Car won't start, HV battery "empty" on MFD

The HV battery is fully disconnected from the system when the car is not READY. Also when the car is not READY, no bars show on the MFD. It sounds like you are in accessory mode.

Shut the car completely off. Then with the radio and everything off, firmly depress the brake pedal. Wait for the brake pump to finish. Then press the start button. You should hear audible clicking from the back. These are the HV relays engaging. it will sound like 2-3 short clicks (2 happen almost at the same time, so you may only hear 2 clicks even though there are 3).

If you hear no clicks, the 12v is not flipping the car to READY. Could be as simple as the brake pedal sensor is out of alignment and doesn't register your foot on the brake so it won't READY, only ACC mode.

If you hear clicks, then your MFD should show something. At a minimum the car should illuminate the READY light. If the engine starting goes wrong 1-7 seconds later, then the dash should also light up like a christmas tree.

Report back with whether or not you hear the clicks.

 

Well your battery just isn't charged. You know it isn't charged. Forcing it to boot multiple times on an uncharged battery will fail eventually, in your case 3 times after. You need to fully charge the aux battery. Go for a drive or even just sit it in READY mode in park. Since it works off the inverter it does not charge faster or slower depending on speed. Driving on the interstate to get higher alternator output doesn't apply in the Prius because it doesn't have an alternator or work off even a remotely close principle.

It takes a half hour at a minimum to charge a slightly dead battery up. If the battery is so weak it can't flick Prius relays, it is really really dead and will take a long time to charge. Might be better to hook it to an external smart charger overnight. Your overall capacity is reduced, but it should still work as a good battery for years to come.