2008 Toyota Prius would not start without jump. No codes.

My experience is you need to clean and inspect all the connections around the 12v battery. And yes it's normal when there's no 12v power for the Prius to act weird and try to start up partially once it's 12v power is restored and this is easily fixed by turning the vehicle off then back on again the way you normally do.

 

how old is your 12 volt. 'good, needs charging' may not tell the whole story. did you charge it?
what did you test it with?

 

Check battery power at jump point under the hood. You may have a good battery in the trunk, but no power going to the front of the car.
You still need to perform battery test, to rule that out as the issue.
You don't have a DC-DC converter issue, otherwise the car wouldn't run - once started. DC-DC converter is same as alternator in a regular car. If it's dead, car wouldn't keep running once you disconnect the jumper cable.

 

You'd need to sit in your car for 12hours to get that battery charged because Prius has a weaksauce DC to DC inverter that's barely able to charge the 12v.

Not sure why you're acting like you can avoid charging the 12v when the tester said battery needs charging? It's entirely illogical.

 

You need to rule out the 12VDC battery by properly testing it. It could be a battery issue or a parasitic draw on the battery. If you don't have the knowledge, I'd strongly suggest you take it to a professional. Problem solving requires elimination of one known problem at a time. An experienced professional can troubleshoot around minor problems and get to the root cause, very quickly - that's why you pay them $100+/hr.
Attempting to troubleshooting the problem using Chatgpt or AI is a bit futile, if you don't even have basic knowledge of how the car is suppose to work. You won't even be able to describe the issue properly. You know what they say; garbage-in, garbage-out (very old computer slang).
As for what O-Riley's parts guy is telling you, I wouldn't hitch my wagon to anything they say is actually factual. I worked in an auto parts store in high school. Training consisted of someone showing someone else how to do things. In most cases they just think they know what they are doing and rely entirely on what the little back box is telling them. I actually grew up in my dad's repair shop, so I knew how those test equipment was suppose to work and why. More than half the equipment wasn't functioning properly, the other half - someone trained people wrong - so wasn't being hooked up properly. You can't complain much, because it's a FREE service; but you need to remember that the store's purpose is to sell you parts.......

Good Luck.....

 

Point being a low charge on a 12v battery in a Prius creates all kind of weird electrical anomalies, sometimes even super low MPG, even though the car appears to be functioning normally.

As in, when I'm working on diagnosing someone's Prius problems or advising someone on how to diagnose their Prius problems the first thing on the list to do is a full and thorough charge of the 12v battery.

If you decide to disavow that step you're setting yourself up for a way bigger nightmare than you realize...

And seriously, your Prius won't start and no codes and you think its not relevant to get the 12v fully charged even though that's a known problem in Prius in winter months for cars that are not driven often?

 

Lol...