how much juice can I safely pull for extended time off 12V battery on a Prime?

The DC-DC converter, which provides 12-volt power and recharges the auxiliary (12-volt) battery, operates only when the car is in READY, not in the “Ignition ON” state, which is equivalent to key-on, engine-off on other cars. (There is also a sub DC-DC converter to provide 12-volt power during plug-in charging.)

Toyota hasn’t published an electrical load analysis, so it’s hard to say for sure. New Car Features (more info) says the maximum output current of the DC-DC converter is 100 A, but some of that goes to essential loads such as the power steering, brake booster, water pump, and cooling fans. See the Electrical Wiring Diagram for more.

For general precautions related to add-on wiring, I’d suggest looking at the Electrical Best Practices section of the GM Upfitter Integration Best Practices Guideline Manual. Some of the information is GM-specific, but there are many hard-won lessons from GM’s engineers that apply to all vehicles. Toyota has an equivalent Body Builder’s Guide series for their trucks, but it’s not nearly as comprehensive.

All of this information is available using a Toyota Techstream diagnostic system; perhaps others more familiar with the phone apps will comment on their capabilities. Of the parameters you mentioned, only engine coolant temperature is part of the OBD II standards; the others can be monitored through the same physical connection to the DLC3 port, but the protocols are proprietary to Toyota and may or may not be supported by third-party diagnostic tools.

 

The auxiliary battery is only charged when the car is in ready mode.
I had my Corolla hybrid in ignition on mode and it depleted the 12V in about half an hour to the point that it wouldn't go to ready mode anymore. My laptop was charging from the 12V socket as well at the time.
After unplugging everything and waiting for half an hour it recovered enough to get the car in ready mode.
I thought that I put the car in ready mode but I just pushed the button without pressing the brake pedal enough.

 

Ready Mode or Get It Towed!

 

that particular OBD2 caused codes and traction control issues on our car. after I removed it the car fixing normally again

 

The 12v battery question has been answered by others. For using the Hybrid Assistant app with a quality adaptor, you will get the speed, engine RPM, HV battery SOC, temperature (HV battery, engine, electric motor, inverter, external, internal, phone), braking applied (regenerative vs friction), fuel consumption (instant and historical), EV traction, and kW produced by the engine, and those flowing in and out of the battery (note I haven't confirmed this but I think this is only kWh from the engine, not from the regenerative braking), total trip time and EV ratio, HV Battery temperature subpanel for a temp of air flowing into battery, the temp of the battery sensor (highest sensor), and speed of the cooling fan. There are some other subpanels that display HV summary, engine summary, fuel summary, and temperature panel. I don't think real-time monitoring of actual power usage in EV mode is displayed.

You can find out which icon represents what data by consulting this page. Hybrid Assistant: Dashboard

 

Or have a jump pack close by

 

Points for technical accuracy, but maybe work on the rhyme?

 

Ready mode, or get it towed,
Or have a jump pack close by;
Watch the amps that you pull,
and keep the tank full;
in Ready, your car won't die.

 

The max you can draw from the DC-DC converter is 100A. Maybe I should tell you there is a 100 Amp fuse that limits the draw and you DO NOT want to replace that fuse. It's rather expensive because it's located in the DC-DC Converter. There are lot of threads to look through: wattage of DC-DC converter ? | PriusChat.
Prius 12 volt charging current | PriusChat

I believe the consensus in all of these threads is to limit the draw to 80 amps max because other systems in the Prius use some as well.

 

I have used my Prius Prime to back up my house when the power fails by using a 3KW 12V to 120V inverter. It draws power for the 12 volt Prime battery (use large cables - can draw several hundred amps). Turn on the prius as in normal running mode. The traction battery will keep the 12V battery charged as long as there is charge in the traction battery. when that is exhausted, the gasoline engine will turn on as needed to charge the 12 V battery. I have done this, and can run my house several days like this. Although I would not want to run 3 KW continuously this way, the statistics work out because my fridge and freezer only run intermittently. My water heater and furnace are gas, and lights are not much of a draw. Need to disconnect from grid of course. I also have a hose to run the exhaust from the tailpipe out of the garage.

 

Do you have link to recommended inverters. What gauge extention cord do you use to run from Prius Prime to lights, frig? Thx.

 

I have used a 3KW, 12v to 120v power inverter to provide backup power to my home during power outages using my Prius Prime. As long as the car is in READY mode, the 12volt battery will power my inverter, and the traction battery will keep the 12v battery charged (at 100 Amps max). When the traction battery is exhausted, the engine will automatically start to keep the 12v battery charged. So yes, you could use this method to run power equipment from your Prime. But, use appropriate jumper cables with a bolt-on connection to the external inverter because they may want to draw several hundred amps depending on what you are powering (table saw, circular saw, etc.)

 

I can confirm I too used a 2000w rv inverter from harbor freight to power my house during outages. It was on a 2012 Prius plug in that sadly was wrecked last month. I used it for several years and it was very convenient and barely sipped any gas or made much noise compared to neighbors using traditional generators.
i am looking to reinstall my same setup in a 2017 Prime. I’m very weary of installing the inverter under the hood bc of fears about moisture, pests, dirt, high temps all down there under the hood. Anyone have suggestions as to where I can snake the two super thick gauge cables from the battery to the inverter?

 

You already covered this, I just want to emphasize it further for future readers who might have skimmed over that part:

There is a 100 amp limit to the car's ability to charge the 12 volt battery, period. It's 100 amps max when it's coming from the traction battery, and 100 amps max when the engine is running to charge. (The engine charges the high-voltage system, and the 12 volt battery still has to get its charge via the 100 amp max DC/DC converter from there.)

And you get less than 100 amps, because the car uses some of that. The car's probably using around 30 just to stay READY, with no power-hungry accessories in use. Because Toyota doesn't really oversize the DC/DC converter beyond what the supplied accessories in the car might use, there can be very little left over for you at times when you have several of those (headlights, defrosters, seat heaters, etc.,) in use.

When you fit an oversized, 2kW or 3kW inverter, the advantage can be that it has enough temporary grunt to start up a circular saw or similar large motor where the power draw is much higher to start than to run. But aside from those temporary peaks, you still have to budget for your overall, ongoing power demand to be within what the car is able to supply. If you are steadily drawing more than 1kW or so, you will be steadily draining the 12 volt battery faster than the car can keep it charged.