EV Regenerative Braking using the "B" drive in EV Mode

True story: A friend of mine rear ended a truck on an expressway. He was given a ticket as was the truck for not having working brake lights. He beat the ticket in court because the vehicle he hit did not have a brake light. Extrapolating to your theory that it's OK not to use brake lights you stand a good chance when you are rear ended that it'll be on your nickle. For me, it's not worth the risk.

 

There is a very big difference between rear ending a car without working brake lights and one in working order.

 

What I'm interested in most is if slowing down in B mode charges battery more or less than braking in D mode. Are any data available about that?

 

To much variation between drivers to call it one way or the other.
If you apply the same amount of braking in D as what B does on its own, then the amount of energy captured should be the same. The trick is in being able to match B's braking amount to see that.

 

I’ve noticed that b provides extra braking force. Try lightly braking then select b. You’ll feel that braking force increases.

 

Well, I observed that when I drive in hybrid mode for a while, then switch to EV (meaning that engine is warm now), then I switch to B, I notice that engine gets turned on then to help with decreasing vehicle speed when braking (especially if going downhill). I did not notice such behavior when driving only in EV mode (engine is cold) - engine does not get turned on then when braking in B mode. Thats why I'm thinking that in the case when engine gets turned on, then probably less energy is regenerated in B mode (compared with D) because some energy is lost because of engine compression that must also be overcome. Or am I wrong?

 

The engine isn't really on; it is just spun by MG1 to waste incoming energy, and that is how the Toyota hybrids operate under B. Others have seen this in EV mode, with a cold engine, when the battery charge is near full. Do you recall the battery charge at the time?

So B can waste energy in a Prime. Those using it for regular driving, and not for controlling speed on a steep decline, are mostly shifting to it once some charge is used, to avoid that wasteful operation. When done right, being in either D or B isn't going to be more efficient than the other. It is just user preference.

 

I'm a little reluctant to comment, because with the plug-in version of Prius B is maybe a bit different? Anyway, my take: B primary purpose IS to reduce charging, and it accomplish this by sub'ing engine braking.

FWIW, the only time I use it here (with a 2010 Prius) is coming down protracted downhill runs, where there's a distinct possibility the battery will get completely charged, and there's still plenty of downhill to come. That's pretty much Mount Seymour, nothing else, around here.

Too: the plug-in's have much greater battery capacity, so maybe it's hardly ever warranted?

 

This is so, but the hybrid battery pack is far too small to regen the entire distance of a long down slope. So shifting to B has the ICE spin up right away. The effective capacity of the Prime battery on the other hand is roughly 12 times that of your gen2. So if the pack charge isn't near full to begin with, the car has the option to increase regen before spinning up the engine while in B.

 

Here's some real data. I just returned from a Blue Ridge Parkway trip from Boone, NC to Asheville. Elevation changes from 3300 feet to over 5800 feet, both up and down thoughout the 100+ miles finishing at 2130 elevation. Started with 0 EV miles. Toggled between D uphill and B downhill. Rarely touched my brakes. Gained EV miles downhill, and used them going uphill. Final downhill section gained and finished with 22 EV miles. I never used B before, but can tell you it was seamless and now I understand how efficient and useful not braking is for long downhills runs. I could smell hot brakes on several cars at the turnouts. Prime performed beautifully. 70mpg with no charging.