicating your DC to DC converter is malfunctioning, I would not be concerned about the fact that you are seeing 15.9V while driving, when the converter should only be supplying 14.4V or thereabouts. I would guess the 15.9 is the surface charge, which is not the true voltage of the battery.

 

14.7V? That would be what the charger was putting out, not the voltage of the battery once it was off the charger. It should have been something like 12.7V at 100%, open circuit.

 

Where on the car are you measuring voltage while the car is running? When I connect my leads to the battery clamps, I have never seen a Gen2 that was outside the 14.0-14.4V range while "ready".

If you do measure over 15V at the battery, I would use a scantool to look at data in a few ecu's (hybrid control, ECM) to see if that matches what in in the data. Maybe there is a voltage drop between an ecu and the 12V system- resulting in the target V for the DC-DC converter (falsely) being set too high.

There are a number of posts concerning key off draw. You want to manually close the tailgate latch while the hatch is open so that you can access battery connections while the car is at normal "fully off" conditions (doors also closed and SKS key far away from car). Securely attach an ammeter (usually most digital volt-ohm meters have this function - make sure the internal fuse is good!) between the battery negative clamp and a clean metal (ground) on the body. Remove the battery negative connection bolt at the body (while the ammeter stays connected). About 15 minutes after the car is turned off and the T/G latch is closed, the draw should be below 25mA (0.025 amps).

Don't forget to release the tailgate latch BEFORE trying to close the hatch!

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I grabbed the data for a 24 hour stretch (midnight to midnight) from my 12V. It'll be a little different from a Gen 2 due to being a wet lead acid rather than AGM as well as the small charge it gets when I plug in my Prime to charge the traction battery. But you can still see the spikes when it's being driven as well as the surface charge that bleeds off after driving or charging. This chart shows two wall charges of the traction battery and two drives of roughly 45 minutes each.