EV Power vs Speed

jcmith · Nov 25, 2007

Does anyone know of a simple formula that could be used to estimate the amount of power in watts to move an EV?If wind resistance and rolling resistance are left out it would be incorrect but a possible starting point. I have a 450 lb car with a 125 lb driver.I want to select a Crystalyte motor to operate at 30+ mph. I was looking at the 5304,5303 or the 5302. The faster the better. I have no hills to climb so that will not be a problem.Any suggestions??

TylerDurden · Nov 25, 2007

Even small slopes are significant with that weight. Slope can also be interpreted as acceleration.

The 'Zotter puts power at about 6KW, ignoring drag from air and wheels.

http://www.kreuzotter.de/english/espeed.htm

jcmith · Nov 25, 2007

Running 36 volts (45 V max) we never saw more than 100 amps. Only when we started to roll were the amps up to 100. Our controller shuts off at 100 amps. I used an 100 amp fuse as well.Even at 45V and 100 amps our power should not be even close to 6000 watts,Should it? The car weight was 1005 lbs during that test run. We did about 250 miles but lost our amp gauge very early in the test. I changed the gauge to a analog type and it is to sensitive to give good readings.I have now got the total weight down to 450 lbs but have not selected a motor and controller. We could use the Perm 132 but the brushed motor uses to much power. I will keep working on getting some solid numbers and will post those when I get them . Thanks for the help!!

TylerDurden · Nov 25, 2007

I punched-in numbers for 15% slope, to account for acceleration. (The frontal area of the vehicle might be a bit more than the 'zotter provides in their profile selections.)

At 45V x 100A, yer talkin 4500W.

You might hit 6KW, if you open the current-limit on the controller.

6000W/45V = 133.333333333333A

It appears in the photo that the underbelly is not shrouded. You can reduce drag there too... plus wheel disks... the whole nine.

Brushless motors can be very efficient: >90%. Also, you may be interested in this data-logger:

What are the goals & constraints of the project, and which PV modules are you using?

Miles · Nov 26, 2007

jcmith said:
Does anyone know of a simple formula that could be used to estimate the amount of power in watts to move an EV?

Maybe you could use this? http://endless-sphere.com/forums/viewtopic.php?t=538 [scroll down to version3]

Miles

fechter · Nov 26, 2007

An X5 series motor might do it. Two of them might be better for that weight. 30+ mph should be no problem.

Are you limited to 36v?

Jay64 · Nov 26, 2007

TylerDurden said:
It appears in the photo that the underbelly is not shrouded. You can reduce drag there too... plus wheel disks... the whole nine.

If you use the wheel disks like they use on taxi cabs in LA, you could put sponsor logos on there to help get money for your project.

jcmith · Nov 26, 2007

Yes we have disc wheels but had beed removed at the time the photo was taken. We are still looking for some type of heat shrink plastic for the underside.I had some once to wrap boats for winter storage and the stuff was super tough. Our solar cells are shell 85 watt with a AERL mppt controller. Input voltage max is 176Vwith 8 panels.Output is adjustable from 24V to 200V. So we can run any voltage we want as long as the motor and controller can take it . We are only limited to a 5KW battery pack and Lead -acid batteries.We have run the car at 48V before and it ran about 6mph fasterthan 36. Our first Motor was an ADC 96V monster. It was 66lbs and the controller was 18 lbs. Add on 8 optima batteries and the car weight was about 1250lbs. At 108v and 400amps it would fly but not for long.The race next year will be a track race on a 1.5 mile oval. We run from 9:00 to 5:00 for four days.We can stop to charge anytime we need to but only by the solar array.

TylerDurden · Nov 26, 2007

You kids get all the fun. :evil:

re batts: 5KW or 5KWh? Still using 6V or now 12V? BTW, there are some new 8V FLA batts, if that's any help. If you can use SLA or AGM, you get lotsa choices.

The skin could be any sheet material... lycra is fine, if pulled tight enough to not luff. The boat-guys give used shrink away in the spring, if sourcing is an issue... it is often still usable.

Speed... range.... budget.... what's the goal(s)?

BTW... Still got the ADC? You might be able to sell-it / swap-it to a helpful EV'er. 8)

P.S. Congratz on the Silver. :mrgreen:

jcmith · Nov 26, 2007

The (New) rule states 5 KWh is max on the batteries.Last year our Optima yellow 51 batteries flopped two days before the race. A professional fishing team gave us six Orbital blue AGM 50 a/h batteries. Those had 100 minutes reserve at the 20 hour rate. They are about 41 lbs each. We ran six at 36v until the last day and we ran four at 48 to lose weight and recharge faster.The batteries only started to work well on day four and got better each day.It was nice to win Silver considering we lost 5 hours of driving the first day when the brake caliper fell off the car. The parts spread over 300 yards or so.The first time we went over 2 miles at one time was the first day of the race. The average speed says it all.We do have the record for the most tickets given to a solar car!!Most of the teams all share info and parts so it is all very friendly. We gave the winning team our mold last week!
We are looking to win our class next year. All the teams will average over 20. Our ultimate goal would be to try for 35.We will have 28 total hours to drive.The track is Texas Motor Speedway 1.5 mile oval. The car withthe most laps wins. Another team got 30mph, 399 laps and only lost the overall due to a horn failure.They got 30 on 48v, no hub motor and 600 watts of solar.Budget very low. The hotels and food gets the most. Anything left is for upgrades.We are mainly waiting to get new solar modules that do not have the glass or metal frames.

fechter · Nov 27, 2007

To go 35mph with an aerodynamic shape like that and no hills shouldn't take that much power even with the weight. With a lower powered motor, the acceleration will be slower, but the power consumption at crusing speed will be significantly less due to lower motor losses.

A PMG 132 might be overkill (but have snappy acceleration).
It would be WAY better than a series wound ADC motor.

A pair of Crystalyte X5 motors might be a good option.

Jay64 · Nov 27, 2007

jcmith said:
We are still looking for some type of heat shrink plastic for the underside.I had some once to wrap boats for winter storage and the stuff was super tough.

What about trying some of that industrial cling wrap that the shipping companies use? You can buy a whole role of it for pretty cheap. I used to use it to secure some of my spares in my race transporter. That stuff is pretty strong, not as strong as the boat wrap stuff I'm sure, but easy to get, cheap, and strong enough for 30mph.

PhoenixOSU · Jul 10, 2009

I tried once before to find a good measure of how much power is needed for a given speed, and its hard. There are a lot of claims of speed that range from incredibly optimistic to somewhat pessimistic. So I put everything I could find into an excel table and put a trend line to it. Here it what I saw, looks like at higher speeds and powers, there is quite a variation, must be wind resistance effects kicking in.

I am wondering what other peoples thoughts are on this, if anyone else has made such an effort?

Malcolm · Jul 10, 2009

Quite a spread there. I don't think the fit of the curve is quite right. Over 30 mph, most of the power goes into overcoming drag, so power depends on speed cubed. According to your curve, 5 kW gives 50 mph (which is about right for my motorbike). At 100 mph, I'd expect the power requirement to be roughly 40 kW.

Differences in drag between vehicle types naturally show up much more at high speeds, so individual results show much more spread.

If you want more detail on calculating power requirements I'd recommend this: http://www.inference.phy.cam.ac.uk/sustainable/book/tex/ps/253.326.pdf

PhoenixOSU · Jul 10, 2009

Very cool. I've tried to do it with math from what I learned back in university hybrid vehicle classes, but the math doesn't really make a good real world model because there are so many tiny things that can cause the results to swing everywhere. That's why I'm starting to just make a database of power and speed numbers and use what I know from an engineering standpoint to put the line of best fit where it makes the most sense. I agree it is high and should be farther down, specially at the 2-4 kW range, but Excel is somewhat limited with its included options for the line of best fit. I may try to get more data points from forums etc and manually create one for myself.

Vanquizor · Sep 9, 2010

I'm contemplating building a touring e-bike, something that I can travel significant distance with as work seems to be taking me all over the province. One question that keeps rolling around in my mind is what is a realistic power consumption figure to use for an e-bike travelling at 70-90kph (45-55mph). For thoese travelling at speed a quick responce with what your steady state power consumption on level ground is at what speed, what setup you are using, and maybe a pic of your ride would be extremely helpful...

Thanks in advance!

-Vanq-

FMB42 · Sep 9, 2010

Welcome ES Vanquizor,

I'm not sure of where you plan on riding an e-bike at 45-55 mph, but here's a link with some information on the street legal aspects pertaining to e-bikes:

http://electricbicycleconversionkits.net/2010/02/electric-bike-laws-canada/

BTW, doing 45+ mph on a electrified bicycle would take some real skill and experience IMO. bicyclee brakes, frame rigidity and suspension, etc, etc, would be hard pressed to handle anything near these speeds without careful design and use (hitting an "everyday" pothole at 45 mph could be disastrous). Having said this, have you considered an e-motorcycle or scooter? I say this because either would be far better suited to the speeds you desire.

oatnet · Sep 9, 2010

FMB42 said:
I'm not sure of where you plan on riding an e-bike at 45-55 mph, but here's a link with some information on the street legal aspects pertaining to e-bikes:
BTW, doing 45+ mph on a electrified bicycle would take some real skill and experience IMO. bicyclee brakes, frame rigidity and suspension, etc, etc, would be hard pressed to handle anything near these speeds without careful design and use (hitting an "everyday" pothole at 45 mph could be disastrous).

Welcome to E:S FMB42,

Boy are you visiting the wrong site if you are preaching street-legal speeds! :lol:

I commuted on my ebike at 44+mph every day for about a year, potholes and all, and my xtracycle really shows no signs of problems (other than old flogged cells), so I don't think it is too hard to build a bike to handle 45mph. I don't think I would attempt it on a wallmart special, but it is doable if you build with the right components. I stopped travelling at those speeds when a low-speed accident made me worry about what would happen at higher speeds. Then I got a Vectrix, and that has led me to travel faster on ebikes again. I think a full-face helmet, motorcycle gloves, and a good leather riding jacket are the minimum needed on an ebike at higher speeds, and since we are travelling at speeds faster than drivers might anticipate, we need to ride like we are invisible.

However, you are correct that a motorcycle is more suited to significant distance at 55mph. Vanquizor, I think Markcycle reported @100wh/m on the motorcycle hub motor he has manufactured. My vectrix weighs a whopping 500lbs, 200lb of it from the 150v30ah NiMh battery, and I estimate it is getting about 130 wh/m. On an ebike, even Lipo would get massy and heavy for just moderate range, so you would probably be dependant on opportunity charges and heavy pedaling.

-JD

veloman · Sep 9, 2010

What sort of clothing are you wearing, and what is your position on the bike?

Also, a good full suspension bike is practically a necessity at such speeds.

If my question above confuses you, then do some research on aerodynamics (there's a thread here). At 50mph you will have huge wind drag. You will need some SERIOUS battery to make this work. I'd estimate at least 4-5kw drawn for 50mph, but it's really hard to say because a baggy jacket and pants could easily add another 1000+watts drag at that speed. An upright position could add even more. I'd suggest aero modding a vespa style scooter. You'd have the room for all your battery down nice and low, proper suspension, and if done right, could take less power than a typical ebiker.

Vanquizor · Sep 9, 2010

I guess I should have considered the section I am posting in more carefully- but to clarify by e-bike I did not mean to exclude scooter type (I own a couple of them), recumbent (not yet but it looks fun), or traditional bicycle style (also in my collection and 2 tidalforce frames on the way). Legality can be ignored as well- it is very easy to licence a 'motorcycle' in saskatchewan, in the past me and a few friends strapped a chainsaw to my old BMX and had a plate on it in by mid afternoon (and thoroughly trashed it by drinking time that evening).

I'm more interested in what folks are seeing in the realworld with what kind of rides. I am well aware of the impact aero has, hence the request for a pic.

Note to mods- given my broad definition of e-bike if there is another forum this would be more appropriate in please feel free to move it.

John in CR · Sep 9, 2010

I recently did some 10 mile runs on the highway to get consumption numbers at different speeds. At WOT 50-60mph, 55mph avg my bike used 61wh/mile or just above 3kw. At a fixed speed of 40mph it used 45 or 1800W. Though upright, my bike is long and low with a 16" rear extension on a normal bike frame ala Xtra Cycle, and my seat height is 26". Those consumption numbers were using a good tucked position, though at the time I had a passenger backrest on the bike, which I'm sure canceled much of the benefit of the tuck. Also note that me+bike is over 360lbs, and the 10 mile run was 5 miles out and back with slightly rolling hills.

icecube57 · Sep 9, 2010

At 72v 45A nominal I was doing 80ish wh/mi and barely getting 45-50mph with mixed riding with hills. Alot of that was dumped as heat into the motor which wasnt very efficient at high voltages. So mark is about dead on with his figures. I think his motor takes 96v and no telling how many ampts to do 50-60mph.. High speed takes high power and capacity. Im planning to do a 100m ride....200m round trip near the end of Sept early Oct. Im planning on packing atleast 44v 25AH if not more 35Ah would be a blessing.. All of this just to maintain a speed of 15-20mph for 100m. My X5 5303 can gobble up about 1.25ah a mile at 66v 45-55A and do 50mph if i let it. Pick you most efficient speed for the battery voltage you are running and change the wheel size if possible. High torque/wind motors will offer more current limiting that their low wind counterparts. Aim for a lower speed 35-40mph if you can. It would be alot easier on everything in the system. The higher you go in speed power consumption gets to be exponential just because of air resistance and drag. My button up shirts end up acting like parachutes when riding 40mph+

John in CR · Sep 9, 2010

Once you get above 30mph or so, power consumption is all about aerodynamics, so there is no "dead on" answer. eg The electrathon 1 hour record is currently 62 miles, and they use slightly less than 1kwh of battery, so there consumption for a 62mph average speed was just under 1000W continuous. That's the optimum end of the spectrum. On a normal upright bike the power consumption for the speed you want can vary greatly with, as Veloman eluded to, rider height, body position, even clothing and tire type.

Here's a good bike power calculator to play around with that has a variety of bike types and riding positions along with other variables that you can use to estimate the power required. http://web.archive.org/web/20030606033346/kreuzotter.de/english/espeed.htm

veloman · Sep 9, 2010

1800w at 40mph sounds pretty good to me. I was expecting power usage to be higher than that. I guess that's about right, since you have a low seat height. Myself on my road racer with spandex(195lbs total), requires about 1000w to sustain 40mph, while seated and crunched over (I can only get to such a speed with a little downhill lead up or tailwind).

I'm really interested in aero moding either a scooter or full suspension bike to bring those high watt draws down considerably. I just hate how there is such a big penalty for going a quick safe speed (keeping up with traffic).

El_Steak · Sep 9, 2010

I commute every day at speeds around 40-45 mph. At those speeds, I get around 65-80 wh/m, depending on terrain and throttle aggressiveness.

My bike is a hardtail MTB with 24 inch wheels.

I run 24s LiPo so 88V nominal, 100V hot. Current is software limited at 120A on the phase wires and 80A on the battery side.

I weight about 175 pounds and ride in a normal/casual position (not tucked down).

I did a lot of test runs with that bike when testing different air cooling mods. I recorded all data, including power usage. You can see it here:

http://endless-sphere.com/forums/viewtopic.php?f=2&t=9791&start=405#p278852

All numbers are in km, so you'll have to convert them.

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