Here and there, I've read that the main rechargable battery in the Prius has an expected life of between 188,000 and 250,000 miles. After the main battery has finally died, the full operation of the HSD would obviously be impossible. However, is it possible for a Toyota mechanic to "turn off" the HSD and use the ICE as the sole powerplant? I know that sounds odd, and I know that the car would be less efficient, and very slow, but what if you have your Prius and not a lot of money and don't want a new car? We've all seen ancient Toyotas puttering along with 6 or even 7 digits on the odometer, but can the Prius do that? My take is that the ICE is too integrated into the central computer and HSD system that this would be impossible, but I know someone out there knows a lot more than I do about this...
No the HSD cannot be "shut off." Your engine is started by MG1, and MG2 is absolutely necessary to get the car moving from a stop. These cars have been on the road overseas since 98. Toyota has a factory in mothballs waiting for these batteries to start failing. To date, this still has not been an issue. Depending on how "bad" the HV battery gets, you will likely only notice a drop in overall fuel efficiency, or power when demanding 100% power from the system. If the battery gets so bad, that it cannot hold enough power to start the engine, then you will be dead untill a new one can be installed. While I don't ever expect the HV battery to truly become "cheap," by the time you need to worry about its replacement, the price will have dropped significantly. I've already seen a few of these batteries for the classics pass through the auction process on Ebay.
The way the ICE and the two MG's are integrated with the PSD, they are a unit, and all three are required for operation. However, with a 100,000 mile / 8-year warranty, and an expected average lifetime much longer than that, by the time anyone has a non-accident (i.e. non-insured) non-warranty battery failure, there will be used batteries available from cars that were in accidents. So battery replacement should not be an issue.
