Traction battery life

There's about a 99% guarantee that the original traction battery will be long gone by 20 years. Most barely make 10 years. Many fail in seven or eight years.

If the traction battery is dead, the car is dead - it will not start. There is no 12 V starter. One of the two high voltage motor generators is used to crank the engine at 1000 RPM initially and many more times during any drive cycle.

The transaxle design requires the two electric motors to be powered by high voltage in order for any drive power to be transmitted to the wheels.

Battery life is more than miles. It's a combination of miles, charge, cycles, heat and age. The batteries don't suddenly go dead. They get progressively weaker. This progression can be observed and acted on without an unexpected emergency failure. However, most will wait until the dreaded battery code appears and then treat it like an emergency.

Realistically, if someone wants 300, 400 or 500,000 miles out of their hybrid they should expect and plan on replacing the HV battery. The good news is it doesn't require $4000 anymore to get new cells for a standard Prius hybrid. Many have replaced the cells in a diy mode for less than $2000. That process is not for the faint of heart as the the battery assembly must to be completely disassembled and then rebuilt.