How much power do in need for my car...

4616 Views 16 Replies 8 Participants Last post by jeremyjs, Nov 24, 2010

gabl136

Discussion Starter · Nov 15, 2010

Hello!

I'm new on the forum so please be patient

I'm planning my first conversion, the question is how much power (dc motor) do i need (peak & continuous) for a car that weights (original setup) between 1800-2200lbs (cca. 800-1000kg). Top speed of 60mph & 20 mile range is enough to get me to work and back home.

Thank you for your answers in advance.

Gabl 136

1 - 17 of 17 Posts

gabl136

Discussion Starter · #2 · Nov 20, 2010

anyone ???

Ziggythewiz

· #3 · Nov 20, 2010

I have a D+D ES-31B that meets that running at 120V. I get up to 70 in 3rd gear with a range around 26 miles with cheap batteries.

david85

· #4 · Nov 20, 2010

Welcome!

Here is some info that others posted to give you a basic idea:

http://www.diyelectriccar.com/forums/showthread.php/freeway-speed-distance-survey-28716.html

Most of them don't seem to need much more than 20kw to sustain 60 MPH but the size, shape and weight makes a big difference from one car to the next.
I suspect my car wasn't able to deliver much over 20kw either and at roughly 3000lbs with me in it, I could just reach 60 MPH.

DavidDymaxion

· #5 · Nov 22, 2010

Rough rule of thumb I came up with:

Max battery Amps * Pack Voltage / 2 / 1000 ~= drive wheel horsepower

The "/ 2" takes care of voltage sag, conversion from kW to hp, and efficiency losses. The "/ 1000" converts Watts to kW. It's rough, but a good starting point. Let's do some examples:

400 A * 48 V / 2 /1000 ~= 10 hp -- golf cart!

500 A * 144 V / 2 / 1000 ~= 36 hp -- old VW bug speeds

1000 A * 144 V / 2 / 1000 ~= 72 hp -- Economy car speeds

2000 A * 336 V / 2 / 1000 ~= 336 hp -- Drag racer!

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Duncan

· #6 · Nov 22, 2010

Improved Rough rule of thumb I came up with: for Lithium

Max battery Amps * Pack Voltage x 1.33 / 1000 ~= drive wheel horsepower

Max battery Amps * Pack Voltage x 1.33 x 2 / 1000 ~= equivalent IC engine horsepower

The 1.33 converts from Kw to Hp

400 A * 48V x 1.33 x 2 / 1000 ~= 51 hp -- old VW bug speed

500 A * 144 V x 1.33 x 2 / 1000 ~= 190 hp -- Economy car speeds

1000 A * 144 V x 1.33 x 2 / 1000 ~= 383 hp -- Sports Car

2000 A * 336 V x 1.33 x 2 / 1000 ~= 1780 hp -- Loony machine

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JRP3

· #7 · Nov 22, 2010

Duncan said:
Improved Rough rule of thumb I came up with: for Lithium

Max battery Amps * Pack Voltage x 1.33 / 1000 ~= drive wheel horsepower

Max battery Amps * Pack Voltage x 1.33 x 2 / 1000 ~= equivalent IC engine horsepower

The 1.33 converts from Kw to Hp

400 A * 48V x 1.33 x 2 / 1000 ~= 51 hp -- old VW bug speed

500 A * 144 V x 1.33 x 2 / 1000 ~= 190 hp -- Economy car speeds

1000 A * 144 V x 1.33 x 2 / 1000 ~= 383 hp -- Sports Car

2000 A * 336 V x 1.33 x 2 / 1000 ~= 1780 hp -- Loony machine

Those numbers don't make any sense. For example, 400A 48V is 19.2KW, or 25.75HP. Nice converter http://www.mr2ownersclub.com/converter.htm

Duncan

· #8 · Nov 22, 2010

Hi JRP3

The idea was to compare IC engine horsepower with electric, a 100Hp engine has 100 Hp at ~ 5000rpm and 50 hp at 2000 rpm

Max battery Amps * Pack Voltage x 1.33 x 2 / 1000 ~= equivalent IC engine horsepower

In driving a 50Hp electric motor will "feel like" a 100 Hp IC engine,
bit like a diesel, if you rev the nuts off a petrol (gas) vehicle it is faster - if you drive normally the diesel feels quicker

It was also an over the top responce to DavidDymaxion

500 A * 144 V / 2 / 1000 ~= 36 hp -- old VW bug speeds

DavidDymaxion

· #9 · Nov 23, 2010

Duncan, tell me where I can buy those infinitely stiff (in Voltage) lithium batteries you used in your formula!

Let's do a few examples. Let's assume these efficiencies: 98% controller, 85% motor, and 85% tranny/diff/bearing losses.

Typical floody conversion: 0.010 Ohm resistance per battery, 500 Amps, 12 twelve V batteries for 144V nominal.
Voltage sag for each battery: 12V - 500A * 0.010 Ohm = 7 Volts

Lossless power: 500A * 144V = 72 kW = 96 hp
Power with sag and losses: 0.98 * 0.85 * 0.85 * 500 A * 7 V * 12 batteries = 30 kW = 40 hp
Formula "Max battery Amps * Pack Voltage / 2 / 1000" = 40 hp
Wow, exactly right!

Same thing for 1000 A controller and 12 Optima batteries with 0.003 Ohms of internal resistance each:
Power with sag and losses: 76 kw = 102 hp
Formula "Max battery Amps * Pack Voltage / 2 / 1000" = 72 hp
Underestimates by 25%

Some thing for 2000 A controller and 12 Optima batteries with 0.003 Ohms of internal resistance each:
Power with sag and losses: 102 kw = 136 hp
Formula "Max battery Amps * Pack Voltage / 2 / 1000" = 144 hp
Overestimates by 6%

600 A controller and 144V of 200 Ahr LiFe batteries (0.003 Ohm internal resistance each for 44 batteries)
Power with sag and losses: 28 kW = 37 hp
Formula "Max battery Amps * Pack Voltage / 2 / 1000" = 43 hp
Overestimates by 16%

336V of A123 batteries, 170 A each battery so 12 in parallel, 100 in series:
Power with sag and losses: 230 kW = 308 hp
Formula "Max battery Amps * Pack Voltage / 2 / 1000" = 336 hp
Overestimates by 8%

Rich persons setup: 2000 A, 336 V of A123 batteries (0.010 Ohm internals resistance each, 24 parallel and 100 in series). 0.010 Ohm / 30 = 0.42 milliOhms per 24 in parallel. That's 70 A per A123 battery.
Power with sag and losses: 349 kw = 466 hp
Formula "Max battery Amps * Pack Voltage / 2 / 1000" = 336 hp
Underestimates hp by 39%
This pack would cost about $40,000 USD

So not perfect, but pretty good for something you can do in your head.

Agreed that electric hp are "stronger" than ICE hp for day-to-day driving, as electrics typically have a fatter torque curve.

Duncan said:
Hi JRP3

The idea was to compare IC engine horsepower with electric, a 100Hp engine has 100 Hp at ~ 5000rpm and 50 hp at 2000 rpm

Max battery Amps * Pack Voltage x 1.33 x 2 / 1000 ~= equivalent IC engine horsepower

In driving a 50Hp electric motor will "feel like" a 100 Hp IC engine,
bit like a diesel, if you rev the nuts off a petrol (gas) vehicle it is faster - if you drive normally the diesel feels quicker

It was also an over the top responce to DavidDymaxion

500 A * 144 V / 2 / 1000 ~= 36 hp -- old VW bug speeds

Duncan said:
Hi JRP3

The idea was to compare IC engine horsepower with electric, a 100Hp engine has 100 Hp at ~ 5000rpm and 50 hp at 2000 rpm

Max battery Amps * Pack Voltage x 1.33 x 2 / 1000 ~= equivalent IC engine horsepower

In driving a 50Hp electric motor will "feel like" a 100 Hp IC engine,
bit like a diesel, if you rev the nuts off a petrol (gas) vehicle it is faster - if you drive normally the diesel feels quicker

It was also an over the top responce to DavidDymaxion

500 A * 144 V / 2 / 1000 ~= 36 hp -- old VW bug speeds

See less See more

Duncan

· #10 · Nov 23, 2010

Hi DavidDymaxion

Duncan, tell me where I can buy those infinitely stiff (in Voltage) lithium batteries you used in your formula!

Why its simple!

Let's do a few examples. Let's assume these efficiencies: 98% controller, 85% motor, and 85% tranny/diff/bearing losses.

The same place that you get an IC car without these or equivalent inefficiencies!

The purpose of this comparison is to help gabl136 to see what sort of system he needs to replace his IC engined car

a 500Amp 144 volt system in most cars will be equivalent to a "normal" car and will blow the doors off an VW bug and be the equivalent of a 150 -190 hp car

a 1000amp 144 volt system will be quite sporty!

See less See more

DavidDymaxion

· #11 · Nov 23, 2010

If you are talking about off-the-line torque, I agree. An electric car often has 2x or 3x the torque at low rpm of a gas engine with equivalent horsepower. They have so much low end torque many folks routinely start off in 2nd gear, even with a low end system. Folks with high end systems burn rubber from a standstill taking off in the equivalent of 3rd or 4th gear! This is great for stop-and-go driveability, and lessens the need to shift. I have both an electric and a diesel, and I agree this is great for city driving.

If you are talking about top speed or 1/4 mile times, hp becomes the more important number.

The 500A 144V cars I have driven or ridden in were pretty sluggish, they felt closer to the 1961 VW bug I used to drive than a modern 4 cylinder.

Anyway, getting back ot the original poster's questions: A 500A 144V system in a 2000 lb car should be adequate for his 60 mph needs. If money is no object go for more!

Duncan said:
Hi DavidDymaxion

Duncan, tell me where I can buy those infinitely stiff (in Voltage) lithium batteries you used in your formula!

Why its simple!

Let's do a few examples. Let's assume these efficiencies: 98% controller, 85% motor, and 85% tranny/diff/bearing losses.

The same place that you get an IC car without these or equivalent inefficiencies!

The purpose of this comparison is to help gabl136 to see what sort of system he needs to replace his IC engined car

a 500Amp 144 volt system in most cars will be equivalent to a "normal" car and will blow the doors off an VW bug and be the equivalent of a 150 -190 hp car

a 1000amp 144 volt system will be quite sporty!

JRP3

· #12 · Nov 23, 2010

My 115V nominal 550A max AC31 in my 2500lb Fiero is certainly not sluggish, though not a speed demon either. I'm surprised to hear 144V 500A DC systems described as sluggish or marginal for a 2000lb vehicle.

DavidDymaxion

· #13 · Nov 23, 2010

"Feel" is hard to quantify, and what feels fast to one person might feel slower to another. (For instance, after my first autocross run in my V8 car I was shaking with fear it felt so fast! After some experience it doesn't feel as fast now, even though I'm significantly faster with more practice.) How about some hard numbers (from anyone): What times do you get for 0 to 60 time or 1/4 mile time?

Look at NEDRA records for 144V cars for 1/4 mile times and speeds: ( http://www.nedra.com/record_holders.html )
22.5 seconds at 61 mph
19.4 seconds at 68 mph
19.2 seconds at 67 mph
18.5 seconds at 71 mph
14.7 seconds at 89 mph (I'm sure this is a 2000 A car)

Those first 4 times are consistent with cars that have about 50 to 90 hp. I don't know if they ran 500A or 1000A controllers, but 18 to 23 seconds in the 1/4 is pretty sluggish. The last one is consistent with a car that has 176 rwhp (per a drag race hp calculator, I assumed 3000 lbs for each vehicle).

I couldn't find a spec for the bug, but I found a spec for the 44 hp Karman Ghia for the 1/4:
21.7 seconds at 61 mph

Two items: The vehicles I drove were probably 3000 to 4000 lbs, so that certainly hurts. Also, they had cheap floodies, lots of internal resistance, lots of voltage sag. Good AGMs or lithiums would have helped.

Here's another data point: My 48V (but as much as 2000 Amps!) car feels like it would out accelerate the 500 A 144 V systems I have driven, maybe up to 30 mph. Passengers express surprise at how quickly it takes off. However, my car is much lighter, top speed is low, and hill climbing has to be done at low speeds. It would be an adequate city car, but not safe for the highway or hills.

JRP3 said:
My 115V nominal 550A max AC31 in my 2500lb Fiero is certainly not sluggish, though not a speed demon either. I'm surprised to hear 144V 500A DC systems described as sluggish or marginal for a 2000lb vehicle.

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JRP3

· #14 · Nov 23, 2010

My 0-60 is about 16 seconds, just leaving it in second gear. 0-30 in first is about 4 seconds. I think I could improve my 0-60 by using first, second, and third, but I haven't had a working foot throttle in quite a while to try it. Clutchless with regen and a trigger throttle strapped to the gear lever means shifting at speed is difficult to get right. I really need to get my throttle issues sorted.

DavidDymaxion

· #15 · Nov 24, 2010 (Edited)

Thanks for a real life number. You would destroy an old VW bug.

Here's a link for the 10 slowest modern cars: http://blogs.motortrend.com/top-10-slowest-vehicles-recently-tested-by-motor-trend-1768.html The 10 slowest are mosly 10 to 12 seconds 0 to 60, with the Smart lagging alot at 15 seconds. 1/4 miles are 17 to 18 second range, in the 70's for mph. My 48V car has an infinite 0 to 60 time!

JRP3 said:
My 0-60 is about 16 seconds, just leaving it in second gear. 0-30 in first is about 4 seconds. I think I could improve my 0-60 by using first, second, and third, but I haven't had a working foot throttle in quite a while to try it. Clutchless with regen and a trigger throttle strapped to the gear lever means shifting at speed is difficult to get right. I really need to get my throttle issues sorted.

azdeltawye

· #16 · Nov 24, 2010

DavidDymaxion said:
The 500A 144V cars I have driven or ridden in were pretty sluggish, they felt closer to the 1961 VW bug I used to drive than a modern 4 cylinder.
...

DavidDymaxion said:
Look at NEDRA records for 144V cars for 1/4 mile times and speeds: ( http://www.nedra.com/record_holders.html )
22.5 seconds at 61 mph
19.4 seconds at 68 mph
19.2 seconds at 67 mph
18.5 seconds at 71 mph
14.7 seconds at 89 mph (I'm sure this is a 2000 A car)

Those first 4 times are consistent with cars that have about 50 to 90 hp. I don't know if they ran 500A or 1000A controllers, but 18 to 23 seconds in the 1/4 is pretty sluggish. The last one is consistent with a car that has 176 rwhp (per a drag race hp calculator, I assumed 3000 lbs for each vehicle).

I couldn't find a spec for the bug, but I found a spec for the 44 hp Karman Ghia for the 1/4:
21.7 seconds at 61 mph
....

The numbers you quoted do not apply to Lithium.

My 3400 lb 144V ThunderSky Lithium Jetta conversion will do 0 – 60 in 12.5 seconds. This is with the battery current limit set to 540A…

jeremyjs

· #17 · Nov 24, 2010

google has a built in KW to HP calculator. Multiply the amps times volts "2000 A * 336 V" for the Kw's then Just type in, for example, "672 Kw to hp" and it will output "672 kilowatts = 901.166844 hp". Obviously that's a baseline number not taking into account voltage drop and motor and controller efficiency.

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