regen braking versus regular braking

Braking is all regen unless one or more of the following is true:

1) Too slow for effective regeneration (around 7 mph).

2) HV battery high SOC limit is reached.

3) HV battery charge limit reached. Note that this will vary depending on battery temperature and other factors.

4) Panic stop.

5) Loss of traction forcing transition to ABS friction braking.

In other words, the Prius will always try to brake using only regeneration. Panic stops and loss of traction force the Prius to switch to friction-only braking. Normal braking will use only regeneration unless more braking is required. Then it will use a combination of regeneration and friction as needed. At very low speeds only friction braking is used.

Tom

 

Does the regenerative braking decrease brake pad life or not affect it?

 

Give you a guess

One of the benefits of the Prius is that the friction brakes should last a very long time compared to 'normal' cars. Personally I've gone through a set of front pads at 35k miles but most of my driving is in town and a lot of it below the 7 mph limit. They weren't fully out but were below the minimum for my annual taxi test. If your driving is mainly highway then you could probably make 80k - 100k miles on one set of front pads. I've read on the forum that some have even passed this mileage!

 

Holy crap, that is way more than I am used to. I thought there may be some kind of degradation but I guess not!

Judging how I drive and my short commute I'll probably get around the same, I love to regulate my braking a whole lot instead of just making a kind of gradual slow stop (bad habit I've had forever).

Thanks!

 

No. In fact once regen is reduced (full battery) the charge indicator will go down to "0".

 

Fine to use regen instead of friction when you can, just know that each use of regen is still cutting fuel economy compared to not braking. So don't use regen as much as possible (literally). Try to anticipate traffic flow ahead and glide from a lower top speed, being reasonable with other cars on the road.

 

I think what Harry really wants to know (or at least I do), is how can you tell, gauge wise, when you've surpassed regen and gone into friction braking. AFAIK, you can't. Scangauge?

There are a couple of times on my daily route where I have to "brake hard" because I can't glide due to road speeds (hwy) and traffic conditions. Since the braking side of HSI screen doesn't go into a red zone like it does on the PWR side, I'm just guessing each time. Kinda' like folks who live at the bottom of a hill, this is a daily situation I encounter and want to maximize my regen efforts.

 

Some of us get that on non-hybrids. And yes, many (though certainly not all) longtime Prius owners have reported far greater distances. I don't expect to know my results for many years.

 

I'd love a simple explanation of how exactly braking (regen and friction) works on the Prius! Do the actual brakes (brake pads meet rotors) not apply under light braking? Do they kick in at a specific pressure of the pedal? Are they actually being applied simaltanously with the regen braking?

And what exactly is regen braking? Is it resistance of the electric motor as it acts as a generator? Resistance of the ICE (engine braking)? Am I asking too many questions? Am I better off not really knowing?

 

All regen comes from the MG's working as a generator. Engine braking is used to slow the car sometimes, but the energy shifted there is wasted as heat. That is why B mode should not be used except where the pack is or will be full as on a long descent. Engine braking is used even in D above 45 mph (gen2 42 mph), to protect MG1 from overspeed condition.

 

Yes regen braking is the resistance of the electric motor as it acts as a generator. The force varies with brake pedal pressure. You have to press quite hard to make it necessary for the pads to engage at speeds above ~10 mph and regen will still be active. Below 7 mph it's only using the pads.

In the Prius shift to B mode for engine braking on long and/or steep descents. A rare event for most people.
For engine braking the new 2011 Lexus RX450h has an S position that has 6 "speeds". You use the gear shift like a sequential shifter to go up and down. Not sure what is actually going on mechanically yet with this. The rate of deceleration under heavy but still regen only braking in the AWD model is quite stunning and unexpected. You get a power meter to see what level your getting as you press the brake.

 

I need to ask a question which is maybe already in this thread and maybe answered, though I haven't seen a clear answer to this.
The CHG area what does it exactly indicate? Meaning: if I am braking within the CHG area, above 7mph, am I braking only and *always* (when above 7mph) with regenerative braking?
What does the length of the CHG bar indicate? Does it show that when I am completely filling it and brake further, when above 7mph, that I am using friction brakes? does it, in a way, show the amount of current being generated while braking with the full CHG area indicating, as per above posts, the battery limit of 55amps?

 

Typically, but not always. I believe if ABS switches on due to a slippery surface, the CHG area will still illuminate, but it has to use friction brakes. There are various other cases as well (battery full or overtemperature, for example) that will result in regen being reduced or disabled, but I don't think the car will indicate this to you. If you must know, you can buy a ScanGauge - but Toyota didn't want you to be confused by the CHG bar not always behaving the same. You can't really do a whole lot about most of the conditions that cause it to use friction brakes instead of regen, so I wouldn't worry about it.

Yes, more or less. Again, it's not a direct indicator of battery amps, but it's a good first approximation.

 

Regen takes a moment to come on to full strength, so if you apply the brakes quickly (but a light amount) you sometimes get a burst of friction before the ECU transitions fully to regen braking. But you apply the brakes smoothly and keep the "CHG" indicator as full as possible (without filling it up) you are not using any friction brakes at all.

Once you approach 7mph the ECU smoothly ramps up friction and removed regen. (I think by 3mph you are fully on friction.) Also, as other people have stated if ABS comes on it dumps all regen but the CHG indicator is swill still fill up.

To force dump regen, drive about 25mph and fill up the CHG indicator to almost full, then switch to Neutral without changing the brake pedal pressure. That will dump all regen and move to full friction. You will feel a slight pulse as this happens, and if the friction brakes are totally cold braking force may change. The ECU auto calibrates friction braking each time you use them... Sadly friction brakes are quite inconsistent depending on their temperature and if the discs are dirty or wet... so usually the first few times friction is used it will be strange, especially during the transition from regen to friction at 7mph. You will feel this as a braking surge or perhaps less braking force as friction is applied. After a couple of stops, though, the ECU learns the feel for the friction brakes (plus the discs and pads get cleaned off) which will make things happen smoothly.

I make it a habit to always use full regen and as little friction as possible. This will maximize your mileage and battery SoC. When I get home from work (a commute in city traffic, 35mph or less....) my front brake rotors are totally cold to the touch. The pad and rotor life will be really long. I often go a long while between car washes (I park in covered areas at home and at work) and I have never seen any brake dust on my wheels because the friction brakes are used so little.

Note that you can charge a max of120amps into the batteries using regen (80-90 is a more typical max through) ... But at low speeds this regen force will feel much stronger. At highway speeds 120amps can be attained with barely any slowing down (the CHG bar will fill up quickly) ..... but as you slow down towards 7mph you will notice that the CHG bar will empty on it's own. You can push down harder on the pedal to try to keep the CHG filled and you will slow down more quickly but you are still not using any friction brake. Just keep CHG a pixel or two from being filled... applying more pressure as you slow down to keep it filled and maximize the charge. It's fun and will become second nature.

It's a pretty amazing system and what the ECU is doing is so complex but it does it in such a seamless way.

 

For those who live in northerly climes, with lots of snow and heavily salted roads, getting the pads and rotors cleaned off regularly is a good idea. Whether you wash them off with water (laborious) or apply friction braking (wears down the pads and rotors) is up to you. At 60k miles I had to have the rotors turned because they were rusted and pitted (from lack of use!).

 

I check about once every 1-2 months during winter if the brakes have ever been really used (read abused... ) and if not I will brake strongly 1-2 times, say on a highway and/or drive a few meters with the had brake on at very low speed to simulated a rear-friction brake "event".

 

OK, I am confused....how can using regenerative braking cut fuel economy compared to not braking, or coasting? Using the regen charges the battery more..allowing me to rely more on the traction motor and less on the engine.

 

Your regen charging energy originally comes from the gasoline engine. It is more efficient to not burn that gasoline in the first place, if you have a way to avoid it.