2017 Prius Prime Plus wouldn’t start the other day. Gave me a message “Low 12-volt battery”. So, I put in a new 12 volt battery. But once new battery was in, car was totally dead. But I jump started it connected to my friend’s car with jumper cables. And I can drive no problem. Problem is that after I turn it off, car needs a jump again to start. Any ideas???
Did you check the cable connection to the new 12v battery? Also, more importantly, was your 12v battery fully charged when you installed it? Where did you buy it and who installed it? Sound like you have a dead new 12v battery that could be a defective one. Another possibility is the master fuse is blown? But, if jumping from the battery post starts the car, there must be at least continuity from the post to the car. I would check the new battery first. If you have a battery tester, that would give you the health of the battery as well as SoC. If you don't have a battery tester, but a simple multimeter, then at least check the resting voltage of the new 12v battery. If it is not reading above 12v, then most likely you got a defective battery.
How about if the wiring to the 12V is somehow defective and prevents charging and otherwise not powering the vehicle. Once the car is jump started the DC-DC converter powers the vehicle until it gets shut off and the cycle repeats. Just spit-balling.
At the risk of sounding condescending (no offense intended) are the battery terminals uncovered on the new battery? Batteries often are shipped with thin plastic caps on one or both terminals and (especially black or clear ones) can easily be missed during installation which prevents a 12v connection:
I'd flip through the owners manual for hints about what to try next. Some suggestion to start ya with are Section 8 * When trouble arises 688 If the hybrid system will not start see pg 689 sub heading Emergency start function 694 If the 12-volt battery is discharged Section 9 * Vehicle specifications 743 Items to initialize
