# [EVDL] Coast down test for determining watt hours per mile ? (Or motor power ?)

2023 Views 3 Replies 1 Participant Last post by  EVDL List
I'm trying to come up with a way to gage how many watt hours per mile an EV
candidate will need.

Has anyone done a coast down test to determine how much power or how many
watt hours per mile a candidate will need ?

Energy = 1/2 mass * velocity ^2.

Take the car up to 70 MPH on a flat straight road. Put it in neutral. Time
how long it will take to coast down from 70 to 60 MPH. Calculate the
energy at 70 MPH. Calculate the energy at 60 MPH. Take the difference.
Divide by the time, should give you the power required to roll the car at
the mean (or the geometric mean) speed. Yes/No ?

Then to figure out the motor power, add in a factor for the differential and
gearbox and the motor efficiency itself. Would this work ?

I'm finding it hard to gage how much power a candidate will need at 65 MPH.
I was using fuel consumption numbers, but they vary for the same vehicle
depending which engine it has and what gearing it uses, so I don't think
they would be that reliable.

--
View this message in context: http://www.nabble.com/Coast-down-test-for-determining-watt-hours-per-mile---%28Or-motor-power--%29-tp16834875p16834875.html
Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
See less See more
1 - 4 of 4 Posts
Sorry bro, I'm great on the math, but I'm going to opt
for "real world" experience.
65 mph at 144V in a 92 Honda Civic sedan will require
about 200-250amps. Sorry I can't be more accurate,
but the slightest hill makes a difference. And
without compression of cylinders working against you,
there is little coast-down. Ie, I can try to hold a 65
at 200A, and eventually I need to pump it up to 250A
for a few seconds, so likely on a perfectly
straight/flat, it's more like 225.
Similarly, 40A to 100A for a 40 mph cruise.
Best to you,

--- me2 <[email protected]> wrote:

>
> I'm trying to come up with a way to gage how many
> watt hours per mile an EV
> candidate will need.
>
> Has anyone done a coast down test to determine how
> much power or how many
> watt hours per mile a candidate will need ?
>
> Energy = 1/2 mass * velocity ^2.
>
> Take the car up to 70 MPH on a flat straight road.
> Put it in neutral. Time
> how long it will take to coast down from 70 to 60
> MPH. Calculate the
> energy at 70 MPH. Calculate the energy at 60 MPH.
> Take the difference.
> Divide by the time, should give you the power
> required to roll the car at
> the mean (or the geometric mean) speed. Yes/No ?
>
> Then to figure out the motor power, add in a factor
> for the differential and
> gearbox and the motor efficiency itself. Would
> this work ?
>
> I'm finding it hard to gage how much power a
> candidate will need at 65 MPH.
> I was using fuel consumption numbers, but they vary
> for the same vehicle
> depending which engine it has and what gearing it
> uses, so I don't think
> they would be that reliable.
>
> --
> View this message in context:
>
http://www.nabble.com/Coast-down-test-for-determining-watt-hours-per-mile---%28Or-motor-power--%29-tp16834875p16834875.html
> Sent from the Electric Vehicle Discussion List
> mailing list archive at Nabble.com.
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

Thinking about converting a gen. 5 ('92-95) Honda Civic? My \$23 "CivicWithACord" DVD (57 mins.) shows ins and outs you'll encounter, featuring a sedan; a del Sol, and a hatchback, each running 144V/18 batteries. It focuses on component/instrumentation/battery placement and other considerations. For more info, http://home.budget.net/~bbath/CivicWithACord.html
____
__/__|__\__
=D-------/ - - \
'O'-----'O'-'
Would you still drive your car if the tailpipe came out of the steering wheel?

____________________________________________________________________________________
Be a better friend, newshound, and
know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
Bob Bath wrote:
> Sorry bro, I'm great on the math, but I'm going to opt
> for "real world" experience.
> 65 mph at 144V in a 92 Honda Civic sedan will require
> about 200-250amps. Sorry I can't be more accurate,
> but the slightest hill makes a difference. And
> without compression of cylinders working against you,
> there is little coast-down. Ie, I can try to hold a 65
> at 200A, and eventually I need to pump it up to 250A
> for a few seconds, so likely on a perfectly
> straight/flat, it's more like 225.
> Similarly, 40A to 100A for a 40 mph cruise.
> Best to you,

Here is why I dislike "real world experience". 144V x ~200A = 28.8 KW.
Call it 40HP. Now there is no way that it takes 40HP to power a 1992
Honda Civic down the road at 65 MPH.

The other thing is that that number is the power input to your
controller and electric motor, not how much energy it takes to push the
car. I want to know how much energy its taking to push the car.

How big is your electric motor ? Does it get hot at 65 MPH ?

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
I've come across a roll down test that was presented on this list some
time ago and put into a nice PDF format. It may be helpful:

http://www.2shared.com/file/3194217/d7d573b5/Roll-down_test.html

-Jon Glauser
http://www.evalbum.com/555

<quote who="me2">
>
> I'm trying to come up with a way to gage how many watt hours per mile an
> EV
> candidate will need.
>
> Has anyone done a coast down test to determine how much power or how many
> watt hours per mile a candidate will need ?
>
> Energy = 1/2 mass * velocity ^2.
>
> Take the car up to 70 MPH on a flat straight road. Put it in neutral.
> Time
> how long it will take to coast down from 70 to 60 MPH. Calculate the
> energy at 70 MPH. Calculate the energy at 60 MPH. Take the difference.
> Divide by the time, should give you the power required to roll the car at
> the mean (or the geometric mean) speed. Yes/No ?
>
> Then to figure out the motor power, add in a factor for the differential
> and
> gearbox and the motor efficiency itself. Would this work ?
>
> I'm finding it hard to gage how much power a candidate will need at 65
> MPH.
> I was using fuel consumption numbers, but they vary for the same vehicle
> depending which engine it has and what gearing it uses, so I don't think
> they would be that reliable.
>
> --
> View this message in context:
> http://www.nabble.com/Coast-down-test-for-determining-watt-hours-per-mile---%28Or-motor-power--%29-tp16834875p16834875.html
> Sent from the Electric Vehicle Discussion List mailing list archive at
> Nabble.com.
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
See less See more
1 - 4 of 4 Posts