Would this procedure be safe for a Prime to jumpstart another car?

That is close to a procedure that I used when jumping other cars from my 2001, with this slight difference though:

I started my 2001 first, then disconnected the 12 volt battery. Old-timers, thinking of old cars, will warn against doing that, because old-car alternators needed a battery connected to present a stabilizing load. The Prius DC/DC converter doesn't, though, and there is no ill effect from disconnecting the 12 V battery while READY. (Just don't forget it's disconnected and push the off button.)

The benefit, of course, is you avoid the worry that jumping the other car depletes your battery so far that you then can't get your own car started. It's already running, and you just reconnect the battery when you're done, and let it charge.

However, that was a super easy procedure in a Gen 1 or Gen 2 Prius because you could disconnect the 12 V battery just by squeezing and pulling a plug. Starting in Gen 3 you would have to loosen and remove a battery post clamp instead. I'm guessing that's also true of the Prime?

It was never an issue when I did jumps my way, since 12 V power was never actually lost. If it is lost, various things can need to be relearned/reset, like power window travel limits and such; you should be able to find a section in the repair manual (more info) with a title like "initialization needed after 12 volt power restored" or something (I don't remember it exactly). Searching the manual for "initialization" will probably find it as one of the hits. It will list all of the things that could need attention in one place. Usually radio presets and clock, of course.

Big batt is always isolated for safety when the car is off. It is only connected to the DC/DC converter to provide 12 volts to charge little batt when the car is in READY. (There can be some edge-case features, like the remote air conditioning activated from the key fob available in some Gen 3s, that are exceptions.)

The nice thing about a Prime is that if big batt itself ever gets too drained to crank the engine, you can plug it in. When the same thing happens in a non-PHV Prius, it's a headache.

If it's similar to the measurements made by hobbit on a Gen 2 some years ago, it could require a peak of around 30 amps for a brief moment before the DC/DC converter comes on line. If the brake pump needs to run, that could be a similar draw but for several seconds. But you should also expect that if your 12v is so drained that it can't supply that, then the donor car battery is going to be dumping a substantial current directly into your battery at first, with the current needed to make the car start coming on top of that.

I don't think most people work that hard at it. Honestly, if your benchmark is the combined load of all of those loads on high at the same time, that's probably not far off what would flow from your 12 V system directly into a drained 12 V battery if you put cables between them and didn't do anything special.

If you wanted more control, you might cobble something up with a buck/boost converter from eBay, or just string together an off-the-shelf inverter from your Prime's system to 120 VAC and plug an off-the-shelf 12 volt battery charger into it.

Some RC hobbyist chargers have settings for charging a 12 V lead-acid battery and can also be powered from a nominal 12 V supply.

 

In the last two paragraphs I gave you three basic approaches there. If you're an electronics tinkerer, you could select a buck/boost converter from eBay or the like, do the surrounding design yourself, and build what you're looking for. If you want the least fuss, an off-the-shelf inverter to an off-the-shelf charger will fit that bill, or you can look for one of the hobby chargers that accept a 12 VDC input and have a 12 V lead-acid charge setting.

When you compare those three approaches, thinking about the conversion losses, don't forget that all of those options work very similarly under the hood. Every one will invert the 12 volts coming in to make some intermediate, non-DC, voltage and frequency, and then rectify and smooth that to the desired charge voltage. Your 12v to 18v laptop converter also is doing that inside.

The off-the-shelf inverter to off-the-shelf charger approach may be slightly less efficient because the intermediate voltage has to match a specific standard (120 VAC, 60 Hz), whereas the other alternatives got to pick whatever intermediate voltage/frequency would be convenient for their circuit, and maybe eke out some extra % efficiency that way. But it's not as if any of those approaches is fundamentally different from the others.

Meanwhile, one benefit of the off-the-shelf inverter and charger is that you know the intermediate voltage is standard 120 VAC 60 Hz, which means you have the option of using it for other stuff if you ever have a need come up.

 

Of course it is OK to use the Prius battery BY ITSELF and disconnected from the car.
Unless maybe you are trying to start a diesel truck in the cold.
12 Volts is 12 volts.

 

I truly don't understand the question. It would be far safer just to pick up one of the mophie batteries that are able to jump start cars and carry it around in your prius. The Prius 12v battery is more expensive than that...

 

Sounds like you did understand the question, just wanted to answer a different one.

I do carry one of the Li-ion jump packs now, and it is more convenient.

But the answer to the OP's question is, yes, that method works too, and I have used it successfully. The more-convenient balance was different back when I had my Gen 1, because the plug-in battery connection made it convenient, and the Li-ion jump packs weren't everywhere yet.

 

Chapman already said "safety". To really assure first responders that they could safely slice and dice a mangled car however necessary to extricate crash victims, without risk of electrocuting themselves, it was necessary to disconnect the big battery inside its own case whenever the car was shut down. From existing experience in traditional cars, responders already knew how to make sure ordinary 12V is shut down.

The headlights have auto-off, so should not drain the 12V battery. Though there are ways to defeat this battery-saving protection, intentionally or otherwise.

 

Does the US light switch only have auto-headlight as a means to turn headlights on? In Canada there’s the traditional “on” setting; they’ll stay on ‘til the cows come home.

At least on 3rd gen; maybe I’m living in the past.

 

I would not want to chance damaging the inverter engineered for just the oem battery. The battery ECU might complain too.

A jump pack is much cheaper and less risky.

 

The converter supplied in the car isn't engineered just to charge the OEM battery ... it's engineered to charge the OEM battery, run radiator and HVAC fans and brake pumps and steering gears and windows and heated seats and window defoggers and and and ....

The experiences of lots of members on PriusChat who have added 1 kW or larger power inverters sourced from the 12 volt system can speak to the same concerns. Generally there's no issue. People might do well to think before drawing the highest possible extra load while also powering all the car's fans and heaters/defoggers and so on simultaneously. But then, it's rare to want all that stuff simultaneously anyway.

The car's converter can give an 'IDH' signal if it is feeling overloaded, and the HVAC automatically drops the supplemental electric heat, if it is in use, on that signal. Under a further overload, Bob Wilson's experience was that the converter output starts to droop a bit. If it overheats, it probably shuts down. There have been some concerns about converters being killed by reverse-polarity jumps (though more recent posts suggest the most common outcome is blown fuses but no converter damage), but I don't know that I've ever seen a report of a converter being damaged simply by excessive electrical load (within the limits it's fused for).

 

The 2020 Prime owner's manual shows two switch versions for the U.S., one with auto-on, the other without. But either way, it has the 30-second delay to auto-off after ignition shutdown and opening the driver door. And if that 30-second delay is overridden, then there is a backup 20 minute battery saver that shuts them off.

But even our Gen3s have that 30 second (customizable 0-90 seconds) auto-off feature, so the lights won't stay on to deplete the battery. On my 2010, I just left it permanently turned on, set-and-forget. But since my 2012 has DRLs, I often now turn it down to the DRL position. It can go one notch farther to DRL-off, but I never use that, only the shop mechanics set it down that far.