Braking vs. Coasting

[VirginiaRick]

I'm just curious if anybody knows if it makes any difference (when descending a sloping grade) whether it's better to just coast, or to apply slight pressure on the brakes, in order to help to recharge the high-voltage battery.

[Ryan McEachern]

It depends your speed. Air resistance is a big drag (ha ha), so at a high speed lots of potential energy gets wasted pushing air around. - so that would say to slow the car down and store some energy into the battery.

 

On the other hand, there are efficiency losses in both storing the energy as electricity, and then turning it back into mechanical energy.

 

Someone would have already done the science on this, but I have been following my gut feeling by using the brake to regenerate some power, as long as I can tell the hill will still provide enough speed to coast me to where I would normally either apply the gas again, or come to a stop in the case of traffic or lights, etc.

Edited April 17, 2013 by Ryan McEachern

[fotomoto]

Braking adds more energy but there are so many variables. For example, would the momentum be better served going up the following hill?

[Jus-A-CMax]

As Ryan said - speed is key but also consider the people behind u, don't hit the brakes just cause u want to regen and thereby throwing the drivers behind u off with your red lights.

 

Another choice is that grade assist - doesn't give u as much batts as the brakes but more than just coasting. Could be best of both world but experiment with that.

[Laurel]

I use the grade assist all of time in town going down hills to regenerate my brakes.  It works super well.  It doesn't slow me down below the speed limit, but keeps me from run away coasting and builds up my battery at the same time.

[frbill]

I have seen that coasting does recharge the battery. For me when I come into town I top off the battery by coasting from 55 - 40 - 30. This corresponds to my speedtrap town's changing speed zones. If I have managed the SOC correctly I almost always top up the battery to almost full, which allows me to EV all the way home  :). I am not sure I would see a better return in SOC if I hit the Regen Mode as I would have to go into EV mode sooner due to a corresponding reduction of speed. Just my two cents, YMMV.

[DR61]

Definition of COAST.  To me COAST means the car rolls free with NO regeneration.  Either selector is in Neutral or your right foot is at a spot in the accelerator pedal map where no regeneration is occuring.  This is the most efficient mode for going downhill when you do NOT need to slow down or stop at the bottom. Great for rolling terrain.  Of course if you do need to slow down or stop then you want one of the various levels of regen with minimal or no activation of the friction brakes.

[F8L]

It is more efficient to use the vehicles kinetic energy and coast than it is to regen brake to put energy into the battery. There are way too many conversion losses for the later to be as efficient as the former. Coasting (with no regen) is the hypermiler's first rule. Drive like you have no brakes.

 

Now you obviously have to balance this with the traffic around you. With no traffic you can lift off the throttle 1/4mile out and coast to a stop. With traffic you just have to drive with the flow and suffer worse fuel economy but you're allowed to curse about it and post angry things on hybrid forums.:)

Edited April 18, 2013 by F8l