This is in theory possible, but I have no idea how to implement it in practice.

The Prius HSD "Hybrid Synergy Drive" has, at its transmission heart, a planetary gearset with two power inputs: One from the internal combustion engine, and one from the AC motor/alternator. Each acts on a different element of the planetary set (carrier, ring, I don't recall which is which) to create what Toyota call e-CVT or electronic continuously variable transmission. Any combination of electric motor or ICE power is possible, including regenerative breaking, using the traction motor as a starter, and ICE to traction motor/alternator recharging of the battery package.

The problem is that a sophisticated "ECU" or maybe better described in the case of a hybrid as a Powertrain Control Module controls every last detail of this complex dynamic power flow. The Prius, along with other Toyota HSD vehicles, is completely "fly by wire" with no physical driver input to any drivetrain system. All functions are merely software requests to the drivetrain computer.

So, it's possible that a Prius with a dead battery will default to an ICE-only mode. But to force this condition would require very complex ECU programming. And honestly, I don't think you can even start the vehicle if the battery is dead.

Further, I'd think that a used Prius with a dead or "worn out" battery would be quite obvious and avoidable before purchase.

This is in theory possible, but I have no idea how to implement it in practice.

The Prius HSD "Hybrid Synergy Drive" has, at its transmission heart, a planetary gearset with two power inputs: One from the internal combustion engine, and one from the AC motor/alternator. Each acts on a different element of the planetary set (carrier, ring, I don't recall which is which) to create what Toyota calls e-CVT or electronic continuously variable transmission. Any combination of electric motor or ICE power is possible, including regenerative breaking, using the traction motor as a starter, and ICE to traction motor/alternator recharging of the battery package.

The problem is that a sophisticated "ECU" or maybe better described in the case of a hybrid as a Powertrain Control Module controls every last detail of this complex dynamic power flow. The Prius, along with other Toyota HSD vehicles, is completely "fly by wire" with no physical driver input to any drivetrain system. All functions are merely software requests to the drivetrain computer.

So, it's possible that a Prius with a dead battery will default to an ICE-only mode. But to force this condition would require very complex ECU programming. And honestly, I don't think you can even start the vehicle if the battery is dead.

Further, I'd think that a used Prius with a dead or "worn out" battery would be quite obvious and avoidable before purchase.