Regen reduced when friction brakes are engaged

I was investigating a way to have the SOC meter tell you more directly when the friction brakes were employed. I think I found three signals indicated in the service manual. Target braking force, Regen Braking Force, and Brake Pressure. Here is a graph of a simple test where I pressed the brake pedal three times at the beginning while I was at a standstill, accellerated, then slowly increase brake pressure until I stopped. I repeated again but increased braking more slowly.

What is interesting is when you start to engage friction braking, regen is turned off. They do not simply add friction on top of regen. I guess they couldn't get the feel smooth enough. So it tells us it is VERY important to avoid friction braking because you waste *everything* once you go there - not just an incremental amount above the regen capability.

Interesting data. What speed did you achieve before letting up on the pedal and at what speed did you apply braking? From higher speeds (45-50) the energy screen shows a clear correlation between moderate brake pressure and regen. However, as the speed decreases it seems that the system transitions more and more to brakes and no amount of finesse on the brake will increase regen. Almost appears that regen is (assuming capacity in the battery) a function of speed and deceleration (Gs).

TickTock said:

I was investigating a way to have the SOC meter tell you more directly when the friction brakes were employed. I think I found three signals indicated in the service manual. Target braking force, Regen Braking Force, and Brake Pressure.

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Does the service manual give you an idea what the units are for the three signals? I'm a little surprised friction brake pressure appears to have a scaler remapping to brake force, but maybe I'm just not bending my brain right.

Toyota's had a ton of years to try and do this, do we know how they mix between friction and regen? Not having any jockey time in a Prius, I don't even know if the driver can tell. Is there an indicator there?

Since you're bringing it up, I'm assuming the Leaf doesn't just tell the driver when friction braking is applied (I'm a Volt guy). I ask because the Volt is silent on the issue as well, and it's been a great point of discussion and confusion because of the Volt's "Low gear" setting. We've got it a little easier, in that the DashDAQ has an option for Volt signals. Two of them are Target Braking Force and Regen Braking Force. I'm calculating Friction Braking Force the obvious way, but since you've got all three it would be very interesting to apply a scalar translation between Friction Brake Pressure to convert to Friction Brake Force (if it is, in fact, scalar) and sum the friction and regen forces to compare to the target force. There is *some* overlap in your graphs, but is there over/under shoot on the resultant force?

I'm curious to hear about the speed question too. I can't see why the energy screen (or the dots) would be lying when they show the regen blending with what you can feel is the friction brakes kicking in.

That said, Nissan clearly decided that when braking has to happen NOW it's not worth worrying about regen.

Its a fascinating subject.. but a quick reminder that best hypermiler practice is to drive as if you had no brakes... if you want maximum range of course.

I make no claim to be a hyper miler (quite the opposite in fact ) I do like know what tradeoffs I'm making, though. To the first order, hydraulic pressure *will* be proportional to the breaking force. Service manual doesn't indicate the units, but does describe the operation and the various signals that are sent over the can bus. I will do some more tests at different speeds. It would be good to confirm that the regen stopping wasn't due to speed instead of applied breaking pressure. I got up to about 40mph for this test.

Does coasting regen show up in the regen braking signal as well, or is that somewhere else (if at all)?

Rusty said:

Toyota's had a ton of years to try and do this, do we know how they mix between friction and regen? Not having any jockey time in a Prius, I don't even know if the driver can tell. Is there an indicator there?

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As a five-year Prius owner, I don’t know if I could ever really tell when the friction brakes kicked in. The transition is pretty smooth. Prii have display screens where you can see when the regen ends and it's around 8 mph. Anything below that and it is friction braking, only. Friction brakes will kick in at higher speeds if you hit the brake pedal hard enough.

Rusty said:

Does coasting regen show up in the regen braking signal as well, or is that somewhere else (if at all)?

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Good question. I'll add that to the list of experiments to run.

TickTock, you must be misreading the signals somehow. When I look at the energy meters in the car's touchscreen, it shows a full 30 kw of regen when I press down reasonably hard on the brakes (as in a typical slowing to a stop, not a panic stop). Also I have been down some very steep and long descents in the mountains where I can tell you that regen alone is not enough to keep the car at a reasonable speed, and yet I gain a great deal of energy when descending. So in most situations I'm certain that regen is not turned off when friction brakes are applied. Better check those signals again, you must be missing something. I'm really curious to see if you can find out when friction brakes actually take effect though.

thanks for the input. You are right to question theses results since there is a lot off guesswork in which messages correspond to the ones referenced in the service manual. I'll try to corroborate the results with the battery current to see if, indeed, no pwr is going into the battery when friction kicks on. It's possible there is another bit that indicates to use max regen even when this signal goes to zero.

I have had stops where the regen went to zero due to braking so it seems reasonable to me. I have often used the eco tree meter to help adjust regen since it is in the field of view compared to the Nav energy panel and when it starts dropping is where I believe the friction pads begin to absorb energy and spoiling the regen. Maybe this can be verified or the message stream that controls that eco gauge correlated with the brake / regen messages. Keep up the interesting observations ticktock!

Since ability to regen is related to speed, I wonder if the "transfer point" shown in that data is in fact happening due to crossing some lower threshold of vehicle speed.

Imagine that there is some speed X above which regen braking is effective. You can use 100% regen brake above this speed. Once below that, you have to rely less on regen and more on friction since effectiveness drops off as speed decreases. It seems like a relatively smooth changeover from regen to friction braking on that graph so I'd be interested to see how that correlates with vehicle speed.

That kind of logic would also explain why braking + regen can occur when coasting downhill - you are above the minimum speed so you get as much regen as possible, but if that's not enough you get some friction braking in there as well.
=Smidge=

It would be a useful training tool to display when friction braking is happening, apparently it can be done easily by monitoring the hydraulic pressure.

Herm said:

It would be a useful training tool to display when friction braking is happening, apparently it can be done easily by monitoring the hydraulic pressure.

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bingo! This is exactly why I started exploring this. I want to add a little speaker that will emit a tone whenever friction braking is applied (perhaps with a tone porportional to the amount applied). That way I can keep my eyes on the road (particularly important when braking ) but still train myself to optimize the regen better when I need to apply brakes.

I tried a few tests last night. At about 50-55 mph, I initiated very heavy braking (just this side of a panic stop). Observing the center display of energy at that point, I was still showing significant regen even though it was clear that I was strongly in to the friction brakes. I would have loved to have a GID meter at that point to see the actual regen power... In any event, it would seem that there is more going on than the logged graph would indicate...

Some people have trouble hearing high frequency tones, perhaps a geiger counter type system?

Maybe I'm reading it wrong, but the graphs above seem to reflect what I see on the bubbles and SOC meter.. regen doesn't turn off all at once, but does taper off as the car slows.

I'd be curious to see 'speed' plotted against that, since I'm sure that's part of the equation.

I added a "blip" to my SOC meter to indicate when a point is lost or gained. You can really hear the power spooling out on an onramp or hill climb.

That is to be expected, all other considerations aside, as the amount of power the motor/generator can produce decreases with speed, as with almost any generator.

GroundLoop said:

Maybe I'm reading it wrong, but the graphs above seem to reflect what I see on the bubbles and SOC meter.. regen doesn't turn off all at once, but does taper off as the car slows.

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TickTock said:

I want to add a little speaker that will emit a tone whenever friction braking is applied (perhaps with a tone porportional to the amount applied).

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You better feed speed into the equation, or stop lights are really going to be annoying (as your graphs already show, it's all friction at 0 RPM). I use a bar-graph to display the amount of friction braking. I don't think I can get the DashDAQ to do intensity level toning. Hmmm....

TomT said:

I was still showing significant regen even though it was clear that I was strongly in to the friction brakes.

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How do you know friction brakes were involved? It really surprised me how much 30 kW will slow down a car in a hurry, and that's in a Volt which is a heavier car (granting that it'll do 60 kW of regen if you push it).

I would expect 30 kW to really stomp a Leaf very quickly from speed.

Smidge204 said:

Since ability to regen is related to speed, I wonder if the "transfer point" shown in that data is in fact happening due to crossing some lower threshold of vehicle speed.

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Weatherman says there is just such a transfer point in Prii (around 8 MPH). There's a similar transfer point in the Volt (somewhere between 4-7, depending on deceleration rate). So if the Leaf blends regen and friction at all, there'll almost certainly be a speed related transfer point as well.

Ticktock have you found a speed related signal yet? In the Volt they have integer KPH, which is pretty useless. But they also have transaxle RPM (in 1/8th RPM :- ). From that it's straightforward to get a fine grained speed indication...