Hybrid Car – More Fun with Less Gas

battery and driving on a long slope downhill

register ::  Login Password  :: Lost Password?
Posted by king on August 14, 2008, 7:30 pm

I'm going to drive my Prius into the mountains in September for holidays.
I know that there is a B-gear that enables the car to brake on its engine
when driving downhill.
I'm wondering if the battery will fill up all the time when doing so and is
there any danger that the battery will be overloaded?
Kind of expect there to be some overload avoiding mechanism in the Prius but
not sure.

Does anyone have any experiences or knowledge regarding this?


Posted by Thibaud Taudin Chabot on August 14, 2008, 8:07 pm
king schreef:

This summer I drove quite a lot downhill. There were moments that the
battery was 100% full (all bars visible). I checked whether there would
be 'little cars' displayed in the display indicating regenerated energy.
When the battery was 100% full there was indication anymore about the
amount of regenerated energy, and that sounds logic to me. I have
experienced nothing negative in these situations.

Posted by Michelle Steiner on August 14, 2008, 8:32 pm

A few years ago, I had all thirty minutes of those five-minute 100 MPG
bars, and the resultant green bars on the battery.  Driving from Reno to
Sacramento is almost all downhill, so even at highway speeds, you're
charging the battery a lot.

Donate to the Leukemia and Lymphoma Society; help send Michelle to the marathon.

Posted by Chas Gill on August 14, 2008, 11:32 pm

When the battery is partially charged the motor/generator combines with the
engine brake effect to keep the car going slowly downhill.  Once the battery
is fully charged the control systems in the car prevent any further current
from reaching the battery and the car depends on engine braking alone.  The
only thing you will notice is that the engine note on overrun sounds a bit
higher (because it's working alone) and you may have to use your brakes a
little more.

Some Prius owners suggest that it is better to use your regular brakes
rather than the B position anyway - more charging current gets to the
battery under normal braking than engine braking - but this only holds true
until the battery is fully charged, after which the above applies and you
will start to wear out your brakes.  That would be the time to engage B.

However, it's best to do whatever you feel most comfortable and safe doing.
Whatever you decide you need have no fears about damaging the car - it is
fully capable of looking after itself.

Enjoy the ride!


Posted by Pete Granzeau on August 15, 2008, 7:45 pm

What I've read indicates that the "B" refers only to _engine_ braking,
that is, spinning the ICE at a relatively high rate of speed (just like
putting a conventional car into Low while giving it no gas).  This is in
addition to any braking force being applied via the foot brake.

The Prius will not overcharge the traction battery; if the battery is
fully charged in normal circumstances, it will use engine braking in
lieu of regeneration of power via the motor generator when the foot is
lifted; when braking is required on a full battery, conventional brakes
will be used.

It is possible to finally reach a point where neither engine braking nor
conventional braking is sufficient, just as it is possible with a
conventional car; in that case, you may crash, regardless what you are

If you are worried about overcharging the battery, you might want to
ensure that the radio and lights are on, and that air conditioning is
also running.

This Thread
Bookmark this thread:
  • Subject
  • Author
  • Date
please rate this thread