Traction battery needs protection put car in park

Hmm. I've seen a similar message when my 12v in my previous gen4 Prius went out: accessory battery low shift to park with all kinds of clicking and dash lights flashing.

I only mention this because you used accessory mode for a while and that likely drained the very small 12v and when that gets low you can get wonky messages like the one you saw. In acc mode everything gets it's juice from the 12v battery and it drains pretty fast. I honestly don't know why it's an option on the Prius because it is easy to kill it that way and you're stuck needing a jump. If you took a drive and used the EVSE to recharge the traction battery then the 12v likely got recharged some too and if this was a one time thing it's probably not ruined for good from it. I'd avoid using acc mode from now on.

But you specifically said you saw "traction battery" in the alert and that could be a more serious problem. You may need to bring it into the dealership to diagnose what happened. The system should store previous faults even if the check engine light doesn't turn on it could have left a record of it the dealer can pull from the obd port.

 

you cannot listen to the radio in accessory. always leave the car in park so the engine can charge the hybrid battery when necessary.
this has been true for all prius for 20 years

 

The warning was likely:
Traction battery needs to be protected

Plenty of Google results. It seems to be more common on older Hybrids going through a pull-car-wash with the car in N, and likely the AC, heat, lights, etc are on. With some big car washes, it can take many minutes before the car gets even near the water.

If the the Hybrid battery happened to be already low, then the hybrid battery gets drained to very low.

As bicso mentioned, the same thing happens if the car is kept in "ACC", and the radio, AC, heat, lights, etc, etc are running.
It's just a matter of how long the battery will last with a given load, and a given initial charge level.

 

The warning was likely:
Traction battery needs to be protected

Plenty of Google results. It seems to be more common on older Hybrids going through a pull-car-wash with the car in N, and likely the AC, heat, lights, etc are on. With some big car washes, it can take many minutes before the car gets even near the water.

If the the Hybrid battery happened to be already low, then the hybrid battery gets drained to very low.

As bicso mentioned, the same thing happens if the car is kept in "ACC", and the radio, AC, heat, lights, etc, etc are running.
It's just a matter of how long the battery will last with a given load, and a given initial charge level.