The Prius doesn't have an alternator (or a starter motor for that matter). If the 12v battery is dead, the computers won't power up, turn on the high voltage system, and engage I think MG2 to start the engine.

There is a DC-DC converter which takes the high voltage (IIRC 350v?) and steps it down to 12v. It should be active any time the READY light is on in the cluster.

PS: If you want to recharge the traction battery, put the car in D, hold the brake, and floor it. The engine will rev up a bit, and it will charge the battery.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.