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Dangers of rapid regenerative braking?
- Thread starter chuckb
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- Driving Dynamics Model S
The brake lights come on at a certain level of Gs. As I recall that is around .25G.
That may happen to coincide with 30kW.
I have followed other MS drivers and observed my own brake lights at night.
I have not noticed any excessive brake light activity in any of those circumstances.
I also haven't had any issues letting up slightly to get less than 30kW regen. If i do get that much regen, the car is slowing down fast enough that I want the brake lights to light up and warn other drivers that I am slowing down.
If you are experiencing more road rage than normal, I suspect it is not because of the brake lights.
It is nice to see that is an option so you can turn it down to low since that seems to work better for you.
mnx
2018 M3P
Maybe you have a defective foot. :tongue:
Seriously though the Model S has about 10X the pedal travel on the accelerator as my Toyota Sienna. It's very easy to modulate (for me).
My first test drive in a Model S was last September in a P85. I was immediately impressed by the sensitivity and feel of the "go pedal" and the amazing degree of control I had at any speed. It took just a few minutes to get used to. Now it is second nature.
I cannot understand why someone would find it to be "difficult" or "touchy".
Whenever I have to drive my Porsche, or any ICE, I am disappointed by the mushy response of the accelerator pedal in either direction. It seems so last century...
no I'm sure its just me and my lead foot. i'm sure I exaggerated a bit but the point is still valid that letting go of the pedal any decent amount does trigger >30kW regen and brake lights. around here in NJ/NYC if your brake lights come on and people aren't actually stopping in front of you, it's really going to piss off the people behind you and they'll likely quickly try to go around and cut you off to get in front of you so they don't have to deal with the constant false brake checking. and if you're like me, I don't usually let people pull in front of me, especially if they're just trying to move from behind me to in front of me, so that pisses them off more. I've probably got the finger just as many times as I've got the nod or a thumbs up.
I've seen other Model S's on the road and i've seen they're brake lights flicker on and off during their normal driving and not while they are intentionally slowing down so its not just me.
It sounds like driving style. If you try and smooth things out which also may mean leaving a gap in front of you this won't happen. If you are always closing gaps and accelerating and lifting all the way off the ordeal then you will get this.
That'd require a lot of back and forth on the brake and accelerator when in traffic.
oh yeah don't get me wrong, I am very happy with the one pedal driving and the regen on the accelerator. I just wish that the brake lights only came on when I hit the brake and wish it wouldn't come on for regen.
I've watched it a lot and again on mine this morning, and the brake lights came on just before 30 kW. As most "slowing" type braking for me is 15 kW, they won't come on until I press the brake pedal. I'm guessing the difference we're seeing here is following distance. If the following distance is 0.25 seconds, you'll be on and off the accelerator all the time with the resulting brake light flashing.
The brake lights are allegedly tied to the forward G-force, and are not triggered by the kW of regen (although on the same road at the same spot at the same speed and temperature and barometric pressure you would probably always see it at the same kW). I think this is the perfect way to do it, because the brake lights just come on when they "should".
UberEV1
Member
Interesting discussion - curious to know if brake lights are indeed triggered by G-force or an algorithm on regen. Simple experiments yesterday showed the following (subject to a bit of error since I was trying to watch both the speedometer and car icon in battery screen simultaneously) . . .
Speed > 30mph: Brake lights on @ 30 kW
Speed < 30mph: Brake lights on @ 15 kW
Note: Once brake lights are triggered, they stay on throughout deceleration (as long as car is in regen) until car is at a crawl.
Seemed pretty consistent with above whether easing into regen level or abruptly achieving regen level, which suggests it might be more of an algorithm than G-force. On the other hand, who knows - just fun data gathering.
Hmm, if that's true it does suggest an algorithm, even though Tesla has stated that there are accelerometers that are used for this. I'll try it for myself and report back.
Skotty
2014 S P85 | 2023 F-150L
I think most regen on brake systems are smarter than that. Initially they only only use regen, then begin applying friction brakes when braking needs exceed regen braking capability. In which case your points don't apply. But I could be wrong. Would be nice to get a confirmation on this one way or the other from a hybrid systems design engineer.
I don't agree with some people's claim that Tesla's approach is simpler (for the operator). Old school -- gas means go, brake means stop -- is about as simple as it gets.
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Two pedals would be twice as complicated at one :tongue:
With an electric engine, you could get rid of the brake completely and just have one speed control, but feet/pedals aren't really fine grained enough to handle that. You'd have to control speed with a hand actuated control of some sort, such as pushing a hand lever forward to go faster, pull back to go slower. Probably any number of video games basically use that mechanism.
That's basically the model we use for most things. Would you want two volume knobs on your stereo; one to raise volume, the other to reduce volume?
Brass Guy
Active Member
I'm in the accelerator pedal camp. One pedal for the motor, another for the friction brake. Keep a modest buffer space in front and move the pedal smoothly. In stop-and-go traffic on a highway, this is difficult at best in the left lane. I always move out of the left lane if I see a slow-down ahead. The left lane is prone to jack-rabbit starts and brake stomping fests, and almost always averages less advancing than the other lanes. Not worth the aggravation even if you gain a few car lengths anyway.
If you decelerate with regen just to the point of turning on the brake lights, then push the accelerator a bit, the brake lights will go out even if you're still in regen. They don't stay on just because you're in a tiny amount of regen. I've seen this myself on an empty road at night. You can flash the brake lights by moderating regen.
There is a private thread on that other forum, someone posted that they used an assistant to watch the brake light in the car image while the driver watched the regen kW meter at different speeds, and there does appear to be an algorithm. I'm not sure why it's private, so I won't copy any info here. If the curve can be extrapolated though, at some speed (probably above 70MPH though) full 60kW regen may not turn on the brake light at all. I don't get that fast often, so I don't know. It is agreed though that "brake lights turn on at 30kW" does not always hold true.
If you decelerate with regen just to the point of turning on the brake lights, then push the accelerator a bit, the brake lights will go out even if you're still in regen. They don't stay on just because you're in a tiny amount of regen. I've seen this myself on an empty road at night. You can flash the brake lights by moderating regen.
There is a private thread on that other forum, someone posted that they used an assistant to watch the brake light in the car image while the driver watched the regen kW meter at different speeds, and there does appear to be an algorithm. I'm not sure why it's private, so I won't copy any info here. If the curve can be extrapolated though, at some speed (probably above 70MPH though) full 60kW regen may not turn on the brake light at all. I don't get that fast often, so I don't know. It is agreed though that "brake lights turn on at 30kW" does not always hold true.
Skotty
2014 S P85 | 2023 F-150L
I looked into this a little more. It kind of depends on what system a car uses. Here's a PDF from Bosch that talks about their systems some:
http://www.bosch-automotivetechnolo...ty_1/en_4/CC_Regenerative_Braking_Systems.pdf
Assuming you had a car using their vacuum-independent version, it suggests that the friction brakes only come into play when braking needs exceed what the regen system can handle. As it reads:
"Vacuum-independent regenerative braking systems fulfill the highest level of recuperation and are particularly suitable for plug-in hybrids and electric vehicles. The conventional vacuum brake booster is replaced by an electro-hydraulic brake actuation unit and a brake-pressure modulation system. The brake pedal and brake booster are decoupled, allowing the generator‘s braking potential to be fully exploited. The friction brakes are only used for deceleration when the generator brake torque is no longer
sufficient. Thanks to torque blending, energy recuperation during braking is virtually imperceptible to the driver."
That being said, a driver using such a system will still typically benefit less than having regen on accelerator pedal, precisely because of the imperceptible torque blending. To ensure you don't start using friction brakes when you don't need to, you would have to have some other indicator to say when friction brakes start to engage. Some hybrids have this, some don't. For those looking to maximize the benefit of regenerative braking, having all regen on accelerator doesn't suffer from this shortcoming.
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The Prius New Car Features manual indicates that there is always some friction braking going one regardless of regen level. Also whenever going over rough surfaces while braking, if any of the braking safety features kick in, regen stops causing a pedal feel difference. In the 2010 Prius it was so bad that Toyota was forced to recall them.
Tesla's approach is much simpler:
1. No complex braking system is required. There are many pages of the NCF manual devoted to the braking system and how it works. There's a "stroke simulator" that simulates pressing the brake pedal and has to decide how much regen and how much friction to use. I'm impressed that Toyota got it to work as well as they did, but compared to Tesla it feels clunky.
2. It's far easier when driving because braking action starts immediately rather than having to wait to press the brake pedal.
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