This past weekend I went on a long trip (got awesome rockin mileage too, 63.7 mpg, best tank yet in the Prius). On this trip there were some good sized hills. Now, I was just using the cruise control in the Prius. So, when I go down these hills the cruise lifts off the gas completely. This enables a small amount of regenerative braking, but it won't keep you @ 55 mph which is what I had set my cruise set at. This made me start thinking about manual control over regen. At high speeds its not the easiest to tell when you transition from regen to regen + physical brakes. It would be very nice to be able to engage full regen before even thinking about using physical brakes. Of course, this would be even more useful in city driving.
So, I PMed Bwilson as he knows more about these cars than I do and he linked me to an
article on autospeed on modifying regenerative braking. Bingo, excellent reference and a big thanks to Bob.
Apparently, the Prius has a pressure sensor on the hydraulic brake system. This is how it measures brake load.
So, armed with that knowledge, I think its pretty safe to say I should be able to make a small circuit that controls regen for a Prius. You could very easily make this like the Insight's MIMA setup and use a joystick to control the amount of regen.
Here is a diagram of Julian's modification which only works when the brake pedal is pushed. Mine would work just about opposite of his. When the brake pedal is pushed, the relay would close and allow normal operation. However, when the brake pedal is not pushed, the pot would control regenerative braking. Julian seems to think that this may cause problems with the ECU when it sees no brake switch activation, but sees brake pressure. I don't know if this will be a problem or not, but you could always add a relay to activate the brake switch in this situation which would fix that problem. Its also not a bad idea considering you really are braking and might want to let others know.
So, thoughts? Possible problems?
