Start with low 12v battery

Pull out a couple of relays that supply power to the ABS, or fuses. Errors will appear in the brake systems, but the energy will only go to connecting and checking the high-voltage battery.

Do not start driving until the 12V battery is recharged. Turn off the car and put back the parts that were removed. Turn on the car and reset the errors.

 

Disconnecting the brake pump is sort of my next-level cheat if I need to jump start with minimal power available. There's a little work involved, and it's likely to set brake trouble codes, of course.

My first go-to cheat is just to unhook the car's own battery. You can do that in a 2010 simply by unplugging the fat white wire you see in the underhood fuse box. Then when you connect your jump pack to the jump terminal in the fuse box, the jump pack only has to power the car going to READY, and does not have all of its charge sucked out by the hungry drained battery in back.

As soon as the car is READY, I plug the fat white wire back in, and the car itself gets to charge its own battery.

That's usually been enough in my experience. I think there was one time with my gen 1 where I did have to disconnect the brake pump too.

 

Will a healthy battery discharge in a week?

 

As I confessed earlier, I've yet to have any occasion to try jumping a gen 3 without the brake pump. I believe gen 3 buried the pump relays inside the ECU/actuator assembly where you can't get at them.

There are ABS MTR 1 and ABS MTR 2 fuses, but they are part of a threefer. The third one is the P/I 2 fuse, which powers the long white "integration relay", which powers a number of things. So pulling the threefer out would mean losing those things too.

Skimming through the diagram, it looks like what's downstream of the integration relay are things like the back-up lights, low-beam headlights, horn, rear window and mirror defoggers, headlight levelers and headlight washers (if you have that trim), and the transmission control ECU (which is all about getting in and out of Park).

So it might turn out that pulling that threefer doesn't really lose you anything needed to get the car READY (and might even be an easy way of disconnecting several things you'd rather not have drawing power while you're jumping). So maybe it's worth trying. I suppose you would end up with trouble codes from (at least) the brake ECU and the transmission control ECU. And might not be able to go anywhere until the transmission control ECU is happy again.

 

On our second GEN 3 Prius now and first RAV4 HV GEN 1...and I had to jump all 3 "once each" successfully...followed by replacing said battery. First time was with cables and another vehicle...and the other two times with a jump pack. If you are having to disconnect fuses and systems to get your vehicle into Ready mode to jump start it...you need a new battery and/or have other issues.

GEN 3 Prius uses a Group 24 and I went aftermarket with them.

FTR...RAV4 HV GEN 1 has a Group 23 battery and no one but Panasonic makes one. Group 24 or 68 could/would work, but I wanted factory fit.

 

Yep, just this past week for someone with a Gen2 connected a fully charged 12v to the existing dead battery and it sucked the life out of it before the car could fully get into ready mode. Lesson learned! Fortunately, a neighbor had a really powerful jump pack and that was enough to get the car back into her driveway so I could first try to repair/recondition 12v and then installed one of these: 12 Volt (12V) 55 Amp (55Ah) VRLA AGM Sealed Lead Acid Battery 55 AMPS HIGH CAP | eBay because voltage dropped down to 12.5 right away after overnight repair charging attempt.

 

A Gen 3 Prius uses an AGM group size S46B24R battery, NOT a group 24 battery.

 

Lol... Brian is still a bit of a rookie when it comes to PriusChat.

If you want to truly understand your 12volt battery needs based on Prius design so you pay less than 1/2 the price of what the gullible think is the correct battery for Prius then study our ongoing dozen-year debate that the mobility battery advocates continue to win here: Fitting mobility 12volt AGM battery. | PriusChat

 

It is really difficult to extract the composite fuses.
Perhaps in this case it is better to disconnect the connector from the brake pump motor. (A17 A18)
Troubleshooting Guide DTC:C1252/52, C1253/53

Brake Pump by MAX2 posted Jan 27, 2025 at 11:53 AM

 

I keep a cheap plastic fuse puller in the glove box for those. Got it at a neighborhood auto parts store.

Toyota provides a smaller fuse puller stowed right in the fuse box for the smaller low-profile ATM fuses, but it doesn't fit the wide ones.

A simple puller that fits makes them easy(ish) to pull.

 

Gen 3s ARE Group 24 "size" batteries. R, F, H, T or nothing refers to terminal orientation and capacity...as it applies.

Factory's are AGM...but you can use a flooded aftermarket...if you want.

Moving on.

 

Maybe I'm missing something here, but...

Doesn't the pump start pumping after opening the drivers door?
Is it starting after opening >>any<< door?

If you want to get under the hood, you could try to enter the car through one of the passenger doors.
Perhaps the pump wouldn't start then?

Mat

PS. I like the idea with pulling the white cable!

 

It doesn't.

 

You don't need much of a booster pack for the Prius. I used a cheap one that was about the size of an iPhone.
All it has to do is turn on the computer system. It doesn't have to crank any starter.

 

The pump buzzes when you hit the ready button even if you get in dukes of hazard style.

 

If you are not sure, if the battery is completely down, it could be enough to save you from disconnecting the battery to jump start the car or do something even more time-consuming and cumbersome.

Get into the car through a passenger door without starting the pump, get the hood opened, connect your additional small battery, get quickly into the car through the driver's door and try to start the car - maybe it is enough to succeed

Mat