I didn't pay too much attention to the several "Shift to P Position When Parked" messages I got in the past month. Then a couple of days ago the car just won't start showing "Check Hybrid System, Stop Vehicle in a Safe Place". I got U0100 and U019B besides the other 14 unreadable codes that my limited OBD II found. How can I check with a multimeter or any other way if the 12v battery needs replacement? I found posts that the 12v battery may be the culprit for the "Shift to P position" message Please help!
pop the hood when done driving for the day in the morning, go out to the car without the key fob open the hood and fuse box open the red cover on the jump point measure the voltage between the jump point and a clean ground bolt
After assuring yourself that the 12v is sufficient ( at least 12 Volts) then do what the car was telling you. Reinitiate, car must show ready in order to drive. Rebooting battery is also a standard procedure. After all problems seem fixed, go for a good drive to bring the 12V up to a healthy state of charge. Ironically that little 12 V is the most important battery, take care of it.
While that might be "sufficient", under no load 12.0 indicates that it is half discharged and it not GOOD.
What Professor and what part isn't right ? It is indisputable that a reading of 12.0 with no load is about 50% discharged. "All of your posts on the subject" won't change that.
Sometimes these threads go into the weeds right around the difference between whether a battery is charged or discharged and whether it is "good" or "bad." A battery reading 12 volts is not in a full state of charge. Charging it will bring it back to a full state of charge. Batteries are actually built to do this cycle, time and time again. If a battery can't be successfully charged, or self-discharges excessively, or has developed an unreasonably high internal resistance, then it is "bad". That's a whole other situation. A voltmeter by itself can't tell you that. A dedicated battery tester can. It happens less often than a casual reading of PriusChat might suggest. Depending on the Prius generation, going for a good drive might not be adequate to fully recharge a battery. In a Gen 1 or Gen 2, from a fully discharged battery, that would be about an eight hour drive. The good news is, it doesn't have to be a "drive". That conventional wisdom comes from conventional cars where the engine-driven alternator puts out higher voltage when the engine is revving higher. A Prius isn't like that, and will charge the battery just as fast whether you're driving or just in READY in Park. I've been known to just leave a Gen 2 in READY overnight after locking the doors with the key (though in a higher-theft area, I might think twice.) Gen 3 and Gen 4 can achieve higher charge rates, so presumably could do the job in less than eight hours, but might still need more than a cursory good drive. (Some observations even suggest that a Gen 4, or Prime anyway, may even charge more slowly during a drive than in Park!).
I agree on all counts, variabilities occur constantly depending on daily useage, or nonuseage, abuse and other normal factors, loads, weather etc. To go for a good drive was only one suggestion. As for rate of charge under any circumstance, I think most Prius drivers know that if you have more accessory loads, the battery may be getting weaker, that's what I meant by taking care of the battery. Frigid weather requires even more awareness.
