Lost my hybrid battery after
Lost my hybrid battery after 9 yrs and only 84k miles. 2000 year, 1st gen Prius. Replacement was $2500 parts $600 labor at the dealer. Makes me think time on the battery is as important as miles. Replacement is a more modern battery than the original. I do expect good results.
Symptoms were a loss of mileage from 50 MPG down to 35 MPG over about 6 months. Prius could not do 25 MPH without the engine running towards the end. This is a voltage loss under load. Probably higher current too, given the symptoms.
At the end, a circuit breaker would blow after 1.5 miles, disconnecting the hybrid battery, leaving only the engine to provide power. The engine revs up a lot in this situation - triangle of death - car of death icons and amazingly - the parking break light comes on. Simply stopping and shutting down resets everything except the two icons for another 1.5 mile go. Then repeat - parking break light, loss of power, no hybrid battery current, engine revs like mad on depressing the gas peddle. Repeating too many times probably risks fusing the circuit breaker.
I don't think the 12V battery was ever replaced. Toyota wanted $225 for a new one. They must think pretty highly of themselves. In the end, Toyota threw labor for the Inverter oil change, coolant change, belts and yes, the way over priced 12V battery. I guess it pays to embarrass the repair guy once in a while.