High acceleration reduces range. but Why?

[RegGuheert]

One thing that should be mentioned is the likely possibility that accelerating at full throttle in the LEAF causes the battery to degrade faster than accelerating at lower throttle settings.

 

[johnqh]

I think you meant the heat loss in battery (and the whole electric system) is proportional to the square of current flow.

I meant what I said.

Many of the losses in the electrical system are NOT proportional to the square of current flow. Some are constant. Others increase linearly with drive current. There are likely other relationships, as well.

If you look at the electrical system only (battery and wiring to the motor), the electric energy either pass to the motor and become kinetic energy, or become thermal energy, and that thermal portion is proportional to the square of current flow.

Once it is kinetic energy, it can become thermal energy too and that is linear to the RPM(?), but that's outside the electrical system.

There is a little loss to the sound energy too, but that's minimum on an EV.

What other form of energy loss is there in the electric system?

 

[RegGuheert]

What other form of energy loss is there in the electric system?

There is the inverter which drives the motor. This uses IBGT switches which have conduction losses which are fairly linear with current. These IGBTs also have switching losses, which are dissipated in the circuits which drive the IGBTs. This switching loss is largely a fixed amount of power, but it does change with battery voltage.

 

[TickTock]

It is a commonly known fact that a heavy foot on the accelerator pedal will reduce range, but It would be nice to know the physics behind the fact.
In a very simplified point of view, the same amount of energy is required to bring a vehicle to a certain speed regardless of the acceleration. With this in mind, the rule does not make sense.

Can anyone with more knowledge explain this?

It may be commonly "known", but it isn't a fact. Rapid acceleration has only a minor impact to range - almost imperceptible compared to speed.

 

[Ingineer]

This is really simple to imagine, even without taking into account all the complex math, systems losses, etc.

We all know that travelling at a faster average speed uses more power. So imagine you have a 1 mile "course". If you rapidly accelerate and your average MPH over that one mile is, say, 80MPH, it's going to take way more energy than if you accelerated very gradually and averaged 40MPH over that same 1 mile distance. Simple and intuitive to "get"!

-Phil

 

[johnqh]

This is really simple to imagine, even without taking into account all the complex math, systems losses, etc.

We all know that travelling at a faster average speed uses more power. So imagine you have a 1 mile "course". If you rapidly accelerate and your average MPH over that one mile is, say, 80MPH, it's going to take way more energy than if you accelerated very gradually and averaged 40MPH over that same 1 mile distance. Simple and intuitive to "get"!

-Phil

Sure, you will use more energy /hour if you average speed is 80, but you will also spend 1/2 less time than if the average is 40.

 

[Ingineer]

When we drive, we are usually going with a certain destination in mind, sometimes we have a time constraint and plenty of extra battery capacity, so we drive faster and use more of it. More commonly, we have a certain distance we wish to travel, we must then increase our efficiency in order to make the trip without running out of power before the destination.

In the real world there are hundreds of losses in all of the various systems at play here. Some are easy to calculate, some are extremely difficult. I know unequivocally that accelerating faster means my average speed between 2 fixed points is going to use more energy, even if you were to divide it over time.

Of course we don't have the luxury of inventing physics such as "say there was no air resistance", etc, because we live in the real world.

I2R losses in all the electrical systems alone flat out are higher at higher current levels, so that alone will explain why you can never achieve a break-even if you divide out the energy/time.

-Phil

 

[EVDRIVER]

Well, with my front mounted fan generator I actually get more power to the pack when I go faster so I guess that kinda ends this debate. Look at the big picture folks :roll:

 

[ebill3]

Well, with my front mounted fan generator I actually get more power to the pack when I go faster so I guess that kinda ends this debate. Look at the big picture folks :roll:

At last - sanity and something the masses can understand. :lol:

 

[smkettner]

Everything is less efficient at full throttle. Battery, wire, inverter, motor, drivetrain.... all of it.

 

[TickTock]

It is easy for our intuition to mislead us. It certainly *seems* like accelerating faster should be worse but this is only true because generally it is accompanied with faster speeds. If you take take a one mile track and drive such that you average 40mph, it will take 90 seconds to complete the lap regardless of your acceleration. Now, if you accelerate slowly, you will have to drive faster at the end of the mile to average 40mph. If you accelerate fast, you get to speed faster and can actually drive slower to still average 40mph (or 90 seconds). Since speed is a much greater source of losses, you are better off in this scenario accelerating quickly. I, too, didn't believe this when it was first suggested in the thread below so I put a model together to prove the OP wrong but ended up proving him right.

Faster acceleration vs slower acceleration


and here below is come quick math illustrating how small the impact of acceleration is compared to speed:
Rapid Acceleration and Battery Degradation: Facts- Opinions

 

[Yogi62]

That graph of efficiency vs motor speed and torque is cool data! Have to think about a bit...

I don't think it is given that if you accelerate slowly you then try to average the same speed. I am asserting that if you reach the same top speed, you will average a lower speed when accelerating slowly and cover less distance at lower efficiency high speeds.

 

[TickTock]

That graph of efficiency vs motor speed and torque is cool data! Have to think about a bit...

I don't think it is given that if you accelerate slowly you then try to average the same speed. I am asserting that if you reach the same top speed, you will average a lower speed when accelerating slowly and cover less distance at lower efficiency high speeds.

True. Noone is arguing that if you take more time to get there your efficiency won't improve. The bottom line is it all comes down to speed and acceleration has very little impact and even is beneficial for the (admittedly unrealistic) scenario of same average speed.

 

[walterbays]

In theory there should be no difference in energy required to accelerate to a given speed, whether rapidly or slowly. Tests show a small difference. http://www.efficient-mileage.com/acceleration-test.html

It may be that one reason we seem to observe a larger difference is that the battery delivers energy more efficiently at low discharge rates than at high rates. http://en.wikipedia.org/wiki/Battery_(electricity)#Capacity_and_discharging

I imagine a bigger reason is that when we accelerate quickly we are more likely to need to brake soon, for traffic or stoplights, than if we accelerate slowly.

Then there's Berra's Law: "In theory there is no difference between theory and practice. In practice there is." - Yogi Berra

 

[Nubo]

One thing that should be mentioned is the likely possibility that accelerating at full throttle in the LEAF causes the battery to degrade faster than accelerating at lower throttle settings.

Probably not significant. Full throttle is putting less than a 4C load on the pack. It's not even breaking a sweat.

 

[harryjpowell]

Wouldn't the actual test that pertains to the OP be like this:
Make two runs over the exact same distance
Complete the exact same distance over the exact same time
In one run use full acceleration to a defined speed
In the second use a slow acceleration to a different speed

Just end the test while at speed at the distance mark.

Obviously trying to eliminate all the other know variables like different avg speeds, regen etc. Just measure the efficiency difference between rapid acceleration and slow acceleration. To me you would have to lower the top end speed on the rapid test to eliminate a delta in the avg speed..

Someone should be able to math this out to a test...

 

[TickTock]

Wouldn't the actual test that pertains to the OP be like this:
Make two runs over the exact same distance
Complete the exact same distance over the exact same time
In one run use full acceleration to a defined speed
In the second use a slow acceleration to a different speed

Just end the test while at speed at the distance mark.

Obviously trying to eliminate all the other know variables like different avg speeds, regen etc. Just measure the efficiency difference between rapid acceleration and slow acceleration. To me you would have to lower the top end speed on the rapid test to eliminate a delta in the avg speed..

Someone should be able to math this out to a test...

Definitely could be done. If you look at the green curve in the graph in the link below, it tells you what your top speed should be at the end of a 1 mile test for different acceleration profiles to keep a constant time to traverse the mile. Would have to put the Leaf into Eco mode (constant power acceleration profile) and figure out the mph/s --> kW conversion.

Faster acceleration vs slower acceleration

 

[TickTock]

Wouldn't the actual test that pertains to the OP be like this:
Make two runs over the exact same distance
Complete the exact same distance over the exact same time
In one run use full acceleration to a defined speed
In the second use a slow acceleration to a different speed

Just end the test while at speed at the distance mark.

Obviously trying to eliminate all the other know variables like different avg speeds, regen etc. Just measure the efficiency difference between rapid acceleration and slow acceleration. To me you would have to lower the top end speed on the rapid test to eliminate a delta in the avg speed..

Someone should be able to math this out to a test...

Definitely could be done. If you look at the green curve in the graph in the link below, it tells you what your top speed should be at the end of a 1 mile test for different acceleration profiles to keep a constant time to traverse the mile. Would have to put the Leaf into Eco mode (constant power acceleration profile) and figure out the mph/s --> kW conversion.

Faster acceleration vs slower acceleration

Actually, no conversion is required if you have a timer visible to the driver. I would suggest the two tests be at 2mph/s and 7mph/s.

For the slow accel test (2mph/s), you can read on the green curve that your final "cruise" speed should be 53mph and on the red curve you can see that you should take 25 seconds to get there. So the driver can monitor the timer to adjust their acceleration to hit 53mph in 25 seconds and then cruise at that speed for the remainder of the mile.

For the fast accel test (7mph/s), the final cruise speed is 47mph and you should get there in 7 seconds.

Record the %SOC before and after each as well as confirm the total time for the mile remains constant for both experiements (80seconds).

 

[gbshaun]

That's an awesome graph, thanks (I had seen it in your 2011 post).

My anecdotal input is that accelerating rapidly, but slightly tempering the cruising velocity so the overall time is the same, makes no noticeable difference in range. That's how i drive much of the time.
Plus it's fun getting up to speed quickly.
Plus it helps dispel the old myth of EV's being slow.

Did i read somewhere that 2013 LEAFs don't have the acceleration of my 2011? IMO that would be a mistake unless there was something gained elsewhere, such as 5% more range due to different gearing.

Shaun

 

[gbshaun]

Presumably there's also no reason to go abnormally slow when climbing a large hill (unless remaining charge is SO low that every aerodynamic advantage needs to be harnessed), correct?

I'm planning sometime to do the trip via the Level-3 charger at Santa Ysabel down to Borrego (L-2 chargers), meaning a long climb back up Montezuma grade or Banner grade. 3600' gain http://goo.gl/maps/jyHLp" onclick="window.open(this.href);return false;

Shaun