Any time the traction battery is actively charging, the 12 volt system is also being maintained by using some of the traction battery charge to feed the DC-DC converter. Once active charging has stopped (i.e. the traction battery is full), the 12 volt maintenance also stops.
Yes, but the voltage sent to the battery is the lowest of all possibilities at 12.8V. Another time the 12V battery is being charged is when the battery heater is on and the car is plugged or preconditioning is happening. During that time, the 12V battery is being charged at 13.5V. The last remaining time the 12V battery is being charged is when the car is in Ready mode. During that time, the 12V battery is being charged using the highest voltage at 14.5V. However, this might fluctuate to a lower voltage when the 12V battery is already charged. This is what I've noticed using my 12V battery monitor.
I really wouldn't call it a battery charge @ 12.8 VDC. The DC-DC converter is turned on by the ECU, mainly to ensure power to the traction battery cooling fans. Ditto with the battery heater; the only reason your getting a higher voltage there is because those heaters draw a lot of power. The only time the aux. battery is really getting a charge is in ready mode, so a bunch of short hop commuting will eventually kill your aux. battery due to not enough recharge time to overcome the initial power-up drain or keeping the car in accessory mode. My battery hovers just over 12 VDC. Whenever I find it below 11.8, I put it on a charger for 1/2 hour - that'll usually take it back up to >12.75 VDC. It's always started for me, but we have much milder weather down here. I do check and refill electrolyte levels annually (fall, after the long hot summers).
Look at this graph. Right before the heater turned on, the 12V battery voltage was 12.51V. After that heater element voltage upper to 13.5V, it was 12.67V. The battery WAS charged. After the two drives, you can see the voltage being around 12.8V which is higher than the voltage after the charge. Anything above the current voltage level IS a charge.
I just changed our 12 V aux battery last week. So that I wouldn't lose all the personal media favorites, clock, seat & mirror preferences etc - I hooked up a 12 volt source to the little posts many cars now use for jumping purposes. The battery meter quickly went down to nearly zero amps, but by simply opening a door - the meter immediately jumped to 10 amps. It illustrates how much draw a car can have by what appears to only be dash lights, trunk & cabin LED lamps. Obviously there's more going on to necessitate that much draw. On a big battery 10 amps is no big deal. However the little 12v boot up batteries on many hybrids? That might become more significant. .
