Does charging the drive battery charge the 12 volt battery?

From what I've gleaned from this board the answer is "somewhat".
When the traction batter is being charged the 12 volt battery is being charged just enough
to offset any drain due to running the fans etc. but not much more. When the traction
battery charging stops so does the 12 volt charging (if you want to call it that).
So after charging the 12 volt battery will be at the same or slightly higher state of charge
than it was when you started.
If your 12 volt battery is low best to put a low amp charger on it or drive the car a decent amount.

 

If you're storing the car, the worst way to charge the 12V is by charging the traction battery. The manual clearly says to not leave the traction battery fully charged for long periods. It's best to use up all the EV range and store it that way. If you think the 12V needs help while the car is waiting for your return, then use a battery maintainer on the 12V or disconnect the negative cable.

But the dealer is right that the 12V is probably fine if you turn off the SKS. Worst case is that it would need a jump, preferably from a booster pack that monitors polarity so you can't connect it backwards.

 

My off-the-cuff recollection of this video says “yes“:

 

I've measured it and it does charge the 12V while charging the traction, but not as strongly as the PiP does.

 

so the real answer is basically "NO" it really doesnt do any meaningful charging of the 12v battery.
my assumption is that thru use and abuse(leaving a light on, listening to radio in acc mode), etc)
the 12 v slowly becomes less and less charged, and never really gets topped up by the traction battery, or driving.
BUT
since the 12 v does almost nothing, except wake up the computers enough to turn on the traction battery, a low 12 v can last for years and years, until its so low that it wont even wake up the computers. this is similar to how the 12 v lived, and died, in all previous prius generations.
to avoid this, periodically recharge the 12v with a good battery maintainer, or , at least if you know you drained it badly, give it a recharge after the event.
if i had to bet money, all the 12v complaints with our new cars stem from some unknown abuse...the new owner ran it dead detailing the interior, or the dealer ran it dead demonstrating the car to a potential buyer, or somebody left a light on by accident one night. Once its drawn down to 11 amps, its never going to magically revive itself. it will just , sooner than later, die completely. it might wake up your car for another 2 months until it finally stops working, making it hard to know what event caused the discharge...Even worse,by then its probably too late, and the life, even if recharged, will be severely shortened, as it was probably discharged for so many weeks / months, that permanent damage was done.
this is the same for all 12 v batteries in all cars, one deep discharge if not recharged quickly, will shorten the life of the battery.
our battery is smaller so more prone to dischariging, and our recharging is weaker, as the engine hardly runs, and they dont want to use up traction battery to revitalize the 12v. even in a regular ICE vehicle, you might need to drive for 4 hours to recharge a battery, and thats if its designed to recharge, many new cars dont recharge the batteries either. my 535i, killed batteries and was designed to never fully top off the battery, instead reducing load on the alternator at 85% charge to improve mileage, by .05%. result? battery would no longer start the car after 1.5 years, no matter how much you drove it. had to be recharged monthly to stop the battery needing to be replaced constantly.
moral of the story, get a cheap voltmeter, and or a cigarette lighter style volt meter, check the battery voltage regularly. recharge the 12 v when you see a decrease from normal that might be once in 6 months, or longer. or you can ignore all of this and just use it until it is dead. that could be 4 to 5 years anyway.

 

it's too bad, pip does a fabulous job of 12v management

 

Hmmm... It’s been a while since I watched that video, but I seem to recall that they showed a cable specifically going from the charger to the 12 V battery.

 

I don't believe this is the case. Whether or not the engine is running, the charging of the 12V battery is the same - power comes from the traction battery via the DC-DC converter to the 12V battery. The 12V battery is tiny in comparison to the traction battery and fully charging it will deplete the traction battery very little. This video shows the 12V charging test and the rate of charge of the 12V battery.

 

I believe that the "inverter/converter," which takes the DC voltage from the traction battery and converts it to 3-phase AC for the motors which propel the vehicle, also provides 12-volt DC to charge the battery and to power the vehicle's small motors (windows, seat adjuster, fans, etc.), lights and electronics when the Prime is turned on. It also provides power to the heat-pump for climate control.

When the Prime is turned off, so is the "inverter/converter" -- which is no different than the alternator on a conventional ICE stopping when the car is turned off. The major difference is that the 12-volt battery on a conventional ICE must provide a big surge of power to the starter motor and ignition system when the ICE is started. The 12-volt battery in the Prime only powers the Prime's electronics so that things like the electric door locks work (so you can get back into the car) and any volatile computer memory is maintained.

I am not certain, but I believe the 12-volt battery does not receive any kind of trickle charging while the Prime's traction battery is being charged. That is because the "inverter/converter" only operates when the Prime is in "ready" mode.

 

That’s true of the DC-DC converter (with 100-ampere output) in the inverter with converter assembly. Toyota’s manuals (more info), however, describe a sub DC-DC converter, mentioned in my previous posting, that does operate when the car is charging.

 

The traction battery charger also charges the 12V battery while the traction battery is being charged. See the following video at time 20:00.

 

Ah, yes, I pointed to *the second* of the two Prius-Prime videos earlier, and that circuit is described in *the first* of the two!

 

with all this "charging" one would think the 12V would be in tip top shape all the time, and have awesome longevity. they would have no doubt been using the trac battery to upkeep the 12 V for many years on many previous generations. but history shows that no prius generation took good care of the 12V. they were left out of the loop somehow. probably to maximize mileage. the 12V is sacrificial. its not integral to the hybrid system. if all this charging occurs when the EV or hybrid mode are running, and when we are plugged in for 5 .5 hours a night, why do we get low charged batteries, failing prematurely? that huge trac battery should be able to keep the 12V at a solid 12.7 all the time, and even if discharged push 30-50 amps at it until its recharged and then taper down to maintenance charging...that doesnt happen since dead battery , once dead, stays dead. 1 yr old battery with single inadvertent discharge should be fine if recharged in a reasonable time frame, like a day or two. but without a little battery maintainer being plugged in to charge at 2 /4/8 amps max... the battery never gets recharged?? when there's 240 amps available from the Trac ??? if it really charged the 12V, we could use a battery 1/4 the size, as it would be easier to maintain, cheaper to replace etc. a little laptop battery would be good enough to wake up the electronics. let the Trac do the rest. i dont know why they dont just do that...
they do it this way to isolate the drive system from the electronic system to maximize mileage... dead 12 v is at your expense and they get to continue to claim 133 mpge !!

 

It does, as the video posted by @jb in NE shows and I have measured it myself.

You can. People have done that.

 

This is likely due to cheap batteries or degradation in transit from Japan and while sitting on the dealer's lot.

The Weber videos show this is not the case.

 

however, that has nothing to do with charging the 12v when charging the hybrid battery from the wall

 

Lead-acid batteries are pretty cheap and can be purchased “anywhere.” I at least am glad they used a “normal” battery in the P.Prime, rather than the semi-proprietary thingie they used in our Gen-2 Prius.