Best way to brake / decelerate?

You will get better mileage coasting to a stop. If you maximize charging, you are also maximizing conversion losses: kinetic energy to mechanical energy to electrical energy to chemical energy and back to electrical energy to mechanical energy to kinetic energy. Each of those conversions eats a piece of your energy pie.

Coasting uses the kinetic energy directly without any conversion losses. Of course the problem with coasting is that you need room to coast. If you have to stop suddenly or have someone riding your rear end, then it won't work.

Tom

 

Gen II braking question

Sorry to take this OT since this is a Gen III thread. Do you know what are "optimal" values for braking on the Gen II? I'm looking for either bps (brake position sensor %) values or BTA (battery amps) values I should strive for.

IIRC (on vacation now w/o my Prius), at say 35+ mph, if I take my foot off the accelerator, on level ground, I'll usually have ~19 amps of regen w/o pressing on the brakes.

 

I highly doubt my battery is failing. My prius is only 4 months old and barely 5k miles. Sure I can alter my driving habit and keep my battery at 4-5 bars maximizing EV but then I would have to drive like a grandma and piss off everyone behind me. Then when I get to a parking lot or a drive through the 4-5 bars will quickly go down to 2 bars and ICE would fire up to charge. I'd rather drive aggressively and keep 6-7 bars on my battery and get 55-60mpg than drive like a grandma and maximize EV then have the ICE kick on to charge the battery and get the same 55-60mpg.

 

My guess is that the most efficient way would be to brake as much as possible as early as possible, then glide (no CHG, no power - light accelerator pressure) to a stop. This minimizes several losses - first, any fixed losses in the inverter are minimized by braking as quickly as possible without using the friction brakes. Second, it minimizes the loss from air resistance, so some of what would have otherwise be lost to air resistance is instead in your battery.

In reality though, I tend to brake moderately, usually preceded by releasing the accelerator to get the natural regen drag for a few seconds. I do try to time the lights as much as possible, and start coasting early when I know I have to stop.

 

yea i get 7 bars alot too. it doesn't mean anything. And surely doesn't mean the battery is going bad like the other poster said.

 

There are several reasons why you can get 7 bars, only one of which is a failing battery. Generally the SOC won't stay that high unless you have cold weather, or some other reason that forces the ICE to run a lot.

Tom

 

I have no idea where I lost you. I realize that every drive must include at least one stop... and likely many more. I also realize that most drivers realize that any kind of stopping or slowing is bad for FE (actually the getting going again, but that's splitting hairs). All I'm asking - the question that remains unanswered (though I can at list infer the answer now) was this:

Do you wish to maximize battery charging, or do you wish to maximize FE. They are not the same. You would drive differently (and driving includes slowing and stopping) to accomplish one or the other.

Really I'm trying to help here, though it sounds more and more like I'm just trying to be annoying. This question is important to understand... and I can't understand why it hasn't been answered!

Of course there are! We're just trying to figure out if you wish to maximize battery charging or FE.

Yay! I'm glad I'm not the only one who's noticed. Thanks Bisco (and you others who are dilligently trying to answer the question that we all *think* has been asked!)

Agggh! Who has said this? Where? If the question is, "how do I maximize battery charging," then you absolutely should maximize charging when stopping. But does anybody here think that's the question being asked? If, as I suspect, the question is, "how do I maximize FE when stopping," then the answer is not as black and white. What is best for charging is not always best for FE. And it will depend on the specifics of the situation - both before, during and after the braking.

Indeed! These are the (seemingly) relevant posts about how to save fuel... not to maximize battery charge. And again... we still need the question before we can come up with a relevant answer.

Dr. Miki - Help us help you! Right now all we seem to be doing is answering different questions or arguing over what the question might be!

You specifically asked about maximizing battery charge. But I suspect that what you wish to do is maximize FE. Is that correct?

 

Never mind. Of the options presented for the question asked, I choose answer C.

That was way easier.

 

i think ggogle has a language program. but seriously, who thinks about charging their battery vs mpg's? gotta be talking about the same thing. i hope. i think yor avatar says it all.

 

I understand that B uses engine braking, not motor braking, thus if anything it may decrease battery regen.

I always stop if I can by maxing out by not exceeding the chg meter.

I can only hit full bars on either a long hill OR aggressive driving if part of that driving includes long braking periods, thus the gas motor disproportionately motivates me to speed and regular, long slow downs not exceeding chg.

 

Above a certain speed, it uses both engine and regen braking. That is why Ken suggested switching to B only below 25 mph, where it uses regen only (unless the HV battery is full).

 

I know that battery charging is less efficient at higher charge rates due to internal resistance and polarization.

I'd think that you want to regen brake at constant power (not constant decel rate), said power to be that which gets you stopped at the right place.

This would involve gentle regen braking (lower decel rate) at higher speed, progressing to more decel rate at slower speed.

You can do this by varying pedal pressure to hold the regen bar length constant.

Below 7 MPH, it doesn't matter what you do, you're on friction brakes at that point.

 

It's clear that no one knows the answer. I'll be the first to state that I don't know. (I'm using the thoroughly-researched-Bob-Wilson meaning of the word ).There are surely many factors affecting this. I think if we could watch our brake temperature then we'd know if friction braking is happening. But I think we would have gotten a lot farther here had the question read something like this:

I'm going down the road, at posted speed, say 40 MPH (or 25 km/hr, OK?). There is a "T" some distance ahead. I will need to stop when I get there. Better yet, the road has a gentle slope downward, and I am in a stealth glide and my speed isn't changing. What's the most efficient way to stop?

 

yes! the doc finally admits he wants to max fuel efficiency. thank you!

 

I want to rephrase the question for OP, and maybe my answer will make sense:
is it best to slow down over a longer distance ?

The answer is yes, because it allows the driver to do the following, in decreasing order of maximizing efficiency:
1. Use road friction to slow down ('gliding', meaning *very* slight fuel pedal depression)
2. Use 'engine braking' (mild regen) to slow down ('coasting', no pedals depression)
3. Use light braking (maximal regen) to slow down
4. Use heavy braking (wheel friction) to slow down

The best way to brake to a stop is mostly a matter of anticipatory driving.
If you want to talk subtleties, here are two:

ICE spin regardless of petrol burn takes about 2 kw. So if I am traveling over 41 mph in my G2 Prius and a light turns red ahead, I slow down relatively quickly to 37, then start the sequence I outlined above.

The discussion whether too light regen exists is interesting but I ignore it because of the risk of burning off too much speed, either because the road condition changed (the light turned green!) or my misjudgement of distance.


Overall in light to moderate city traffic driving I usually hit low 70's mpg in temperate weather when the car is warmed up, so I do not think I am missing too much opportunity.

 

Sage has summed it up nicely. And now that you're stopped in the most efficient way relative to the circumstances... the next task is to get going in the most efficient fashion (hint: It isn't using much - if any - battery power).

 

SageBrush's answer is a little surprising; he seems to be saying that if a light turns red you should lightly hit the brake and then lightly press the accelerator (to use regenerative braking to reduce speed by 5 mph or so and then "glide"). Almost all of my experience driving has been in conventional cars, so maybe I have the wrong intuition, but I would never touch the accelerator if going at normal speed and a light turned red up ahead. Facing a red light, can it really be better to "glide" and then stop abruptly instead of coasting and then gently braking?

 

Yes. Most cars have a fairly significant amount of engine drag when you release the accelerator, which people are used to using to slow down a little bit at a time. The Prius will coast with the engine off, so there's no engine drag. However, to simulate the engine drag, it applies a bit of regen with the electric motors. By pressing lightly on the accelerator so the HSI shows nothing in either the CHG or the green part of the bar, then you're truly gliding - there's no engine drag, and you're not applying any power (either electric or gasoline) to the wheels. If they hadn't created the fake engine drag, that's what the Prius would feel like when you let go of the accelerator.