Does charge mode also charge the 12-volt battery?

Perhaps just have someone start the car and let it sit in READY Mode. That will charge the 12v battery using the traction battery. Do that once a week for 30 minutes or so. If the traction battery is topped off before you leave town the ICE need never start.

 

Since Charge Mode uses the engine to charge the Traction battery which requires the car be in READY Mode the answer would be yes. The car always charges the 12v in READY Mode .

 

If that's the original battery and no annual battery preventive maintenance had been done - This is a good question to ask.

Your 12v aux. battery is charged while in ready mode or actively charging the traction battery. This battery does not start the ICE, it merely wakes-up the computers and closes the heavy-duty relays to the traction batteries.

You've got three options here:
1. clip an automatic battery maintainer onto your aux battery for the month.
2. get a jump box for the car
3. preemptively replace the battery, with a new one

Many people leave their Primes untouched for a month, without issue - but the older and colder the battery gets, The more likely it will require a jump, when you get back. You can also try to top-off your 12v aux. battery before you leave. Put it on an automatic charger over night, before you leave. Do Not leave the traction battery charger plugged in for a month - you will definitely have a dead battery, when you get back.

Hope this helps

 

For the "Blinka" original poster:
You have a few options:

NOTE: If you get in the car, and just put the car in "Ready", the car will turn off in ~5-10mins.
That's in your owner's manual.
Or, just try it for yourself!

Options:
1)
Have the car/traction-battery charge the 12V battery.
First:
Charge up the Prime until the traction-battery is full.

Then...
To have the car stay on longer than ~5mins (again, in the owner's manual):

A)
- Get in the car.
- Put the car in "Ready mode"
- Put the car in "D".
- Put the car in "P".
- Wait for ~15-30mins.

or B)
Charge up the Prime until the battery is full.
Have someone, once a week:
- Get in the car.
- Put the car in "Ready mode"
- *Lock the Doors from the Driver's Lock Button* ("Camping Mode")
- Wait for ~15-30mins.

or C)
- Get in the car.
- Put the car in "Ready mode"
- Get out of the car.
- *With the Mechanical Key, Lock the driver's side door* ("Keep kids/people/dog/cat cooled/heated/comfy Mode")
- Wait for ~15-30mins.
- Get back in car, turn off car, get out of car, lock the car.


Note: Putting the Prime in "Charge mode", does not mean that the ICE always stays running. Once the traction battery if fully charged, the ICE will turn off. Iirc, the Prius may even automatically switch from "Charge Mode", to "EV Mode".



2)
Get a Good Solar battery charger (~$150 -> $300), and put it on the dash (on top of a blanket, to protect the dash).
**And, add a 12V power connection, to an always-on 12V power circuit.
That will work fine, as long as there no snow blocking the windshield.
Fwiw, the cheapest I saw as ~$210 on Amazon (Battery Tender 35 Watt Solar Battery Charger with Charge Controller).
Imho, I'd suggest to build one from components - a top quality PWM solar charger, a good quality ~35watt to 50watt panel, then the needed wires and a 12V power connection wired to a constant 12V source .
Note, the Prius does not have an easy to connect to constant 12V source. A 12V power-connector, would need to be added to the car's wiring.
Note: A good PWM solar charger will have less standby current draw (<~10ma), than the more efficient MPPT solar chargers (~25ma-100ma).

Code:

Example Suggested parts:
https://www.amazon.com/gp/product/B07NPDWZJ7/
Renogy 10 Amp 12V/24V PWM Negative Ground Solar Charge Controller Compact Design w/LCD Display
$17
Note: Less than 10ma standby power draw.

Renogy Solar Panel 30 Watt 12 Volt Mono Monocrystalline Power Charger for RV Battery Boat Caravan and Other Off-Grid Applications,
$46
Note:  Good size to put on the dash.

https://www.amazon.com/Shirbly-10AWG-Electrical-Tinned-Copper/dp/B09XMD3QXN/
Shirbly 14AWG Electrical Wire 20FT Tinned Copper Wire - 2 Conductor Parallel Wire Insulated Stranded Wire for Solar Panel Car Audio Auto, Low Voltage Electrical Wire for LED Lighting (14AWG 20FT)
$15
Note: Use bigger/thicker wire, for any solar panel over 35watts.

https://www.amazon.com/Cable-Matters-Replacement-Cigarette-Light/dp/B07S9H8HY9/
Cable Matters 2 Pack 12V Replacement Cigarette Light Plug Cord with Leads - 15A Fuse and Green Indicator Light
$9

https://www.amazon.com/Custom-Accessories-18003-Under-Mount/dp/B01ACD3FS8/
Custom Accessories 10240 12V Auxiliary Power Outlet
$7

Saying, "many people leave their Primes untouched for a month", is "correct", but it is also extremely misleading.
Define "many"? Is it 10 people, 1 million? Or, is it "the majority" - (as "many" people infer)?

There are more people in New England, NY, and NJ (~45million), verses all of Calif (~40 million).
Without looking at the populations of every State, it's likely that over 50% of the people get snow in the winter - just like in Northern Calf.

Very few batteries will last ~month when the temps are below ~32F.
That's also true for new batteries.
The Prius has a Very Small 12V battery. So, a Prius battery won't last long, in very cold temperatures.
Many Mercedes vehicles have huge/massive/large-capacity batteries. That's because many will sit for a month+ in a garage.



The statement "12v aux. battery is charged while in ready mode or actively charging the traction battery.", is not fully correct.
For the Prius, the ICE/Engine, will charge the traction battery.
The traction battery connects to a DC-to-DC converter, which then charges the 12V battery.

For Prius, when the car is in "Ready", the traction battery will charge the 12V battery (as long as the traction battery voltage level is above some voltage level).

For a Prime, you could throw the ICE/Engine in the garbage (and the gas tank), and other than many error codes , loss of max power and heating, the car would be okay as a limited mileage EV (~~15-40miles) in milder temperatures (no real front defrost, and limited heat). Btw, the car would also obviously fail a smog check.

 

the car will not charge the 12v any faster in charge mode than any other mode in ready.

 

@ntm01450 I see a couple of alternatives for you to think about. First, you may get lucky and have your 2017 switch on without a hitch after a month of sitting, and that might be telling about how much to worry about your specific cars 12 volt system. 2 You could leave the car sit for a month untouched and when you get back take a reading of your 12 volt battery with a voltmeter both before trying to switch on the car - and if the battery reads anywhere above 12 volts before you try to start the car - take another few readings of the battery after you switch on the car. 3 If you take a reading of the battery and it is a few 1/10s of a volt below 12 volts you can put a smart charger in the battery for a few hours before switching on the car. 4 you can leave a smart charger that has a ring terminal pigtail connected to the battery where the pigtail has a quick connect plug that attaches to the smart charger - hopefully there should be no need to worry about the wife having to disconnect anything directly from the battery. 5 you could have a friend or family start the car once a week , having them turn on the windshield defroster to start the gas engine and leave it running for 5 10 or 15 minutes each week. 6 any variation of the previous 5 suggestions.
Isn't it comforting to know you have a few choices ... and even if the one you choose first doesn't work, there is still a backup plan or two you can follow with,

 

Remember, the owner is leaving and is proposing to have some neighbor/friend keep the 12V battery topped up! What you're proposing is asking an awful lot of this neighbor/friend. Just not practical, IMO. I'd say the OP should just leave the car unattended and take his chances when he gets back. Worst case, a 12V booster pack will bring it back to life when he returns, but most likely, it will be a non-issue and the car will power up just fine.

 

We are still running the original battery in our 2016, and it has gone down three times in the last year requiring a boost to power up the car--I know, I'm living on the edge. I have a NOCO GB40 on standby to get me running in a no time. I also keep a 12V voltmeter in the 12V outlet in the car and check the reading before trying to power up. Currently, the first press of the power button without the brake pedal (ACC mode) will show about 12.12V and the second press (all 12V consumers powered up) will show about 11.48V. The car powers up just fine.

 

The solar panel idea is twice as expensive as a good reliable jump-pack; that will work every-time. That's assuming the OP has the ability to wire-in the unit and hopefully snow doesn't cover the panel or some thief doesn't smash the windows for that panel.

Just my 2 cents.

 

@FuelMiser I was out of town for a weekend Oct 2021 in the mountains and the morning temps were below freezing. Rob43 had just started a thread about the Primes 12 volt system to get a better handle on how the thing actually worked. So I'd been taking readings of the 12 volt battery at different times of day and night.

I had my multimeter with me in the mountains and I took a reading of the battery that cold morning and it was a few 1/10s of a volt below 12 volts and I was concerned. But the Prime started normally. I switched on the defroster to start the engine and let the car warm up, that's it, drove home later that day and almost made it home without having to stop for fuel. I probably could have made it home without stopping and probably would have tried if I were alone. I hope you know where I'm coming from about that.

I know our Prime only pushes 14 volts into the 12 volt battery for a short time and it *the 12 volt battery) rarely get much above 12.6 volts, at least from the readings I've taken.
The Primes 12 volt system is not like the 12 volts system in any other car I've owned.