# Help me understand

The biggest problem I have with understanding EVs is correlating characteristics to an ICE, in which I am well versed. For example, looking at the post for the Dodge Zeo concept, it states that it has a 200KW motor, and a 64KWH battery pack. It also states 250 mile range.

Now the place I’m having a problem with is actual power use. How does a 200KW motor get a full draw of power from a 64KWH battery pack? Obviously it’s possible since they are doing it, but there’s not enough detail to explain it there. How much actual power must be produced on a second by second basis to power this 200KW motor? [B]How does the KW correlate to the KWH?[/B] And how are they basing a 250 mile range? Does it have a 250 mile range whether the car is going 50MPH or wide open? I’m sure I’ll have more questions based on the answers I get. Thanks.

The motor is not drawing 200KW on a constant basis, that’s its peak power.

And there is no correlation between the 250 Mile range, the 64KWH pack and 200KW motor because we don’t know the specific details of the test where these numbers were obtained, if they were actually tested in the first place or just estimated by designers.

We can make assumptions based on the numbers, like if the car had a 250 mile range at 30MPH, that means the power required to move the car was equivalent to just under 11HP, and that’s with 100% discharge of the pack. Or if the test was done at 60MPH we get 4 hours or 22HP. Problem is, this relationship is not linear because the faster a car moves the more forces resist its movement… unless they really cheated and used a dyno for testing. It gets either 11HP/30MPH/250 mile range or 22HP/60MPH/250 mile range, but not both.

OK. But, this doesn’t answer the question of how the numbers correlate to each other, in general. I’m not trying to find out how the specific Zeo numbers correlate, that was just an example.

What I’m looking for is how to look at the numbers for an electric vehicle and be able to know what I’m looking at, and be able to look at a motor or a battery pack and be able to figure out what kind of acceleration and top speed I can expect, and what kind of range, for the purpose of selecting components that will work well together. I do realize there’s not going to be a single formula that will work in all conditions every time, but such a formula would allow one to start selecting components in a halfway intelligent fashion.

I’m also looking for information on what to look for. For example, why would a 36V forklift motor mounted in place of the transmission allow an S10 (much like my own) go 180MPH through the quarter mile in 9 seconds, when a 380V motor SHOULD be a more powerful motor? How would I figure the range according to battery selection? What determines range vs acceleration where the battery pack is concerned?

First, let me give the accurate but mostly useless answer:
1 kWh = 1kW * 1hour. The kW is a unit of power, the kWh is a unit of energy (power applied over a period of time).

Unfortunately, that isn’t of much use to us for the purposes of your question.
It’s probably more helpful to you to know that 1hp = 746 watts, so 1kW = .746 hp.

What about the battery’s rating of 64kWh? That’s a little like fuel capacity in an ICE vehicle. The speed and acceleration components of vehicle performance aren’t really related to the size of the fuel tank, assuming the tank is much larger than a thimble but also much smaller than a Semi-load of gasoline, so that you wouldn’t run out of gas before reaching maximum speed or have weight of the tank playing a huge role in performance.

So the 64kWh battery pack has more to do with the distance you can travel (like the size of your gas tank). The thing to remember about chemical batteries is that you can get more of the total stored energy out if you withdraw it at a slower rate. That means you can go farther if you go slower – and that’s [I]on top of [/I]the effects of drag that affect EVs and ICE cars alike.

But I still haven’t really answered your question, I know. How much power can I get out of a 64kWh battery into a given electric motor? The answer there is: we don’t know. You’ll need to know deeper specs of the battery back, like maximum discharge rate vs. time. And that is not part of the basic specs that a battery manufacturer usually publishes.

As a (rather overly-simplified) rule of thumb, you can take the battery energy spec in kWh, and drop the hours to get a power number (kW). If you are using a lithium type battery, then you can probably expect to discharge the battery at a small fraction of this rate – maybe a tenth – without damaging the battery, causing catastrophic disassembly, or worse. With a Nickel battery, like a NiMH or NiCd, you can draw power at a multiple of this value, perhaps even 2 or 3 times the value for a limited time. Of course, YMMV. Battery people are working very hard to improve these numbers, but progress is slow.

Battery technology is either a very interesting or really dull subject. You might consider getting a book on batteries from the library to find out which one it is for you. Don’t worry if it’s an old book – batteries have probably gotten like 3 times better in the last 100 years – the lessons still apply.

Enjoy!

Disclaimer: this post is both too simplified and too technical. It almost certainly contains inaccuracies. Sorry.

may want to rethink your formula for KW vs HP

[QUOTE=parts68;2928]may want to rethink your formula for KW vs HP[/QUOTE]
Right you are! I sure did write the second part of that backwards.

1hp = 746W, so 1kW = 1.34hp (or .75kW = 1hp)

I wouldn’t have been offended if you’d just corrected that for me, honest.

Unfortunately, electrical engineering is a bit more complicated that can be explained in a few paragraphs on an internet forum. Not meant to criticize or seem snotty, just stating the facts. I am an electrical technician, by trade, have worked on all sorts of electrical equipment, and still wouldn’t consider myself close to being competent to design a BEV. I salute all the DIY/hobbyists who take on conversion projects. I don’t know that there is a calculator that you can plug info into that would give any kind of reliable estimate of the performance of an EV. I suggest you look at what others have done on this forum, or whereever you can find info, and use the experiences of others as a guide.

So far we have determined that the size of the battery bank is analogous (sp) to the size of the gas tank on an ICE auto. Absolutely right. This is the major limitation of ALL EV’s, that the battery pack to move you several hundred miles down the road would weigh and cost a huge amount, and increasing the weight of the battery also causes you to need more battery energy to move the additional weight, so adding batteries is governed by the law of diminishing returns, i.e. not really worth the cost and weight at a certain point.

Another issue relevent to the gas tank analogy is that the S10 dragster that was linked to is very interesting, but it unfortunately is irrelevant when discussing a useable road vehicle, because he is only going 1/4 mile at a time. So basically he can afford to use his whole gas tank to go really fast for an extremely short distance.

As far as the voltage of the drag truck being 36VDC, voltage in and of itself has nothing to do with power output. Generally speaking higher voltage cuts down on weight and cost, due to reduced current (amps) for a given power output, which means smaller diameter wire for your battery interconnects, and other power components. Smaller wire means less copper, which translates to less weight and cost…to a point.

I have never (in industry) heard of batteries being rated by KWh. Normally batteries are rated in Ah (amp/hours) which means the battery will output so many amps for an hour (or half that rate for 2 hours, etc). This is just a rating though, and as previously stated depends on rate of discharge, temperature, age of battery, and a few other lesser variables. A 40 Ah battery, you would think could be discharged at 10 amps for 4 hours, 20 amps for 2 hours, and so on, but due to a variety of factors this is not really the case.

Another important factor to consider is the gvw of the vehicle. Simple physics tells us that work=mass x distance. That is why a 5000 lb. SUV gets much lower gas mileage than a 2500 lb. compact car. So a given battery pack will move different vehicles different speeds and distances, depending on the overall mass of the vehicle, not to mention the terrain (hilly or flat) efficiency of the vehicle (rolling resistance, air resistance, heat losses, electrical losses)

That S10 electric drag truck also benefits from being fairly stripped of creature comforts (weight). You could get an EV really light by stripping the interior, but it might get unpleasant or impractical to drive everyday really quick.

It is interesting to me that many large SUV’s are actually more efficient than a lot of the compact/economy cars on the market. Yes, you read correctly, Big SUV’s achieve efficiency through economies of scale. It is actually more efficient for my to drive my truck with 4 people and 800 pounds of stuff in the back @ 20 mpg than it is to ride my motorcycle alone at 40 mpg. The truck will go 80 miles per PERSON per gallon, whereas the motorcycle with just me on it will go 40 miles per PERSON per gallon. Obviously if I drive to work alone in the truck it’s a different story, but you see the point.

I am thinking of an EV conversion using a full size pickup truck/SUV so I can carry more batteries and get more overall range, because my daily commute is beyond the range of a 40 mile per charge vehicle. Municipal power at my residence would make charging cost negligible. Battery cost would be much more, and I wouldn’t gain as much range as you might hope because I’d be wasting a lot of energy to carry around all those batteries, but I’m not sure I have a choice, if I want a vehicle that fits my needs.

Thanks, the posts help some. Sorry it took so long to get back, just got back from vacation.

The battery pack determines the range, but on an ICE there are several things related to the fuel system. You have the fuel tank size, fuel line size, fuel pump pressure, and fuel pump flow capacity to consider, ie a vehicle with a 10 gallon tank feeding 10PSI at 20GPM in a quarter inch line will be a lot slower than one with a 10 gallon tank feeding 60PSI at 80GPM down a 3/4 inch line, because the larger system can deliver more fuel to the engine. Of course, range is a lot lower with the larger lines. This is another thing on the drag S10, I’m sure his “fuel lines” are allowing a lot more “fuel” to flow.

But, as has been stated, the overall question still hasn’t been answered, even with the HP calcs, and I know that the real problem here is comparing an ICE to an electric motor is comparing apples to the neighbor’s dog. Perhaps if I listed my goals…

What I am looking for is to make a vehicle that weighs approximately 3600lbs be able to run as though it has an ICE that makes approximately 350HP and 350 ft lbs of low end torque, WITHOUT considering what sort of battery pack might be needed to drive it. My target vehicle is a first gen S10 ext cab, which in current trim weighs in at about 3000lbs. Figuring 3600lbs will help take battery weight into account, and will include some other weight savings measures I’ll be taking as the project proceeds.

Looking at the different electric motors and their specs, I’ve not been able to look at the specs and determine what might do this. The specs I have looked at are confusing, take the S10 dragster and compare it to other EVs as an example. It looks to me like that truck should be nowhere near that fast based on the parts used. And, this is where my difficulty with the whole EV concept comes in. Looking at the Tesla specs, what size ICE would a car like that need to perform the same way?

I think that if I could get a few examples of different vehicles with different weights that perform the same, with one being an ICE and the other being an EV, that I could start getting a handle on “this motor in that vehicle will perform as though that vehicle had an ICE with these specs”, which is what I’m trying to do now. Once I get a handle on sizing an engine, figuring out the fueling system won’t be an issue because I’ll be able to figure tradeoffs on my own.

Telco

The is more to it than that. You have to understand that the electric motors produce maximum torque when they start(0 rpm). Compare that to an ICE, where the clutch is typically dropped at 3500rpm for high torque. Another thing to remember is that electric motors like to run at the top end of their rpm rating. Most electric motors are at their highest efficiency at the highest rpm. They also have a much wider useful power band (pretty much 0-max rpm). Now compare that to an ICE. How long will the typical ICE run if you keep the tachometer pegged? How much fuel will it burn if you run it that way. For a typical ICE the power band is what 1500-3500 rpm? So you really have only 2000rpm to work with. Most low end electrics run up to 6000 rpm and I have seem some that are 13000rpm. So what I am saying is that it is extremely difficult to compare the two in any simple manner. To do any real comparison you have to go back to the physics of the problem and that gets complex pretty quickly.

As a easy example, lets just compare a Cummins 350 (typical semi engine) diesel. It has 350hp and 1250lb-ft of torque. A typical semi weighs 80,0000lbs loaded and 17,000 lbs empty. Now try and compare it to the 350hp S10 you have mentioned in any meaningful way. The short version is that there is no easy way to do it.

If the batt is 20AH and the mainline voltage is 400V, the batt have 8kwh energy. If your electric motor need the 8kw to drive your car, the car will run for 1 hour. However, If the motor need 16kw, the car will run for 30min, and the batt will discharge for 40A at 2 times.

[QUOTE=arb;3281]On any modern engine, the fuel pump, pressure, line size do not regulate the amount of power the ICE will make - sounds strange - hold that thought… Here’s why. The engine’s RPM is the #1 factor = horse power is work over a distance, so torque does not make horse power - torque over a distance - RPM = horse power (or watts - 746 watts per horse power)
air volume is the #2 factor (There’s no replacement for displacement) followed closely Compression ratio and air charge pressure (to do turbocharge to get wasted heat/kenitic engergy from the exhust ?)[/QUOTE]

Fuel system size can be a limiter. The size of line limits the size of motor can be supported.

[QUOTE=arb;3281]Now comes fuel - for spark ignition, the fuel must be deleivered at about 14.7:1 ratio to the fuel. If the lines are bigger, pump higher volume, higher pressure, etc it will not change this fact. BUT, if they are less than is required, the engine will not run well and actually produce less power - if it runs at all.[/QUOTE]

Max power actually comes in at around 12 to 1, 14.7 is stoich, best economy comes in at around 18 to 1. These numbers are different according to the engine, fueling method, whether a wide band O2 is available, as with fuel economy you have to balance lean burn with engine longevity,

[QUOTE=arb;3281]For compression ignition, yes the volume of fuel is what controls the amount of power is produced - after the first factors that is, but this is controled by the injection nozzles / computer or mechinical pump. If the lines / lift pump etc are not correctly sized, again power is reduced.[/QUOTE]

Yep.

[QUOTE=arb;3281]I suggest rather than choosing some arbatrary set of numbers for another car that was never produced ( 350 hp S10) http://en.wikipedia.org/wiki/Chevrolet_S-10 - why not set design goals like, 80 mph top speed, 0-60 in 8 seconds, 60 mile range, things like this.[/QUOTE]

This misses the point of the whole post. And there are 1000HP S10s on the road. I don’t care about whether a manufacturer ever made it or not.

Thanks for the replies everyone. Unfortunately, I’m not getting what I need to know from these posts. I know that an ICE operates differently from an EM (electric motor), but there must be a way of looking at an EM performance graph and equating it to an ICE performance graph, same way I can look at the ICE performance graph for one specific motor and know how it will perform in a multitude of vehicles, under a multitude of performance conditions. If I could equate an EM motor’s graph to an ICE graph, then I’d be able to look at any EM motor performance graph and know how well it would perform in any ICE driven vehicle.

I guess what I’m looking for is a Rosetta Stone for speaking translating ICE to EM.

Yesterday I took my daughter to a college prep event at the college she wants to go to, and we browsed the book store. I found a book about an inch thick that covers electric motors, and the cover said that it’s designed to be a bridge between an “Electric Motors for Dummies” book and a professional electrical engineer’s reference manual. After I read it, if I find it to be useful I’ll post up the reference number and title to the book. It is supposed to teach you how to interpret a motor’s parameters so you can select the correct motor for a new application, so it should be just what I need.

Thanks for trying though.

Telco,

I look forward to the review and name of your new found motor book.

LR

[QUOTE=Little Rhody;3311]Telco,

I look forward to the review and name of your new found motor book.

LR[/QUOTE]

It’ll take some time. The reading is dry and I’ve got a lot on my plate right now.

go to diyelectriccar.com and subscribe to the EVDL… and look in all the archives before you post anything else… people ask all the time… and there’s info up there, and its a good read.

As far as comparing ICE to Electric…you know torque @ rpm for both ICE and the electric motor, you know HP @ RPM, you know the rough weight of the vehicle its going into. From this you can calculate the torque and HP through the transmission. you can calculate each point on the graph for ICE and each point on the graph for electric… and see for yourself. No one is going to do the work for you, there’s no magic equation, but there are conversion factors, like kW to HP.

The reason it can’t be corrolated very well, is because we don’t know what voltage you’re talking about, how many Ah the batteries are. Just because its 65Kwh, doesn’t mean its not a 1000V 65Ah pack…Torque is current, speed is voltage. Higher voltage, you can go faster. Higher current, you accelerate faster.

As far as the 200Kw motor, what does that really tell you? Thats peak power dissipation. It will handle UP TO 200kW… but not for very long, thats not continuous… kinda like pegging the throttle on an ICE (without a rev limiter), and letting it sit at 10krpm on a stock engine, it might do it for like 30 seconds, but its going to break eventually. The real thing to look at, is the wh/mile discharge. If nomal driving is 300 wh/mile with that car, and you’ve got a 64,000Wh pack, then that means you can drive to roughly 80% depth of discharge of 64, which is 51,200. Divide by 300 and you get ~170miles. Its a large motor, but peak doesn’t tell you a ton about the motor, you need max current, torque curve, volts/rpm, max rpm, voltage, HP and weight. My motor in my motorcycle is almost 30,000W, but I don’t get that… not for very long thats for sure. Even if a motor is 90% efficient (most aren’t), and you have 200,000W, thats 20,000W of energy that gets wasted mostly in heat… have you got a 1500W hairdryer at home? Take that, and multiply by about 15… and you have an idea of how hot it’l get contuniously putting out that much heat.

200kw is PEAK, and its likely not for very long.

1KW = 1.34102209 HP = 268Hp
you want 350? you need a 261kW motor… good luck finding one…

electric motors don’t corrolate well, mostly because their torque curve is pretty flat, and you get max Torque at 0rpm. Also, as speed increases, torque falls off, it doesn’t “peak” like an ICE. They’re two completely different vehicles. You get lots of torque at low RPM, high current. Once you start going, torque decreases with speed, and amps drop as well. There’s no real rosetta stone, what you have to look at is motor, to tranny, to tires to road. Compare the performance of a car with ICE to the same car with Electric. You CAN calculate the hp and acceleration and top speed in either case, it just takes some elbow grease.

They’re fairly simple physics calculations.

The book I picked up is Electric Motors and Drives: Fundamentals, Types and Applications, third edition. The book has two ISBN numbers, ISBN 13: 978-0-7506-4718-2 and ISBN of 10: 0-7506-4718-3. So far it’s helped me understand electric motors a lot better than I did, and it has the math to back up what it’s saying. It also speaks to you in a no-nonsense manner, and is not condescending to the reader. Nor does it seem judgemental or preferential towards any one type of technology. Kinda like Joe Friday, just the facts, ma’am. I’m only about halfway through, but can heartily recommend the book to help understand the inner workings and the math behind the technology. Really I’ll have to read it 2-3 times before I’ll be able to do the math and understand what I’m seeing as I’m more of a hands-on guy than a book learner, but I’ll be a lot further along once I do get a good understanding of it.

Frodus - I’ll check that out, thanks. Still think there needs to be a way to correlate the EV to the ICE though, because that WOULD increase interest in the EV if people could equate the EV they know nothing about to the ICE they grew up with. Not many are brave enough to sail into the unknown.

There is a way…

its called HP and Torque Both the ICE and EV has that data available.

You have a flat torque curve too, which is great, it means better acceleration from a motor to an equivalent HP ICE.

Frodus - The problem with that is the difference in how the torque and HP are generated. When I first started looking into this stuff I started by looking for an EM that made the same power as the ICE I would use, and found that the motor was 5 feet long, 4 feet wide and weighed about 2500lbs. Nothing like the dinky motors EVs use. This is where the problem lies, and I doubt that we are going to be able to bridge the gap of my questions vs your answers on how to correlate one to the other, either I’m not explaining properly, or you’re not understanding or don’t know the answer. No matter though. Luckily, there was a fellow on that board you recommended that CAN do this, and from what he’s saying I’d need two Warp 9s in series to generate the power I’d want. I’m thinking that I can extrapolate what I want from the advice that other fellow is providing, with a few more questions. Thanks again for that link.

Thats because motor nameplates are rated continuous not peak. ICE’s are rated peak. Many times you can get 2-5 times the continuous rating on a motor. Torque is still torque, and HP is still HP. You don’t need to convert. What you NEED to do, is do what I said, find a motor who’s PEAK matches the HP rating of the ICE. You may not even need to do that, because the torque will be MUCH more at lower RPM’s and provide TONS better accelerations.

Try not to be so condescending… I don’t really like being told that I’m not understanding or that I don’t know the answer… when I’m actually perfectly clear. You’re not the only one that’s asked these questions. I’ve explained the physics of it all, the calculations and the conversion. They DO correlate, just not the way you were thinking. I think you aren’t hearing an answer that you didn’t expect to hear. Keep an open mind, EV’s are uncharted territory for most. But for me, I’ve worked as an engineer in the Industrial sector on small/med/large motors and can tell you, don’t understimate the size of a motor. A small motor can produce tons of torque and high speed. If you gear it right through the transmission, you’ll be able to tear the asphault off the road

Its not that I don’t understand, Its that you’re expecting a different answer. Peak HP on an ICE is not even that great of a measurement to begin with. The motor in my motorcycle maybe peaks around 30+HP, but its nameplate is more around 5 or 8 continuous Hp. The torque is 50+ftlbs, same as the original engine, but a useable 5000rpm. Some engines don’t even PEAK until 5000rpm but go to 10000 rpm. Thing is, I have constant torque to 30mph with a 4:1 gear ratio.

DC or AC motors are rated continuous duty because they’re made for machinery, and meant to be on constantly. You need to get data from the manufacturer of a motor, ask them about the peak HP, or kW. Then you can compare apples to apples. But since you haven’t done that… I can’t search through thousands of motors just to find one for you. I spent the last year reading, researching and chosing my parts to match my application. You’ll have to do the same. EV’s are in their infancy, you’re gonna have to sit back a little a little and research like a lot of people have. Most people that drive cars understand HP/Torque of an ICE, but those same people don’t know ■■■■ about electric motors.

I made many calculations on acceleration/torque/speed/HP so that I could match the motor as close to the original as possible. Its not the HP and Torque you want to calculate, its the acceleration and speed you want to hit. That is very easy… if you know where to start.

HP/Torque is no different for an EV motor than a car, but the range/curve is much more usable for electric. If you can’t find ONE motor that will do it, use two, as the other fellow stated. I think thats a great idea. There’s a guy who put 2 WARP’s in a fiero or something, looked badass. I bed he could spin the rubber off his tires. I think you’re underestimating the torque/HP of a motor… you nee to FEEL it… and that feeling comes from acceleration and speed.

What is the speed and acceleration you want? Work backwards.