# Simple Formula



## aggiemarine07 (Nov 26, 2013)

I was wondering if anyone out there can provide me a simple formula to figure out the basic needs for converting a vehicle to electric.

I've seen the EV calculator spreadsheet elsewhere on the forum but its way to complex for someone like me who was a non-engineering major in college.

My question is this, is there a simple way for me to input the weight of my vehicle, max speed I want to go, and the max distance I want to cover on one charge to help me figure the correct electric motor in volts (max speed) and the total KwH I need in batteries (max distance)?

It would look similar to this:

Variables/Requirements:
Weight = 1500 lbs
Max Speed = 55 mph
Max Distance = 50 miles

You will need:
Max voltage = 144v motor to obtain 55 mph speeds with that amount of weight
Max KwH = 10,000 kwH to obtain 50 miles on one charge with that amount of weight

I know the voltage/KwH numbers are wrong but something along those lines is what I am imagining? Can it be done that with that amount of simplicity? Thanks!


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## dcb (Dec 5, 2009)

I usually search/browse evalbum.com to get an idea of what batteries+voltage/motors/controller will do for top speed and range in similar sized vehicles. No engineering required, just lots of clicking.


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## aggiemarine07 (Nov 26, 2013)

Thats another great reason. It would prevent a lot of clicking and give people a good starting point to go find the right equipment rather spend a lot of time trying to figure out what power requirements they need.


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## dcb (Dec 5, 2009)

Well that is the thing, there is a large element of DIY here, so a fair bit of homework comes with that. There are plenty of sites selling conversion kits, and they have the "right" motor, though they may have different "right" motors and budget is a huge factor as well. I enjoy learning about this stuff, and saving money, but I expect to screw up a few things along the way (part of learning), so that is part of the cost of my "education". 

But one can go to evalbum and ape any vehicle/system there and expect similar performance. S10's are especially well covered, though their weight and size bump up the equipment costs vs performance as compared to a lighter and more streamlined vehicle.

i.e. based on: http://www.evalbum.com/1994
his s10, with 144v and a warp 9 (24hp cont), 100ah batteries looks like about 55mph top speed and 30ish mile range. If you used 180ah cells that would get you more like 50 mile range.

The same batteries in a fiesta, with a E-31B motor (18hp cont), 144v 100ah look like a 70mph top speed and a 50 mile range
http://www.evalbum.com/3832


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## aggiemarine07 (Nov 26, 2013)

So what your saying is that it cant be condensed to a simple formula? 

It cant be as simple as:

-Im driving a 2000 lbs truck what voltage of motor do I need to get 55 mph?
-With the same vehicle, how many KwH do I need to maintain this speed for 50 miles?

Thats the basis for my original post so that way when people start to research they know that no matter what they get, it needs to be a minimum of X.


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## sergiu tofanel (Jan 13, 2014)

Sure you can condense it into a formula. Just have to be careful about the old "garbage in, garbage out" adage.

Required battery capacity:


C = range * (1.98 * weight * Cr + 0.05 * CdA * speed^2) / (De * Pe)

Cr = rolling tire resistance (typically about 0.015)
CdA = vehicle frontal area (in square meters) x drag coefficient (ranges from 0.15 to 0.75
De = drivetrain efficiency (transmission, differential, bearing losses), approx 0.90
Pe = powertrain efficiency (motor controller + motor efficiency), approx 0.85

speed is measured in mph
range is measured in miles
capacity is in watt-hours


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## PhantomPholly (Aug 20, 2008)

It can be reasonably simple for a rule-of-thumb.

First, 8Kwh of batteries is approximately equal to 1 gallon of gas for purposes of propulsion, including some overhead so you don't discharge too deeply.

Second, the motor and controller don't weigh much compared to the batteries, so you can simply select a combination that gives you the performance you desire. Only challenge there is $$$.

Third, as you increase the pack you increase weight, causing "mileage" to go down and creating a situation of "diminishing returns."

Using just those three rules and estimates of weight / anticipated ICE-equivalent mileage based on pack size, you should be able to figure out about what you need to hit your target.

Roughly.

Mileage may vary....


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## dcb (Dec 5, 2009)

you have to know the coefficient of drag * the frontal area of the vehicle, plus the rolling resistance of the vehicle to have a good approximation of the power requirements for a given speed.

Here is a calculator with a fair bit of fudge in it to guess your power requirements for maintaining a constant speed (not acceleration) http://www.wallaceracing.com/Calculate HP For Speed.php

So if I put in (just guessing) .35 for my cda for my 94 civic, and 35 sq ft for the frontal area and 2522 lbs, and 65mph as my speed, I get an at the wheels power requirement of 29hp. Then I would need to find a motor that can sustain that, and figure out what gearing is required and what voltage, and account for additional weight of the conversion.


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## blahphish (Apr 24, 2014)

I'm brand new to EV and have been using this calculator for some rule of thumb figures: http://www.evsource.com/battery_calculator.php This might help answer the original question. I would love to hear if some of the more experienced guys thinks this is a good tool for ball park calculations.


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## dcb (Dec 5, 2009)

probably not since it doesn't consider the aerodynamic drag, which is exponential with speed, and is what cars/trucks spend most of their energy on while cruising at speed. It will give you a very rough guess, and could be off by a huge amount at higher speeds.

As in the case of the fiesta and the s10 in my previous post. Same batteries, very different result. Some of that due to weight but top speed is mostly limited by aerodynamics.

The bigger and boxier, the worse it gets:


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## blahphish (Apr 24, 2014)

Good info. Thank you.


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## PhantomPholly (Aug 20, 2008)

Ya, my prior post presumes you have a baseline for mileage at different speeds for the donor vehicle, and can adjust for weight increases. Weight doesn't have quite as much of a detrimental effect for cruising as high velocity, but it should also be obvious to most people that you can't stuff 2,000 pounds of batteries into an MG Midget.


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## dcb (Dec 5, 2009)

I guess I'm kind of on-message that smaller is better $$ wise too  The s10s were popular partly because they were affordable and could carry a lot of lead. But now the energy density of batteries is such that you can fit them into much smaller vehicles, and have less power demands (so you need less kwh). Your vehicle choice has a large impact on your system requirements, costs, and performance. Enough so that it is often well worth it to get a third hand econobox and convert it and sell your current vehicle (I live in a world surrounded by single occupant SUVs trucks and minivans).

i.e. per the attachment, a civic needs 26hp at 60mph, and an s10 need 66hp at 60mph. Which means the s10 needs at least 2.5 times the kwh pack for similiar range at 60mph (plus 2.5 times the current capacity if the system voltages are the same). Which very roughly means it will be 2.5 times as expensive to convert to electric and use 2.5 times as many kw/mile @ 60mph.

Also some cars are reasonably streamlined but heavy, which will slow down your acceleration and hill climbing ability.


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## PStechPaul (May 1, 2012)

The basic physics of energy and power requirements are simple, but there are all sorts of actual values for such things as drag, rolling resistance, drive train efficiency, losses due to the electronics, and possibly most important, driving style. My EVcalculator seems to come up with reasonable numbers for what is actually needed, but then you must add to that based on real-world efficiencies and experience.

You may also check the specifications of various DIY vehicles in the "garage" listing. Otherwise, it seems that most "smallish" EVs average about 250-350 Wh/mile. As they say, YMMV.


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