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Originally Posted by Schwa
Couple things I can add, 1st gen Prius (2000-2003) doesn't have an EV mode (as far as I know) so extra battery capacity would be used in assist or normal stealth driving mode. You wouldn't really be able to do 10 miles of EV since the engine fires up when you turn the car on, and stays on until it's warmed up the systems.
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The biggest 'problem' for the DIY person is dealing with the HV interconnection. Obviously Toyota doesn't want people messing with that part of the system, so there's no easy way to connect / disconnect the secondary battery systems, they have to be (to some extent) hardwired.
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I've been looking at the other end of the speed range, cruise speed. It looks like a turbo-alternator, a turbine driven alternator, could generate serveral kWh, 2-5 hp, of power from the ICE that could be dumped onto the traction battery bus. Sensing the higher voltage as a higher SOC, the extra power would be dumped into MG2 to provide more motive power. This becomes a compound engine where part of the waste heat dumped out the exhaust is turned into motive power. The result would improve highway MPG without having to do 'ugly things' to the existing system.
As for an interface, my plan is to follow Richard's PriUPS approach. I need those connectors for my whole-house UPS but that is another project. The main thing once you have reasonable access to the traction battery bus, power can flow both ways. A turbo-alternator would be a really nice way to improve highway MPG.
This would also allow me to investigate air-metal battery technology. Rather than try to build a 200 or 273 VDC battery pack, I would use a 48 VDC bus system and a step-up, switching power supply. The high voltage, ground isolated interface is fully contained in the switching power supply. Meanwhile, I can deal with the high temperature, less lethal voltages from a smaller set of air-metal cells.
The same switching power supply could be used for those who want to start with a smaller battery pack investment and add cells as needed for more capacity. Properly designed with battery specific control units, mixed chemistries would work just fine. But I don't have much interest in these batteries.
I like air-metal cell chemistries because of the high energy density and their high temperatures support rapid reactions in small, hot packages. There is a heat management and oxide removal challenge but these have already been solved, in reverse, by reduction cells.
In one of the "Back to the Future" movies, the 'Professor' fuels his "Mr. Fusion" by dumping in aluminum cans. Aluminum is high on my list of metal-air cell chemistries.
Bob Wilson