Battery charging on long downhill

Yes. The system switches to engine braking, which discards the excess electrical energy by spinning the engine. Obviously there are limits. If the required braking exceeds the electrical capacity, friction brakes are brought into play.

Tom

 

And on really steep and long downhill sections your friction brakes work like every other car. This means they heat up and become useless. So next time you decend Pikes Peak, just remember that the regen braking is cool for the first few hundred feet, then the friction kicks in and they get hot. I see lots of cars coming down from Pikes Peak where their wheels are glowing red or smoking because they didnt stop at the required brake checkpoint and cooling stations...

 

It is better to use "B" to help friction brakes make their job.

 

Under some circumstances the car will use MG1 to spin the engine, just as it does to start it but with no fuel applied, to use excess energy.

As far as I know the car will not automatically apply friction brakes in the B mode. Cruise control is canceled when you select B.

Also of interest, the B mode does regenerate but not at the maximum rate. You can increase regeneration, in B or D, by using the brake pedal and putting the HSI indicator bar farther to the left, if you put it clear off to the left you start using friction brakes in addition to regeneration.

 

+1
Nice to hear someone understands how the regen works.

 

I tested cruise control in long tunnel, 5.5 km downhill, in Norway, "D" mode selected. The car was not able to maintain preseted speed - 75 kmh - just with regen braking, no friction brakes applied if I can say.
I had to use friction brakes by pedal to get the tunnel speed limit.

 

The engine in the Prius is always "into the drive line", as are MG1 and MG2. There are no clutches or shiftable gears which would allow otherwise.

Regenerative braking uses MG2 as a generator. Normally the electricity from MG2 is used to charge the HV battery. Engine braking occurs when the SOC of the HV battery is too high, or B mode is selected. During engine braking, MG2 acts as a generator, but instead of charging the HV battery, the electricity from MG2 is used to spin MG1 which in turn spins the engine as a big air pump, thereby dissipating excess energy. Note that this is not an all or nothing proposition - engine braking and battery charging can occur at the same time.

You miss-understand the phrase "apply the friction brakes as needed." The braking system in the Prius responds to pressure on the brake pedal. Increasing brake pedal pressure tells the Prius to regenerate more power, thereby provide more drag. Eventually regeneration hits a limit and can provide no additional drag. If more is requested by increased brake pedal pressure, the Prius has no choice but to activate the friction brakes. This is what is meant by "apply the friction brakes as needed."

Tom