Long Descents and Regen Techniques

What RPM was the ICE spinning? Or do you even have a gauge / monitor to show it?

Note that even D mode (without applying the foot brake) will spin up the ICE for engine braking when the battery is full, but the degree is quite different. On a sample hill that I occasionally traverse, D spun up to about 2200 RPM, helping but not sufficient to hold the car to the speed limit. B spun up to a much louder 4600 RPM, and did keep speed down to the posted limit.

Give me a bit of time, and I'll dig up the thread from a mountain Hawaiian member here who believed this advice from others. Until he had to repair burned brakes TWICE!

P.S.: Here is that thread:
3,500 foot decent = crazy HOT brakes and Burning Smells | PriusChat

 

Here are some more numbers --

Gen3 Prius has a battery charging limit of 27 kW. IIRC, the Gen2 is very slightly lower. Because there would also be some conversion losses in MG2 and inverter before this energy even reaches the battery, I'll just take a swag at 30kW of actual regen drag at MG2.

Take a fully loaded Gen3 at highway speed, applying full force braking. This is roughly 3900 pounds, 70 mph, and 1g of deceleration. A little physics and math computes out to 544 kW, far far above that battery limit. Less than 6% of that can be regenerative braking, the rest goes to the friction pads. Regen-only mode can provide only very light braking in this condition.

For a different scenario, try a lightly loaded Gen3, driver only, on a residential street. 3200 pounds, 20 mph. A 1g panic stop is just 127 kW. Regen can provide 24% of that, or 0.24g. This is good moderate braking, and drivers can reasonably keep under this limit for a very large amount of their (non-emergency) brake use. This is how they get long brake life.

For additional support for my argument, look back to the so-called 'brake surge' issue raging here in about 2010. It was even the subject of a recall somewhere in there, reducing but not eliminating the issue. This was related to the ABS-triggered switch-over from regen-only braking to friction braking. It was (and still is, sometimes) reported happening only from low to moderate braking at low to moderate speeds. It didn't initiate at high speeds, or from hard braking, because those were not using regen-only braking.

 

Thanks for the link. That was very good read. While I rarely see any thing mare than just hills in San Diego. I will be using B mode in any mountain passes where I may see 6% grades. Sounds like you have experienced 12% grades.. something I have never seen.

3,500 foot decent = crazy HOT brakes and Burning Smells | Page 3 | PriusChat

 

I think Gamma742 said "friction brakes" when Gamma meant to say "regenerative brakes". Assuming Gamma said

...that makes sense, if by "80% full", Gamma means that the totally filled battery indicator really represents "only 80% full because the Prius never lets the batteries fully charged"

 

Thank you, breakfast