Regenerative Braking Details?

Toyota was leery of introducing the Prius and making it feel too different from a conventional car. So it has that forward "creep" programmed in as soon as you let up the brake, because that's what conventional cars with automatics did. And the computer is full of rules for what it should do when you press the brake this hard or this fast, with the idea that you should feel as much as possible like you're pressing a regular brake, and not notice what's happening behind the scenes (the one time it gets noticeable is in light braking over potholes, etc.).

Tesla had the luxury of coming along later and saying, well, people are used to new ideas now, why don't we just forget the silly fake creep and build a car that doesn't go when you're not pushing the go pedal? By now customers will probably accept it.

 

The new Porsche Taycan uses two pedal control as well. Just like their ICE cars; one pedal for "go", and one for "slow". They do have an additional feature; The driver can switch between zero regen with no pedal being pressed, or light regen ala the Prius.

Aloha,

John

 

When I test drove a Model S, they put it in normal mode, and I switched it out as soon as the first time I came off the accelerator. Very disconcerting to me at least, but maybe could get used to it. Won't you get the same effect in a Prius when you are in B mode? Maybe not as strong as the Tesla, but some "drag" every time you lift off.

 

Does it really "turn on" the engine? I am under the impression that it uses the MGs to spin the engine without burning gas, a real engine brake.

 

Maybe the BMW i3 does this also. I remember hearing folks talk about the drag experienced when they lift off the throttle. But who cares, nobody buys an i3.

 

Only in EV mode. Otherwise, it'll spin the engine and use it as an air pump in B mode to increase deceleration w/o using the brakes. At least, that's the case in the Prime. Not sure what the other Prii do in EV when put in B mode. Either way, the regular Prius battery will not hold much energy on a long mountain decent.

 

Either way, it results in you feeling some drag when you let up, so if that's the effect in question, then yes, you get that effect.

 

Not sure, but my recollection at least is that the engine didn’t sound much different from when it runs normally.

 

Indeed it does sound about the same. Four cylinders, 1.8 liters, pumping air through some pipes into a muffler.

 

From what I am reading....
Pressing the brakes lightly will NOT engage the brake pads, but will regenerate to the traction battery.
Sliding the gear shift into "B" to slow down will also regenerate (assuming EV mode is active).

So, if I just need to slow down coming to a traffic light (ie not slam on the brakes, which would definitely engage the pads), either way will do the same thing? I'm trying to regenerate the most energy without using the brake pads. And sliding to B is a little annoying if just tapping the brakes will suffice.

Am I correct in the above?

Thanks!

 

This brings up an interesting question. Does the adaptive cruise control turn on the brake lights when it brakes if someone in front of you slows suddenly?

 

That's a hard yes.

 

Lots of great discussion on regenerative breaking. I'm on my third Prius now, 1st was a gen III, 2nd was a gen iv, and 3rd is a prime. So here's what I have learned in nearly a half a million miles of driving the Prius. Regenerative breaking is simply using the motors (MG1, MG2, and IG) to slow the vehicle. A mild to moderate amount of braking will transfer all the kinetic energy into the motors to product drag and produce power to the traction battery. Moderate to Hard braking will do this as well, but you'll also have the traditional disc brakes kick in for additional stopping power (wasted kinetic energy). This is why the eco tracking systems on all the models love a nice long coast and a very slow deceleration. On the Prime, I find that the sweet spot is when the bar is just shy of reaching the bottom on the energy level meter, which is a slightly higher than modest level of braking. The traditional brakes also kick in when you are at about 4mph to finish the stop. You can hear them kick in if you decelerate slowly and have the car in EV mode with the radio and AC off. Now, I must admit, I haven't done any mountain driving with the Prime yet. Oddly curious to do so though. But in the previous models, when you are driving mountains, downhill is where life gets interesting. Coasting or even using braking to maintain speed shouldn't need the traditional brakes, because all that kinetic energy is being routed through MG1 and generating power for the traction battery. I suspect the Prime does the same, only with a charge max of 80% SoC (same as charge mode?), but like I said, haven't gotten to play with that yet... On normal Priuses, the traction battery is smaller, so it will max the charge to the max allowed SoC. Once it hits that limit, the car will transfer the kinetic energy to the IG. The engine will sound like its running, but there's no gas being used in the process. This is a similar method to what you'll see semi's doing. If you need additional stopping beyond that, then the traditional brakes are used. The benefit to all of this is that if you don't do sudden stops, then you'll only use the traditional brakes occasionally and when stopping at low speeds. My Gen II, I went 150k miles on the OEM brakes, and probably could have gone another 50k more. It was just coming up on the second change when I traded it in.

 

In the mountains, you'll likely hit the Charge Current Limit before the battery fills up. Normally the car allows around 40kW into the battery when it's below around 80%, and more like 20kW when it's fully charged. That's at summer temperatures. If the battery is below freezing, it might be as low as 2kW. If you charge the battery too long and/or too fast, the charge current limit will rapidly drop. If you call for braking that is close to or over the limit for too long, the engine will come on. The car will still put energy into the battery up to some percentage of the limit, and the engine will dissipate the rest of the power.

It is possible to charge it past 80%, but the higher you go, the more the charge rate is limited. You'll be wasting more energy to engine braking and friction braking as you approach 100%.

There is apparently an algorithm that reduces the charge current limit based on state of charge, charge rate, how long it's been charging, battery temperature, etc., to protect the battery. Contrary to popular belief, the problem is almost never that the battery gets too hot. Even on long mountain descents, the battery temperature might only rise a few degrees. Maybe some internal part gets too hot and they have a thermal model to set the charge current limit, but the reported temperature is very stable. The cooling system is weak in my opinion, but apparently the battery is efficient because it doesn't seem to generate that much heat when charging or discharging.

I use the Hybrid Assistant app to watch the charge current limit (actually shown as power not current), battery temperature, etc.

 

Do keep in mind that lithium ion battery chemistries tend to be endothermic during charging -- the charging process actually cools the battery. This may help control battery temperature while charging. Though I don't know how this is expressed for the Prime's particular chemistry.

There is no free lunch, the heat comes back out during discharge

Also keep in mind that this doesn't negate the I-squared-R heating component, it would just offset (some or all of) that resistive heating. And if I recall correctly from my work days, the endothermic cooling may not be uniform throughout the full charge cycle, but may be weaker or even switch to exothermic near the bottom or top ends.

 

When I let off my feet from gas padel, speed of Prius v lowering automatically. It seems something is slowly pulling back from the behind. Is it affected from the regenerative brakes?

 

When I let off my feet from gas padel, speed of Prius v lowering automatically. It seems something is slowly pulling back from the behind. Is it affected from the regenerative brakes?

 

When I let off my feet from gas padel, speed of Prius v lowering automatically. It seems something is slowly pulling back from the behind. Is it affected from the regenerative brakes?