echoreply said what I was going to. The standard approach in an EV is to go at a normal speed until you get reasonably close to the stop and then let regen slow you down. This typically corresponds to a comfortable level of braking, so it’s considerate to other drivers as well.
In any car, if you are engaging the friction brakes, you are wasting energy. In an ICE, to avoid that you have to coast and give yourself a very long distance to slow. But EVs have regen, and can slow much more rapidly without using the friction brakes. They gain all the advantages of coasting without the downsides.
The BMW i3 can coast. It has a display that shows when power is coming from the battery during driving and when it is going to the battery during regen. In the middle is the sweet spot where you are coasting.
The inverter is a fundamental part of the motor and shouldn’t really be thought of as a separate thing that can just shut off the power.
If the coils are left open, then you’ll get something akin to coasting. But that’s a specific action the inverter has to take, not like just cutting off the flow. Another way of shutting off the power would be to close the coil circuits, in which case you’d have a massive braking effect (and probably destroy something in the process).
This isn’t some theoretical distinction, either. The new Tesla inverters include a pyrotechnic disconnect to open the coil circuits. Why? Because the inverter may fail in a way that the coil circuits are closed. If that happens on the highway at speed, it could be dangerous (like slamming on the brakes). If the inverter detects a failure like that, it blows a fuse so that the coils are forced open.
So the inverter is always actively managing the power delivery. Yes, it could maintain a “coast” by ensuring the net power going into the motor is zero, but that’s just a special case. No reason to think there’s anything magical about that setting as compared to any other.
what would make it seamless for me is coasting with the accelerator pedal released and a stepped brake pedal that starts out regen. It’s just swapping the process between pedals. I think it’s better to have the brake pedal control braking as a part of a well defined function.
Something that seemed unrealistic was the regenerative rate of between 17% and 70%. If anyone has a good source for real numbers that would be an interesting read. 70% just sounds impossible.
Do you mean the efficiency in energy recovery? 70% is not unrealistic. Round-trip, regen loses about double the overall drivetrain losses, because the motor acts as generator and then a motor, and the battery is first charged and then discharged. But 15% losses are reasonable for one-way efficiency (the motors are easily >90% efficient, and so is the battery). Doubling that gives ~30% losses, or 70% efficiency.
It’ll vary based on a bunch of factors of course, but 70% is probably a reasonable average.
Maybe I completely misunderstood the numbers. I thought they referred to the amount of charge that went back to the battery as a percentage of full charge. I always assumed that was a pretty small amount.
That’s really what I’m looking for. What do I get in return for regen braking.
Yes, that’s around 70%. It’s just that most of your driving losses come from air and tire resistance, not braking. Regen helps the most in city driving (constant stop and go) and going downhill.
If you lose 10% of your battery going up a big hill (slowly, so air resistance isn’t a big factor) and regen all the way down, your battery will charge back up by about 7%.
Checking how I drive yesterday - yes, I keep a light pressure on the pedal to maintain speed, not much different than coasting. letting off the pedal is like medium-light braking.
Ignoring the comfort/expectations/old man yells at cloud side of the question, if there was a power/range advantage to coasting versus regenerative braking then electric car manufacturers would be doing it. Even in IC engines the equivalent of coasting in neutral is only advantageous in very specific circumstances because coasting in gear can keep the engine enough above idle speed that it cuts off the fuel injectors. That’s called “DFCO” or deceleration fuel cutoff, which happens somewhere around 1,000 RPM, and again, it saves more fuel than is otherwise lost because of the additional engine braking and reapplication of power that’s necessary.
so if you lost 50% of the battery going uphill then you would arrive at the bottom with 50% +.7*50% for a total of 85% charge. that would be substantial. If that were the case then highway driving would greatly reduce the range over city driving.
Yes, that’s right. EVs see most of their losses from air/tire friction, so they generally get the worst efficiency at high highway speeds. The best efficiency I see is actually in dense highway traffic at low speeds, say when it’s slowed to 30 mph or so.
Not something I can easily answer, since regen is always on in a Tesla. I’d have to brake super hard all the time so that regen isn’t having much effect.
In my experience, brake usage is still required once or twice on most drives. Someone cuts you off, you misjudge your speed, nearly pass a driveway, etc.
My hybrid actually does get more milage off the highway. It’s rated at 41/41 but in practice with the ac off I get about 38 at 70 mph and can get up to 50 while driving around on regular streets when there isn’t a lot of traffic and I can drive efficiently. I’ve actually got up past 60 round trip when hypermiling just to see. Air resistance scales up fast with speed. Most cars are most efficient around 35-45 mph and even the difference between 60 and 70 can be substantial.
Regarding regen efficiency: I’m not sure if this information is terribly useful, but it’s an EV thread where I haven’t posted in a week, so I must speak!
This is a real world example of regeneration on big hills. These are real numbers, but I’m not exactly sure how to manipulate them to get to a regen efficiency number.
From my house to the Loveland Ski area (on top of the continental divide) is 65 miles and about a one mile elevation gain (5500 feet).
Driving to Loveland uses: 23kWh which is about 100 miles of range or 35% of my battery
Driving from Loveland to home uses: 5kWh which is about 22 miles of range or 8% of my battery
Both directions the average speed is between 55 and 60 mph.
If I only look from Idaho Springs to Loveland, where it is mostly a constant climb, I use about 20% of my battery, going from 75% to 55%.
If I only look from Loveland to Idaho Springs, I use about 1% of my battery.
The graph of the kW being used bounces above and below the 0 line.
An ICE car with DFCO will use very little gas from Loveland to Idaho Springs.
OK, I get it now. 70% recovery is of the kinetic energy recovered. So if you’re flying down the road at 60 mph and you brake for 10 seconds then you recover 70% of the kinetic energy needed to maintain 60 mph in those 10 seconds.
Using really bad math you’re traveling at 60 mph or 1 mpm. 10 seconds of braking would equal 1/6 of the energy to travel a distance of 880 feet. 70% would be 616 feet’s worth of energy in that 10 second span.
I’m trying to mentally see how much free energy could be recovered. If over the course of a 300 mile full-charge range it recovers 5 miles of travel that adds up over time.
I’m not sure what the criteria/formula they use to calculate energy recovery.
In my car, it tells me that I recovered between 0% and 100% energy during a brake, but I assume the meaning is “the percent of the potential energy you could have possibly recovered during this stop” - this is part of the “brake coach” that tries to teach you to drive efficiently.
100% means “you braked slowly enough that the generators slowed the car fully and the brakes never engaged the brake pad” and 20% would mean “you stopped hard and used almost all mechanical brakes and the generators barely had time to recover any energy”
I realize you guys are talking about something else, which is the theoretical efficiency of how much energy you can recover by using a regenerative system, but I was just pointing out the numbers the car may give you, if other brands to give you numbers like that, are probably related to potential energy recovery and training you to drive more efficiently.