FFE 12V accessory battery charging

[NightHawk]

I was curious about when the FFE charges the 12V accessory battery from the HV battery.
I measured the voltage at the 12V battery and when its not being charged the voltage is about 12.5V

With the car in PARK and parking brake on in the garage:
With the car in OFF mode and all lights,etc off I get 12.5V on the battery as expected.
With the car in ACC mode I get 13.5V on the battery indicating a modest active charging of the 12V battery in this mode.
With the car in Ready to Drive mode (green indicator on) I get 14.5V on the battery indicating a more significant active charging of the 12V battery in this mode.

I haven't tried to measure the 12V voltage while actively driving the car in motion (via a 12V accessory port) but I suspect it would be the same as in Ready to Drive mode (~14.5V)
When I asked about when the 12V battery was charged before at the dealer they said only when the car is actually moving, but my tests seem to indicate it happens any time the car is not in OFF mode even sitting in PARK.
Thats a good thing since you can ensure your 12V lead-acid battery stays charged even if you aren't driving for a week just by having the car in ACC mode for awhile during that time, with the charging cord plugged in so as to keep the HV battery charged up.

 

[davideos]

It has been awhile since I looked at the 12V battery. My results are posted here http://www.myfocuselectric.com/forum/viewtopic.php?f=8&t=1649&hilit=+12V . One thing I remember is that the voltage level to the 12V battery does and will change when the car is plugged in. I even think it may drop when driving. The voltage at the terminal doesn't really say a lot...other than if it is 13 or 14V, it is actively being charged. If the voltage goes up from one reading to another, more current is being pumped into the battery. The voltage read on your meter is a result of the internal resistance in the battery. Voltage at battery terminal = Current X Resistance + internal voltage of battery. So the voltage you read is dependent on the current the DC to DC converter is providing to charge the lead acid battery.

My guess is that when in ACC mode or when just plugged in, the demand can be met by a lower current...hence, a lower charge current to the 12V battery. In "run" more current may be supplied to anticipate a higher demand...electric seats, brakes, steering, etc.