Trick for starting with a dead 12 volt battery?

I have experienced a dead 12vdc battery three times. Each time was because we had failed to properly shut a door, the car thought the door was still open so the interior lights were left on overnight, resulting in a dead battery.

I have not tried starting the vehicle with the A/C button. I think that is wrong.

From my research the only method for the vehicle to recharge its 12vdc battery is for the ICE to be running.

There is no circuitry existing for the HV drive battery to charge the 12vdc battery [though I wish there were].

 

can you link that reddit?

 

This part is true: there is a remote air conditioning feature (Owner’s Manual (PDF), pp. 531–534), and as described in New Car Features (more info), when it is activated, the system main relays are closed and the DC-DC converter operates, so the auxiliary (12-volt) battery will be charged, or at least maintained.

This part is mostly untrue: if the auxiliary battery is discharged, the remote A/C feature won’t work. Energy from the auxiliary battery (or from the sub DC-DC converter during plug-in charging, or from some other external 12-volt source) is required to operate the hybrid vehicle control ECU and other computers and to close the system main relays, without which the HV battery remains isolated and can’t charge anything or start the engine.

Depending on which other electrical loads happened to be switched on and the actual state of charge of the auxiliary battery, it’s possible that someone may have observed that the remote A/C feature worked when the car wasn’t able to be started in the usual way, but I’d suggest not to rely on this behavior.

On the contrary: there is no alternator, and the DC-DC converter, which receives its power from the HV battery, and the sub DC-DC converter, part of the electric vehicle charger assembly, are the car’s only equipment for charging the auxiliary (12-volt) battery.

 

A good trick to starting with a truly dead battery, is to replace it. If the death was somewhat premature, research what you can do to avoid that next time, maintenance regimens.

 

you need a good 12v for the car to start the charging process. if the 12v won't start the car, neither will it allow charging to start.

 

not working unfortunately

 

If you do add enough charge to the 12 volt battery, then just stepping on the brake and pressing the power button—the usual way of getting into Ready mode—will work just fine, no 'trick' needed.

 

This was discussed back in July Trick for dead 12v battery | PriusChat but it was just discussion. AFAIK, nobody has confirmed or debunked this idea.

 

It should be fairly easy to at least get a baseline of how the system works with a voltmeter on the 12 volt battery. As long as anyone doing the testing understands that one test and one reading doesn't give much info on how the system performs under all the conditions it can get exposed to. An app that gives 12 volt readings should be just as enlightening as long as the app reading and the voltmeter reading are at least close enough.

 

You need an adequate 12v to start charging the hybrid battery and the 12v.
That pretty much debunks the idea

 

adequate is the key. and testing is the only way to find that out, per conditions.

 

That's CDN $'s, likely around $50 USD.

 

vvillovv may have meant testing the 'trick'.

The goal is to get the system main relays to click in and the DC/DC converter to fire up, at which point the 12 volt system is no longer running on the 12 volt battery, and the 12 volt battery is getting a charge.

Pressing the brake pedal and the power button is the usual way to do that. But the remote A/C button also does that.

The claim being proposed is that the remote A/C button may still succeed in circumstances where the usual way won't.

A comparison of the startup current profile drawn from the 12 V battery during the few seconds it takes for the hybrid system to come online (in either method) would be something solid to look at.

We already have such a thing, done for Gen 2 about a dozen years ago (at least, as of this writing, the page it comes from is shown by Page Info to have a last-modified date in January 2010).



Maybe the time is ripe for somebody to make the same pictures for a Prime, and both ways, with the power button and with the remote A/C button.

 

I've got an app that shows some sensor at 14 volts while in ready. I haven't gotten out the voltmeter in a year or two, but last time I checked at the battery terminals I could only see 14 volts right after ready on and only for a few seconds before it dropped back to 12.+ whatever .6 or so iirc.
So I'm guessing there are probably at least a few sensors in the Prime that not to many of us even know about much less how they work within the system.

 

This actually worked for me yesterday in my 2019 Prime. I suspect I had lights on that drained the 12-volt. I was about to give up and take a bus or uber to buy a battery when wife saw this post. AC button, could hear the fans running, let it go about 3 minutes, then I was able to start it.
In my case, the 12-volt is still good. By the time I got home, it was at 78% and I was able to charge it to 100% - maybe an hour. I'll get it tested soon to make sure it is okay.
Regarding all the reasons that maybe this is not supposed to work.. I'd say just try it for yourself if you are in a pinch. Have you also noticed that the plug-in battery can receive some charge while coasting downhill? I've seen it go from 0 to 30% descending a mountain. Didn't think that was supposed to work that way, yet it does.

 

I didn’t even notice the AC remote button until trying this trick the other day. It did work in my case. I heard the fan running, let it go about 3 minutes, then was able to get the normal ready mode and got going. So a field test success. But would be interesting to see actual technical test results as you suggest. My success was with 2019 Prime.