Does regen acutally work to help increase range?

looks right to me.

 

This is not correct. Every experienced Prius driver knows that you can prevent regen by putting a light pressure on the go pedal.

The answer to the question in the thread title is complicated: Yes and No!:

Regen recaptures some kinetic energy and stores it back in the battery, which obviously increases range. But any braking involves energy loss, which decreases range.

Putting the Prius in Neutral (a very dangerous thing to do!!!) will prevent unwanted regen, but will also prevent wanted regen. I.e., when you NEED to slow down, you WANT regen. But when you can get by with coasting to a stop, you do not want any braking, regen or friction.

Therefore, regen on the go pedal, which introduces regen when it would be better to coast, will hurt your range; but regen used INSTEAD of friction braking, when you MUST slow down, will increase your range compared to slowing with friction braking. On downhills, when you must apply braking to stay within the desired speed, regen will increase your range compared to the use of friction braking. Likewise when you must come to a stop in a shorter distance than coasting would do. But having the car apply regen when coasting would be adequate will cut your range.

So as a generalization, regen on the go pedal probably reduces range, while regen on the brake pedal probably increases range. Hypermilers know this when they "feather" the go pedal to achieve coasting without regen.

 

Interesting way to generate traffic for his site (along with claiming a BMS is completely unneccesary because LiFePO4 is "self balancing"!) but factually incorrect.

 

I have not watched the video, or even gone to the web site. But it is obvious that regen implemented EXCLUSIVELY on the brake pedal would not reduce range. Regen implemented on the go pedal could easily give you braking when you don't want braking, and unwanted braking could reduce range. This is one reason I advocate regen ON THE BRAKE PEDAL ONLY, and never on the go pedal. Allan disabled regen on his Zenn (which he no longer owns) for just this reason. The very light regen on the go pedal of the Prius probably does not hurt mpg much, but it would be better to get rid of it and use the regen on the brake pedal only.

And of course no battery is "self-balancing," though both my LiFePO4 battery packs came with chargers that integrate a BMS into the charger.

 

I've watched the videos and my first comment is they desperately need
to find an editor. The videos could easily be cut in half and increase
their content ratio by 100% Maybe Count DaMonay can help.

I'm not competent to comment on the purely electrical elements and
calculations, especially when carried out to 4 decimal places.

That said, I think there was some experimental design errors or
misunderstandings in the original 356e tests that has lead to this
brouhaha.

As I understand it, three conditions were tested with the 356e:

1. Brake pedal regen only
Brake pedal - deceleration by regen and friction brakes
No pedals - freewheeling ( I'm not saying "glide" as it has Prius-specific
meaning and includes a small power draw on the HV battery.)
Accelerator - acceleration only.

2. Max regen
Brake pedal - deceleration by "aggressive" regen and friction brakes
No Pedals - constant "aggressive" deceleration by regen
Accelererator - acceleration only

3. No regen
Brake pedal - deceleration by friction braking only
No pedals - freewheeling
Accelerator - acceleration only

The perceived problem came up when the figures suggested that the
car used less power over the 47.6 mi. course for the second condition
over the first, and the third case - no regen whatsoever - over the
second.
Overall conclusion: regen in an EV offers no energy savings/efficiencies.

In condition 1 there is freewheeling and some regen via the brakes.
In condition 2 there is no freewheeling and a lot of regen.
In condition 3 the is a lot of freewheeling and no regen.

My observation is that condition 2 is not about regen at all. It is about
essentially constant deceleration at all times except when the
accelerator is depressed. It would be like driving with a dragging brake
shoe. You would have to accelerate -- that is use power -- to maintain
speed when you would otherwise be freewheeling.

To me, this is not a surprising experimental result.
It would also be less efficient to drive around dragging a 200 lb. anvil behind.

I do not yet understand why condition 3 -- no regen -- looks more
efficient that condition 1 -- some regen.

I suspect that, as is noted later in the series, it has to do the nature of
the road course driven, having to do with stops/starts, topography, and
ratio of city vs. country, regen "friendly" vs. "unfriendly" driving.

As a wannabe EV owner, I am keenly interested in how EV operation
interfaces with real world driving, so I find all this more than just
entertaining.

I'm looking forward to final resolution of this matter... even though it
may have more to do with electrical complexities/inconsistencies than
purely physical considerations - as in Newtonian physics - of how things
"work" out on the road in the real world.

Am I missing something in all this? :noidea:

 

I also suspect the driver, on the "full regen" test kept the throttle up to the lights, and then slammed on the brakes, thinking "i'll get the most regen!"

I've seen people make similar comments, where they are excited about braking because of the regen. we all know this is a fallacy, and that no braking at all is better than regen braking.

To validate the experiment you'd need a repeatable trip, with exact controls on the level of acceleration and deceleration. and it would prove that regen braking is better than not, but that driver behaviour CAN negate the benefits.

 

Rokeby: I agree with you: Condition 1 MUST be more efficient than Condition 3, since both are freewheeling when no pedals are pressed, and some regen is used when the brake pedal is pressed.

Note that the Prius does the brake pedal right, but does the accelerator pedal wrong: The brake pedal uses ALL regen whenever possible, and uses friction braking ONLY when regen would not be adequate (emergency stopping or below a slow threshold speed); but the accelerator pedal applies regen braking when released, which means some braking when braking may not be wanted.

However, implementing Prius-style brake pedal operation requires a LOT of computing power and complex mechanisms to operate the friction brakes reliably when needed without applying them at other times.

 

Thinking about this further, I find two additional things that would
confound accurate energy use and efficiency analysis between
Condition 1 (C1) and Condition 2 (C2)

Both the test runs for C1 and C2 were made on the same day.

1. Someone , I believe it was Hobbit, said long ago that to properly
warm up a Prius for a FE/MPG trial, the car needs to be driven for 20
miles or so before the event. In this way you get, for lack of a better
term, a "whole car" warmup, including all rotating equipment;
transmission, rear axle, wheel bearings, pump bearings, etc.

There is no mention of the 356e being warmed up prior to C1, this
could mean that the the 356e would have been less efficient on the C1
run compared to the C2 run

2. The internal temps of the drive batteries during the runs could have
fuzzed-up the data. I believe that the C1 and C2 test runs were done on
a warmish, 90 degF day. It is well known for the Prius anyway that
when the HV battery gets up over 100 degF, it suffers from both
reduced capacity and reduced ability to receive a charge and deliver
power.

I don't know how effective the 356e's drive battery cooling system
was/is. Without taking temps before/during/after each run there is the
possibility that temps were not equal for both runs.

Which run this would favor would depend which run was at the hottest
part of the day.

 

On my big electric van, I can disable regen, and I have run this test many times mainly due to skeptics. It's night and day!

The van can stop almost completely in normal driving w/o using any friction brakes. It weighs over 7000lb, so that's a non-trivial amount of kinetic energy recovered from the inertia.

My tests show about double the range available in city driving with regen vs. without.

The Prius is also evidence that Jack is wrong on this. It wouldn't be hard to arrange an accurate Prius test, it should be a simple matter to log the actual KWH put back in to the pack. (this is probably the easiest and most accurate) You could also disable regen by setting CCL to zero or High SOC during braking with a little CAN bus hacking so you could add a "regen disable" switch that would make this safe.

 

doing the test with two Prius would be perfect. follow each other and drive the same, but one brakes normally while the other drops into N when braking - no regen is available. who gets better MPG?

 

I think some of us are missing the point that we are talking about the regen when doing nothing with the pedals.

It is quite obvious that using regen for braking is better than no regen! You might be interested in quantifying this but this is not Jack's point, he's talking about the no-pedal regen that is programmed into our vehicles and which we should be allowed to disable in my opinion.

 

Yes, the Prius does a small amount of regen during pedal release to simulate the way most cars with automatic transmission behave. My Electric Van has this feature as well, but I can disable it with a switch. In the van it feels almost as if something is still pushing you along, like a stiff wind, even though it's just coasting. It's sort of disconcerting, so I see why Toyota included this "fake" drag. However on my van, they also included a small deadband (hysteresis) between accelerate and regen so you can "float" the pedal there for coasting. It takes practice to know where this is, but I'm accomplished with it and as such never have the need to use the regen inhibit switch. Still, I too would appreciate the option to turn it off, but I understand why Toyota didn't include it. It would definitely scare uninitiated drivers!

 

Sounds like a ridiculous assumption that somehow "free charge" for the car is a net negative on mileage. Sounds like nonsense.

 

Like Skoorbmax says... Huh?

There's no way that using regen to slow down could NOT extend your range - provided that you were going to slow down with brakes anyway. Now if you drive like an idiot, all bets are off. I recall a Prius commercial from a few years back that demonstrated how to drive like an idiot. He would fly off the line, then slam the brakes on so he could show his kid how cool it was that he was recharging the battery.

As for regen on the go-pedal, I disagree with Daniel. If you don't drive like an idiot (and I don't mean YOU you here, of course) regen on the go-pedal is the most efficient way to drive. You never slow down more than you intended once you learn to drive this way. You go exactly the speed you wish. And in all but the most extremem circumstances, slower uses less energy than faster. As long as you don't constantly yo-yo your driving, slowing down is a net benefit.

 

Yeah, that's key.

To further my earlier statement if regen was a net negative imagine how stupid the Toyota engineers would have to be to have put it into the car. Maybe if it didn't use regen it would get 55 city instead of 51? I bet they never tested that!

OP's test is interesting, though, it seemed to help quite a bit.

Now, I'm sure a badly implemented regen would hurt and maybe the Porsche in question has that issue (I cannot get links to work properly at this time to check).

 

Regen only "hurts" mileage if you use it rather than not stopping in the first place.

In other words, all the Regen energy ultimately comes from the ICE. There is conversion loss in/out of the MG's and the Battery, so you're much better off just going slow and not having to stop, than stopping and using regen.

But, if you have no choice but to stop, it's clearly going to help your mileage to have regen. There's no argument about that!