Jumps to life, but dies completely after use

Did the mechanic remove the battery from the car and place it on a load tester? If not, I would go to someone else!

The 12v battery needs to be in pretty good condition in order to start your Prius.

 

Look at the top of your battery. It will have a date code. Post it here. If it's the original battery, just get a new one.

Verify the + and - terminals on the 12v battery are good connections. verify the - cable screw is tight at the car body.

If the battery has been load tested and verified good (not just a voltage check) then check the voltage at the underhood jump point also. If you have good readings at the battery, but not at the jump point, it's likely you have a bad connection or blown fuse at the battery + terminal.

when you're checking the voltages, have someone turn the headlights on/off. How much does the voltage change at the battery and at the jump point?

 

Oh god how many times have we heard this. Its a bad 12 volt battery. I assume you don't own a voltmeter.

I cant see how you have any blown fuses if the "Car runs alike a dream" after a jump. If you had any blown fuses you would have a dash board full of obd code lights.

Stop jump starting it you can really damage a G2 especially if connecting directly to the battery in the trunk.

 

Load test can be done without removing the battery. Pretty much a slam dunk what the verdict would be, just saying.

 

The 120A fuse at the 12v battery + terminal is a unique situation.

If the 120A fuse/fusible link in the 12v battery + terminal assembly is open, the car will be dead. A simple jump start at the underhood jump point will allow the car to power up and go ready, with no codes. The car will work normally, until you turn it off, then it will be dead again.

Here is a thread with a blown 120A fuse. Basically caused by connecting jumper cables to the prius properly, but then connecting the jumpers to the same battery terminal (+ terminal) on the other car, effectively shorting the prius battery terminals together.

12volt battery/jump starting issues, help! | PriusChat

 

Jumper cables should maybe be pulled completely apart. Leave one away from the scene, out of reach. Take the other (preferably the one with red clamps): clamp one end to dead positive, the other to good positive.

Then go back and get the other cable: connect one end to good negative, the other to dead ground.

I've even seen Scotty Kilmer do this: connect neg and pos leads to the good battery, then grab the other two ends, one in each hand (small blessings), and start moseying over to the dead car. A short circuit is real close, as close as the two loose clamps.