Gen 2 Prius wont turnover

You need to get the car scanned when the master warning light (MWL - Red Triangle) pops up again.

When you removed the 12 V battery to charge it, you wiped out the codes stored in some of the ECUs.

One code that behaves in the way you describe is P0AA6. If you get this code, be sure to get the 2 sub-codes from the freeze frame data. If you only get 526, cycle the car off then on one more time then read the codes again. The second should be there.

When you have the code(s) share them here and we can help further.

 

I suggest to scan again for codes with Toyota Techstream. It might show some codes that the general professional OBD2 readers might not show.

 

And I suggest that you make positively sure that the new battery is FULLY charged before you proceed.

And check the main battery cable connections and test the charging voltage if you can get it into "ready" mode again.

If that 12.9 was measured while the charger was still connected, then either your charger has failed OR the new battery is bad (potential shorted cell).

ONLY AFTER you are sure that there are no major faults in the 12 V system should you pursue any trouble codes that you find.

 

turn the headlights on for five minutes and then read the voltage at the front jump point with a meter, or go to advanced and get a load test.
your mechanic may not have a prius aware scanner, and you want to read the codes when the trouble lights are on, don't disconnect the battery before doing so.

 

Just read the codes, your 12v battery is fine.

 

This is likely true.

BUT the entire 12 V SYSTEM might not be "fine".
ALL of the basic elements need to be checked.....connections and charging......before you pronounce it "fine".

A few quick and simple checks can save you from making a BIG mistake.

 

No. No need to go overboard the other way.
How long after the charging did it sit before you measured 12.57 ?
It is probably OK.