Re: PHEV question
The reason they feed false data to the car's battery ECU is because the car adjusts the amount of battery assist based on the state of charge, so if the battery is full (80%) it will provide more of the propulsion than if it were at 70 or 60%, thus limiting the amount of work the large battery could do at speeds over 34 miles per hour.
There may also be some serious discrepancies between what the car is expecting to read as a voltage on the battery and what you have if you use a lithium pack, especially as far as how it's expecting to measure the state of charge, remember the Prius uses other sensors such as multiple temperature probes on the cells to determine the SoC, so there's going to have to be some re-engineering to make it work smoothly.
My preference would be to develop a system that could interact with the NiMH pack by charging it, actually keeping it full until the Lithium pack depleted, then the car would have it's normal pack to maintain once the charger switches off, otherwise the car might try to charge the Lithium pack up to what it thinks is normal operating voltage once you run it down under heavy EV use, thus wasting gas.
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