Does fast acceleration cause battery degradation?

[DaveinOlyWA]

Wouldn't accelerating quicker use more of a charge out of the battery? Causing you to recharge the battery more often, thus you would blow through quicker cycles and hit EOL quicker. This is the same reasoning that people use to claim that regen braking is bad for the car.

it is possible since higher current caused by quicker acceleration could require more power due to changes in resistance

and regen is "bad" but there is a level of "badness" to all but the utmost of the elite. iow, if it aint perfect, there is "something" wrong with it.

but regen is much better than any other option for slowing the car in most cases since it does regain some of the power expended to get you to up to a level of velocity you desire.

if not for regen, you would lose power in heat due to friction brakes which is a no win situation.

when saying regen is bad is only saying that keeping a constant velocity OR very gradual changes in velocity is better which we all already know.

but the biggest detriment to accelerating quickly is the lack of fine speed control. it is simply easier to overshoot your target which in essence wastes energy due to expending more power than was needed and paying the penalty for outside forces like wind resistance and what not.

either way, in the grand scheme of things, i am guessing there would be neglible differences in the battery life but probably measurable differences in tire life.

what you saved on your batteries could easily be spent on tires. people talk about the expense of replacing a battery pack after 75,000 miles but at the rate some people are going here, they will be putting on their 3rd set of tires by then. how much is that?

 

[Nekota]

Wouldn't accelerating quicker use more of a charge out of the battery?

I don't know. Why would it? If you accelerate to the same speed then, whether you do it fast or slow, you are adding the same amount of kinetic energy to the car. If the battery and motor and drive train were 100% efficient, and you were driving in a vacuum, the amount of energy used would be the same, no matter what the acceleration. Of course none of those things are true, so we have to look at where the losses are. Motor efficiency does drop off very slightly with increased torque, but not enough to notice. (It's actually worst at very low torque.) I would think the same is true with the drive train. That leaves the battery (taking us back to heat) and air resistance. The faster you accelerate the more time you spend at your target speed, so if that speed is high enough that air resistance matters, then the difference might be significant. But only if you weren't going to retain that speed very long.

So, once again, why would accelerating quicker use more of a charge out of the battery? The only significant answer I am seeing is battery heat.

Ray

My take on the physics is the resistive losses go as I^2 x R so high currents lead to quadratic losses in the power train. I-squared losses are present in the battery, cables, inverter and motor. However, if you use lower current over a longer acceleration time the I^2 loss relationship is linear with time. The idea that getting to speed quickly requires the same amount of work as getting to speed more slowly is correct but this is the ideal 'no loss' simplification. Another similar loss relationship is the range loss at higher speeds (also quadratic) where going slower gets you to your destination but takes more time.

And when you combine the two modes, jack-rabbit starts with high speeds then you lose battery energy to resistive losses
and battery energy to wind losses. The idea of going fast and getting to high speed quickly saves energy is flawed and fairly easy to verify in your LEAF.

 

[planet4ever]

And when you combine the two modes, jack-rabbit starts with high speeds then you lose battery energy to resistive losses and battery energy to wind losses. The idea of going fast and getting to high speed quickly saves energy is flawed and fairly easy to verify in your LEAF.

No doubt as to that! The wind losses are quite obvious; the I²R losses not nearly as easy to see. The SAE graph I pointed to claims to cover efficiency of the LEAF "electric powertrain", which I assume includes the inverter. It displays efficiency against torque and speed. Wouldn't torque be roughly linear to current? Above 50 Nm in the graph there is a drop off in efficiency, and the slope does appear to increase quadratically as you would expect with an I²R relationship, but the impression I get is that this must be a curve with a very small value of R. (Admittedly it is tough to judge, since we are really dealing with a third dimension represented by color.)

The real, though partially hidden, point I was trying to drive home in my post was that from a pure physics viewpoint the amount of energy needed to accelerate a body to a given velocity is independent of the acceleration. Some posters seem to assume that if you accelerate twice as fast it must take twice as much energy. I'm not sure whether ztanos made that assumption in his post that I was responding to, but he did seem concerned that the extra energy required would use up more battery cycles, hence aging the battery more quickly.

Ray

 

[TickTock]

<snip>
The idea of going fast and getting to high speed quickly saves energy is flawed and fairly easy to verify in your LEAF.

For all practical purposes this is true since we usually end up at the same final speed (or faster) when we accelerate quickly. However it isn't true if you end up travelling the same distance in the same amount of time. This is a key criteria of the jack-rabbit start method. Losses due to high acceleration are small compared to the drag losses due to speed. Accelerating quickly allows you to travel the remainder of the distance at a slower speed and still arrive at the same time. None-the-less, I agree, this is rather contrived and we rarely have the opportunity to apply in normal driving. I, too, was a doubter until I ran the numbers:

Faster acceleration vs slower acceleration

 

[Nekota]

<snip>
The idea of going fast and getting to high speed quickly saves energy is flawed and fairly easy to verify in your LEAF.

For all practical purposes this is true since we usually end up at the same final speed (or faster) when we accelerate quickly. However it isn't true if you end up travelling the same distance in the same amount of time. This is a key criteria of the jack-rabbit start method. Losses due to high acceleration are small compared to the drag losses due to speed. Accelerating quickly allows you to travel the remainder of the distance at a slower speed and still arrive at the same time. None-the-less, I agree, this is rather contrived and we rarely have the opportunity to apply in normal driving. I, too, was a doubter until I ran the numbers:

Faster acceleration vs slower acceleration

Yes I agree if the equal time constraint is applied as would be for trains to meet and stay on schedules. If you accelerate slow to travel a fixed distance in a fixed time then you lose more energy due to the higher end velocity as you stated. This would be expected since the I^2 losses are much smaller than the air friction losses. Maybe I should have stated the same velocity limit since this is what typical driving represents.

(edited to correct typo - replaced larger with smaller)

 

[SanDust]

My question to you is how does the pack degrade faster from flooring it when heat is NOT an issue?

It's a higher C rate and a higher C rate results in a shorter cell life. You can see that from most any graph of cell life. The reason is that when ions move more quickly they don't fit into the structures in as orderly a way.

The "Nanny" control which limits acceleration may the purpose of preventing this to some extent.

 

[TickTock]

since the I^2 losses are much larger than the air friction losses.

I was with you up until this statement. A typo?

 

[Nekota]

since the I^2 losses are much larger than the air friction losses.

I was with you up until this statement. A typo?

Yes a typo - much larger should have been much less. I'll edit with note.

 

[DaveinOlyWA]

not to dispute anything here because i think we all have the general idea of things but remember; most driving is go, stop, speed up, slow down. iow; its not a constant velocity thing.

accelerating slowly simply allows you to change your speed choices. I coast from light to light sometimes in drive, sometimes in Eco, sometimes in neutral depending on distance, traffic, yada yada.

immediately accelerating to your target speed as quickly as possible only increases your likelihood that you wasted that energy. slowly accelerating to a set speed (traffic permitting) means that a change in traffic flow can be more quickly adjusted to.

daily, i see people zoom around me on a multi-lane road only to slam on their brakes at the red light just ahead not realizing that i am not slowing them down. they only seem to think that i am.

 

[gbshaun]

<snip>
The idea of going fast and getting to high speed quickly saves energy is flawed and fairly easy to verify in your LEAF.

For all practical purposes this is true since we usually end up at the same final speed (or faster) when we accelerate quickly. However it isn't true if you end up travelling the same distance in the same amount of time. This is a key criteria of the jack-rabbit start method. Losses due to high acceleration are small compared to the drag losses due to speed. Accelerating quickly allows you to travel the remainder of the distance at a slower speed and still arrive at the same time. None-the-less, I agree, this is rather contrived and we rarely have the opportunity to apply in normal driving. I, too, was a doubter until I ran the numbers:

Faster acceleration vs slower acceleration

TickTock, thanks for the link to last year's thread. that was interesting, as well as your report later in that thread about trying the "Jack-Turtle" method and how it did indeed improve your efficiency. This is what I have been doing since got the car. I don't drive fast for several reasons, but couldn't think of any reason (apart from tire wear) of NOT flooring it eg when i get the green light at the end of the freeway on ramp. I figured there would be some increased heating for a few seconds, but the difference between a long moderate acceleration and a maximum one to the same speed is so minimal, especially when put in perspective of the entire drive. Anyhow, it's my oportunity to display one of the great features of an EV, and dispel the myth of lame performance. I know I've surprised many a driver as I've silently pulled away from them.
On surface streets, If I do get caught at the next light, well at least i reclaim some of that energy. If my faster accelerating gets me to the next light just before it turns red, then i've just saved myself significant time,... and energy if i'm on a schedule and don't have to drive so fast.

 

[DaveinOlyWA]

<snip>
The idea of going fast and getting to high speed quickly saves energy is flawed and fairly easy to verify in your LEAF.

For all practical purposes this is true since we usually end up at the same final speed (or faster) when we accelerate quickly. However it isn't true if you end up travelling the same distance in the same amount of time. This is a key criteria of the jack-rabbit start method. Losses due to high acceleration are small compared to the drag losses due to speed. Accelerating quickly allows you to travel the remainder of the distance at a slower speed and still arrive at the same time. None-the-less, I agree, this is rather contrived and we rarely have the opportunity to apply in normal driving. I, too, was a doubter until I ran the numbers:

Faster acceleration vs slower acceleration

sounds good but that is not how life works around here. its great if you can "train schedule" your traffic flow but i can not.

fact of the matter; if you touch your brakes that means you are driving too fast for conditions. granted that is another "perfect world" statement but applies much more than your perfect world statement simply because speed adjustments to account for conditions can happen many times a minute and more likely at least several times an hour on more open roads (even on the freeway here with our "failure to move right" WA drivers!)

your scenario is a one time shot. need to be X miles in Y time so if we accelerate to Z in A time we dont have to average X/Y mph since Z is lower and more efficient due to less wind friction, etc which can overcome the penalty of A acceleration of the distance involved is great enough and that is where it fails.

my speed limit is 40 mph but generally only good for a quarter of a mile...sometimes a mile but that is about as far as it gets. so my target speed even on a 40 mph road is constantly changing based on my specific conditions so we want to average 35.

so i can accelerate immediately to 35 mph thinking that might work? well, what happens if my planned one mile jaunt suddenly changes to 5 mph in 3 blocks? i have now wasted my 35 mph velocity with regen/friction braking. now if i had been accelerating slowly, i might only be wasting the energy to get to 25 mph and so on

 

[Alric]

Forgive a non-physics, non-math person but wouldn't Newton's second law argue that rapid acceleration needs a higher force and thus a higher energy expenditure?

F=ma.

I would we would need to take into consideration the momentum of a 1 ton car.

 

[dgpcolorado]

Forgive a non-physics, non-math person but wouldn't Newton's second law argue that rapid acceleration needs a higher force and thus a higher energy expenditure?

F=ma.

I would we would need to take into consideration the momentum of a 1 ton car.

You are overlooking the time factor. If you accelerate quickly you do so for a shorter time (higher F but for shorter time). The discussion above was more about whether or not it is more efficient to get up to a constant velocity quickly versus doing so slowly and thereby keeping the car at a lower velocity, with concurrent lower drag, for longer. Since drag increases as the square of velocity the fact that it isn't linear makes the question interesting.

 

[FalconFour]

Just for laughs...

"I plug in, go 100mph*, cruise hills, and have no engine. (I also dent egos at green lights)"
* - yes, I round up - people generally notice "100mph" more than they'd notice "93mph".

But yeah, I floor it every time I'm at the front of a green. Maybe if I lived in a city that didn't think electric cars were "wimpy golf carts", I wouldn't need to...

I also work on lithium battery cells - built a pack for my electric bike before I had my LEAF, using bare lithium cells and an iCharger 208b+. When they're new, the internal resistance is so low, the only bottleneck in the system being strained is the inverter/motor controller. Also, charging is a higher strain on lithiums than discharging - high-end lithiums can discharge within 3 minutes rated discharge time. Not happening in the LEAF!

Regarding acceleration power, here's what I know. When you accelerate slowly, you're using a smaller amount of power for a MUCH longer time. Consider if you run a faucet slowly for 30 seconds, and you fill a bucket. You use 5 gallons of water. Now if you run a faucet full blast for 5 seconds, and you fill a bucket. You STILL use 5 gallons of water. Just different rates of consumption... and I still get 3.9 miles/kWh no matter how I drive

 

[smkettner]

and I still get 3.9 miles/kWh no matter how I drive

That says it all right there.

 

[Stoaty]

Just different rates of consumption... and I still get 3.9 miles/kWh no matter how I drive

Then you ain't drivin' right... 5.7 miles/kWh here (and no, I don't accelerate rapidly).

 

[FalconFour]

It's also a locale thing. Here, you get run over if you don't keep up with traffic - which is all 40mph and higher speed limits on straight grid-pattern roads and a straight, never-jammed freeway system. The one thing Fresno has going for it is how high-speed the traffic system is... so that probably contributes a bit to the lack of efficiency. It moves really, really quick.

 

[Stoaty]

It's also a locale thing. Here, you get run over if you don't keep up with traffic - which is all 40mph and higher speed limits on straight grid-pattern roads and a straight, never-jammed freeway system. The one thing Fresno has going for it is how high-speed the traffic system is... so that probably contributes a bit to the lack of efficiency. It moves really, really quick.

I would probably be dead meat in that environment. :lol:

 

[SanDust]

Forgive a non-physics, non-math person but wouldn't Newton's second law argue that rapid acceleration needs a higher force and thus a higher energy expenditure?

F=ma.

We've been through this before. Doesn't have much to do with F=ma. Since the terminal velocity is assumed to be the same, if you doubled your acceleration you would need twice the force but only for half the time. Net result is that if you started from rest you'd end up with the same amount of kinetic energy once you reached terminal velocity.

However, as suggested, you'd cover more ground getting to the terminal velocity. Starting from rest, v=at, so if your acceleration was doubled then the time to reach terminal velocity would be half. Given that d=vt and by definition your average velocity is the same in both cases, doubling the time also doubles the distance. The result is that while the energy expended is the same you'd go twice as far while expending it.

Even though the batteries are not lead acid you'll also have a Peukert effect if the acceleration is fast enough. That's something different though.

 

[Nubo]

...you'll also have a Peukert effect if the acceleration is fast enough.

I had that once... :lol: