Does charging the drive battery charge the 12 volt battery?

I am puzzled. John Kelley states toward the beginning of his video (emphasis added):

"The DC-to-DC converter that's inside the inverter-with-converter assembly on Toyota Priuses connects to the high-voltage battery in the back of the vehicle when the ready mode is on and uses that high -voltage, steps it down to a lower voltage right around thirteen-and-a-half or fourteen volts, and uses that to charge the 12-volt battery and to supply electric current to the rest of the vehicle's systems while you're driving down the road."​

John Kelley's metering seems to show a current of 0.8A (800 milliamps) flowing into the 12-volt battery and zero current flowing out of the DC-to-DC converter:



So, where is that 0.8A current flowing into the battery coming from? As John Kelley points out, the only other wires connected to the batter via the battery's fusible link block are two wires supplying the distribution fuse block to various vehicle systems and one wire supplying power to the electric power steering.

I would expect there to be current being drawn out of the 12-volt battery to support some minimal electronics such as the remote keyless entry and the Entune telematics when the Prime is turned off. (How else could you use your key fob to unlock the car or your cellphone to turn on climate preconditionng?)

My hunch is that there is a problem with signs -- i.e., negative indicates power flowing out of the battery and positive indicating power flowing in, not the other way around. Does this make sense given what the meters show later in the video when the vehicle in in "ready" mode with the inverter-converter functioning under load?

 

Purely a guess on my part, but I think that whatever charging is going into the 12v while running or plugged in, is minimal. while running, just enough to run active electronics and maybe a tiny amount to maintain the battery. While plugged in, even less to run electronics since car is mostly shut down, and also minimal to maintain battery. Like I said if there were any decent charge going into the battery every night for 5 hours, or every day while driving. They would not be as discharged as they are.They would be pristine!! Moot point for me as I have several maintainers and I can top up battery anytime I feel like it in garage. Voltage usually reads 12.5 for me in acc mode on my meter plugged into cig lighter. After guys got done tinting, has been down to 12.3 for 6 days...plugged in every night to charge , driven an hour every day. Still 12.3 after 6 days?? Sorry, This car isn't recharging the 12v. I put on my maintainer to bring it back up. 2 hours at 4.3 amps and done. You really think 5.5 hours on the evse should have done the same thing right, it's pulling 12 to 13 amps right?? Every night. But battery shows no increase in voltage. They could easily have built a proper charging circuit into the system to maintain the 12v. They didn't. It would cost $$, and mpg, and kWh

 

Also noticed with my cig lighter volt meter, initial voltage starts off around 13.9 once I am in ready mode, but after some time 5 to 10 min, is dropped down to 13.1.system is monitored and voltage reduced to conserve kW for driving?? If they are that stingy with electric while driving, it seems likely they aren't shedding any extra kw for the 12v battery either. I'm sure the cig lighter is indicative of what 12v is getting.

 

That guy makes great videos, but they are too long to listen to for some nuggets of info. Is there a clear answer in there to our question? I can't watch 3 vom for 20 minutes.what part shows the 12v getting the necessary v and amps to actually charge it?? 12.7 @ .8 isn't going to charge a dishcharged battery. I guess I have a long boring video to watch now ; )

 

Hmmm... Apologies in advance if I just read this description too quickly, but does anything in this statement suggest that it can’t also do this when it’s charging?

 

aye, there's the rub

 

When the car is charging, there is a separate 12V source on the charge controller under the right rear seat. That keeps the 12V charged during the plug-in charging cycle for the traction battery.

 

If the 12V battery is fully charged, there is no need to maintain a high system voltage to continue charging the battery. A fully charged 12V lead acid battery is normally about 12.7 volts, so with a bus voltage of 13.1 volts there is a slight float charge on the battery.

https://www.mmbalmainauto.com.au/PDF/State_of_charge_12_volt_batteries.pdf

 

Yes, that seems to be what the Weber U video says. I gather Old Bear interprets the description he quoted to mean that it only activates that circuit in Ready Mode, and not while charging. Based upon the Weber U video, it charges the 12V in both cases.

 

What I got from the videos is that there are two charge sources for the battery - one is the DC-DC converter directly while in READY mode; the other is the 14V converter in the charge controller for the traction battery while charging from the wall. The underseat charge controller only gets power to it when the car is plugged in. Somebody with access to the Toyota online manual should be able to pull up the wiring diagram.

 

has anyone put a meter on the 12v while the hybrid battery is being wall charged?

 

Ask and ye shall receive. Went to the garage, car is sitting there with charge cord plugged in waiting for next charge schedule tomorrow morning.

Static battery voltage - 12.50.

Open up the driver door, put car in ready mode - voltage 12.01 volts. Note that headlights come on in this mode, along with the interior lights and the puddle light and whatever computers boot up.

Start charging with charge now - voltage rises to 12.48 volts. This is with the driver door open and the headlights on for the 30 second timeout.

Headlights time out, voltage on battery briefly rises to 13.05 volts, then drops back to 12.63 and then continued to drop slowly. I assume after the battery gets the power that was used back in it, then the voltage will drop back down to about 12.47 volts.

 

Yes. I don't remember the precise value, but it was lower than the PiP, but clearly charging the 12V. Maybe something like 13.1 or so, iirc.

 

You might be right. I checked immediately when it started charging the HV battery and it was dropping slowly. I didn't watch very long.

 

That would make sense. It only needs to put a bit of power back in the 12V battery. If I get highly motivated, I'll make a video of the voltmeter and talk through the steps.

The brief experiment shows that the car will charge the 12V battery during traction battery charge from the wall.

 

Many thanks... that explains how there is a small charging current (0.8A) flowing into the 12-volt battery when there is zero flowing out of the converter/inverter.

So, in addition to accepting 120vac or 240vac from the EVSE and turning it into DC to charge the traction battery, the Prime's charging circuitry also includes a "float charger" which maintains the 12-volt battery. Makes sense and is consistent with Toyota's well-conceived engineering.

 

Here is a pretty dull video. I had intended to edit it, but my editing software is not playing well today, so you get it unedited. It appears the hood on the Prime is aluminum, since the GoPro magnetic mount had no stick. Converted the video to very low resolution to not kill your bandwidth.

Dropbox - Prius Prime 12V charging test from wall power.mp4 - Simplify your life

 

i wonder why they lowered the charging rate, pip never had 12v battery trouble

 

What was the charging rate on the PIP?

 

it's been awhile since i looked, i'll check again. not sure if @jerrymildred remembers.

i wonder if toyota thought the larger battery wouldn't need as much charge?