12V battery is dead- can not jumpstart

You don't need to rev the other car at all. All you need is a small amount of current to boot the computers. The car uses the HV battery to spin the motor not the 12 volt battery.

I had just bought a new Yellow-top battery when the wife's car quit. I was able to jump the car just using the new battery. But, it took a few tries pushing the power button before I could see the ready light. Once you get the car in the "ready" state then it will use the HV battery to recharge the 12 volt. It will turn on the gas motor when the HV battery gets low.

 

It's possible that the battery has suffered an internal short that's preventing you from pulling it up to a useable voltage or that one of the cable connections at the battery posts has become bad.
Your best bet is to climb back into the vehicle hatch area and manually unlatch the hatch (I believe you need to open a cover to get to the mechanism).
Once you have the hatch open you can remove things to get to the battery itself. Check to see if the connections to the posts are good.
Be very careful (of course) about the polarity and try doing a conventional battery to battery jump. If that doesn't work the safest path is to connect a known good battery in its place. If you had to, you could try this with jumper cables (with the old battery out of the circuit) but there's a lot of risk in attempting to drive around with the unprotected cable ends in use.

 

Yeah, it should take very little extra power to boot-up the computers assuming all else is normal. In fact one member awhile back used 6 D size batteries to do it. Something seem amiss.

 

could it be a blown fuse? maybe try jumping the battery directly. you have to crawl into the back to unlock the hatch from inside.

 

Another approach:
Try the jumper arrangement you tried before (feel free to try a smaller set of cables for better clip contact) and observe the dome light, try the headlights, etc. If they aren't seeing the voltage there's no sense in even trying the power button. If you can get the headlights to respond, turn them off, open the rear hatch (while it's powered) then try pushing the power button a few times with the brake depressed to get to Ready mode.
It's possible that the car's ECU lapsed from the low voltage of the bad battery. As described before, crawl into the back to unlatch the hatch, uncover the battery then use a wrench to disconnect then resecure one of the battery cables (attempt at a reboot). Then try a conventional jumper start (but don't bother with revving or even starting the boost vehicle).
Good luck!

 

yup, either recharge the battery really well or replace it.

 

It needs to be in ready mode for a few hours to be extra safe.

 

...or get one of these: Battery Tender 021-0123 Battery Tender Junior 12V Battery Charger : Amazon.com : Automotive

 

What happened with the original jumpers was if you look closely at the clamp you will see how the cable itself is only attached to one side of the clamp. It relies on both jaws to be securely attached to the point. If not and only one jaw is grabbing the point and its not the side of the of clamp that has the cable attached then the only current that will flow to the point will be forced to flow through the clamp rivet connection. The connection that holds both sides of the clamp together. Sometimes no current will flow at all through that tiny connection if new and painted.
Or maybe not enough to charge the battery.

Btw, that little battery begins sulphating really fast when allowed to go stone dead. So its usable life was severely degraded. Learn how to do the mfd battery test and keep a close eye on it.

You do not want to ever have to jump start a Prius. Its the car's Achilles heel and if not done properly (racing the donor car ) can cause thousands of $$$ in damage.

 

I was just about to get a set of jumper cables to carry in the car, 200A 10 gauge 12'. Most reviews said they were OK, one review said the cables heated up pretty hot. Was hoping to get a set that's not so heavy to carry in the car.

 

Not to argue with CSS don't get the tender he suggested, get one of these if you are going with battery tender brand:
Battery Tender 021-0128 Battery Tender Plus 12V Battery Charger : Amazon.com : Automotive
That one is a charger, but still a pretty low rate of amps compared to many chargers. The one CSS listed is a tender/maintaner it will not charge your battery up.
I know from experience this week, the wife killed her Venza battery stone cold dead, the only thing I had at home was my battery tender junior that I use on my motorcyle over the winter.
I plug it in and it charged for 9 hours while I was at work, not even enought to turn on the dome light.
Borrowed a friends battery charger and charged it from 3% which is what it was at when I hooked it up after all day on the tender to 100% in 9 hours.
I used a Schumacher SC-1200A SpeedCharge 12/8/2 Amp Charger/Maintainer/Starter/Tester : Amazon.com : Automotive which is actually $15 cheaper than the battery tender one, charges from 6v to 12v batteries, deep charge, and from 2-12 amps rather than .75 of the junior.

 

You should be able to use very lightweight jumper cables for the Prius because they don't need to carry heavy current. The reviewers were probably jump starting cars with 12V starters. The Prius doesn't start the engine using the 12V battery. The heaviest draw is what it takes to pressurize the brake booster (that noise you hear when you open the driver's door) and that only runs for a few seconds.

 

This one I didn't know. So, can somebody explain to me why we have a 12V battery?

Mike

 

Its to keep alive all the ECU's and upon power up & proper certification of the FOB by the Body ECU powers the relays to connect the Hybrid battery to the system. That in turn spins a motor/generator (MG) that spins up the engine. The engine may or may not start depending on the cat temperature or hybrid battery charge level but when the relays close the word READY appears on the dash and you can drive away either in electric or ICE.

 

This happened to me. It was early in the AM in my garage, and dimly lit. Once I figured out what was going on, it was no problem - but it too a bit more time than I was happy about...

 

12V is also a convenient voltage for powering all those other things like lights and stereo and minor motors like window regulators and such.

 

Came across these Harbor Freight 8 gauge jumper cables with inline battery tester. I wonder how these would work in the Prius and the battery tester part. Looking for a set to keep in the car.
12 Ft. 8 Gauge Jumper Cables with Inline Battery Tester

 

Those should work just fine.