Why can't they jump a Prius battery?

If motor/generator 1 is allowed to freewheel, the engine won’t move the car. It’s like jacking one drive wheel off the ground and allowing it to free spin, resulting in the differential not delivering torque to the other wheel.

The Prius transmission is essentially a differential except the torque split is not 50/50. The engine drives or is driven by the planet gear carrier (spider gears in a differential), MG1 drives or is driven by the sun gear, MG2 drives or is driven by the ring gear which is also the output to the drive wheels.
If MG1 is locked up, the transmission is in an overdrive with a ratio of 1/(1+(sun gear teeth)/(ring gear teeth)) to one.

If I knew what the tooth count of the sun and ring gears was, I could give you the actual overdrive ratio but I don’t so I give you the formula for calculating planetary gear reduction ratios.

Here’s a pretty cool explanation and animation:

http://eahart.com/prius/psd/

OK, I have seen the animation. That is a brilliant demonstration. So, I have to admit that there is no starter as such. The planetary system does the job and the 12 volt battery is used to supply the initial push to turn the motor over at startup. Right?

I think the 12v powers the computer, etc, but all power for starting come from the high voltage pack.

You are right. I mentally think of it as a “starter battery” because the 12 V is needed to get the car going when the “Start” button is pressed. If that 12V is dead, the car won’t start no matter how much juice is in the hybrid battery.

The tricky thing about jump-starting a Prius is that the battery is way in the back of the car. The manual says to use a special terminal located inside the fuse box (under the hood) to jump start the car, but I could not get that to work after multiple tries. When I attached jumper cables directly to the 12V battery, however, the car started instantly. The tricky part was snapping out some of the plastic cover pieces in the rear of the car in order to get to the battery – a chore that was made even trickier because the release for the hatch is electric as well and won’t open when the 12V battery is dead! Luckily, there is a manual method for opening the hatch from the inside, although that requires climbing over the back seat and twisting yourself into a Gordian knot.

My MKZ hybrid has the 12V battery in the standard location, but when it died, ‘jumping’ failed to work, don’t ask me why. Once the 12V battery was replaced, everything worked fine.

The thing that baffles me is that if you look in vehicles like motorhomes or TV remote trucks which have dual batteries (one runs the engine, the other runs the “house.” there’s usually a button you can hold down to tie the batteries together. That way if the engine battery is dead, you just hold the button down and start the vehicle off of the house battery’s power.

Now, I know that unlike motorhomes, the voltages of the two batteries are different in the case of hybrids, but… OK? Step the voltage down as part of the crosstie. It seems a very easy way to get around the “jumpstarting these things is a hassle” problem.

That’s not that simple to do with DC to DC, especially considering the current demand. It would be a very costly addition to account for a very limited circumstance.

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There is a DC to DC converter to step down the voltage to 14V to charge the battery and power the accessories but 12 volts is needed to switch on the Power Management Control ECU to operate the inverter/converter unit.

A charging supply isn’t beefy enough to deliver the CCA needed to reliably start the engine if the starting battery is depleted. The size and cost goes up dramatically due to conversion inefficiencies.

I think that the 12 volt battery does not crank the engine in a Prius. That’s done through the transmission by one of the two traction motor/generators inside the car’s transmission. These motors are powerful enough that they can spin the engine at idle speed or faster which helps make the starts so smooth and seamless that the driver is often not aware that the engine has been switched off or has been restarted.

Interesting thread about the Prius. Everyone now agree w/ these conclusions?

  • no starter motor, cranks are done by the main engine electric motor, powered by the big hybrid battery
  • 12 volt battery is used to operate the normal 12 volt gadgets: computers, radio, lights, fuel pump, fans
  • no 12 v belt driven alternator, 12 volt battery is charged by the hybrid battery, using low-loss voltage conversion circuitry.

How do you Jump Start a hybrid vehicle made by Toyota?
First, you will want to make sure both cars are safely parked and turned off before starting.

Step 1: Open the hood and locate the Jump Start Terminal under the fuse box cover
Step 2: Connect the red positive jumper cable to the positive terminal
Step 3: Connect the other red positive jumper cable to the positive battery terminal on the car giving the jump.
Step 4: Connect the black negative cable clamp to the negative battery terminal on the car giving the jump.
Step 5: Connect the second negative clamp to a solid, stationary, and unpainted metallic point that is out of the way.
From here, you just need to start the engine on the vehicle giving the jump and let it run for about 5 minutes. Then you can turn on the Toyota hybrid and look for a “Ready” indicator to come on. You can then remove the jumper cables in the reverse order that they were connected in. https://www.whiterivertoyota.com/blog/how-to-jump-start-a-toyota-hybrid-vehicle/

But that only works @Barkydog if the hybrid battery already has enough charge to crank the engine, right?

No Idea, internet advice is all I can say.

Someone here will know I’ll bet.

Right. This is just to get the 12V system working, the main battery has to have enough charge to start the car. Problem is, if the 12V is dead, it died while being charged by the hybrid system, so jumping may not work once disconnected.

Jumping didn’t work for my MKZ hybrid, had to replace the 12V battery.

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Just like bad batteries in non-hybrid cars. Life’s too short to constantly jump start a car with a battery that has a dead cell. Buy a new one and be done with it.