# Watt-hour Estimate



## Ziggythewiz (May 16, 2010)

Higher voltage is more efficient because there are less amps pulled/burned in resistance. The best estimate would be to ask someone with a similar setup what they use at different speeds.


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## charliehorse55 (Sep 23, 2011)

ginfaxi082 said:


> I can make my daily commute without getting on the freeway


Having to comprise to meet your primary goal is not the sign of a functional design. 

It is easily possible to achieve that range while driving at highway speeds! While you do use more power travelling at a higher speed, there are many EVs using lithium-ion batteries to achieve 100+ mile ranges even while driving at high speeds. 

If you set out to create a vehicle that can't fulfill your needs, you won't be satisfied with the result.


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## Ace_bridger (Nov 22, 2011)

My advice would be

1) go as high as possible with voltage to reduce current and the associatted losses. Power lost due to heat = I^2 x R. So it you reduce the current then you reduce your heat losses significantly. Hence why domestic electricity is tranfer through power lines at 50,000V and not 110V (or 240V if you live where I do)

2) If you want range then squeeze in as many batteries as possible.You can use the following to calculate what you'll need. The range of an EV is determined by how much power your pack has. Power in the pack = volts x Ah. So a 144V 100Ah pack has 144 x 100 = 14,400 Wh. You could estimate how many Wh/mile your car uses, ask people on here who have comparable vehicles what they get. My VW Golf was measured at using ~300 Wh/mile. To get range simply divide Wh by Wh/mile. I.e. for the above calcs Predicted Range = 14,400/300 = 48 miles. Bear in mind that is estimated and also assumes that the pack will supply 100Ah at that current. A 100Ah pack will deliver 1A for 100 hours but it will not deliver 100A for 1 hour. Check the specification of the batteries.

3) have fun building and driving an EV!!!! 

Cheers,

Ace


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## ginfaxi082 (Jan 13, 2012)

> My VW Golf was measured at using ~300 Wh/mile.


Is that a constant power consumption, or is it 300 Wh/m at, say, 60 mph?



> It is easily possible to achieve that range while driving at highway speeds! While you do use more power travelling at a higher speed, there are many EVs using lithium-ion batteries to achieve 100+ mile ranges even while driving at high speeds.


I hear what you're saying, but there is a method to my madness. I'm in college right now, so I don't have the largest budget to throw at an EV project. But I figure I will when I graduate. And that's around the time my oldest kid will be getting her driver's license. So I figured I'd drive the inexpensive, low-speed build until I graduate, then build a faster one and give the low-speed one to my kid. Besides, I'd sleep a little better knowing that my baby is driving a car that can't go faster than 45 mph! 

Anyway, I was going to convert an old beetle, but that deal fell through. Right now, I'm trying to secure a deal on a late 90's Saturn sedan. Has anyone used one of these as a donor for an EV conversion? And, if so, how do they perform? I suspect it would be a good candidate due to it's decent aerodynamics and low curb weight, but I am admittedly a novice at this stuff.

Thanks for all the input!


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## tomofreno (Mar 3, 2009)

The energy stored in a battery pack depends on both the pack voltage and battery capacity (Ah), since the energy per charge is the product of charge and voltage, qV. A 200V pack of 100 Ah batteries has the same energy storage as a 100V pack of 200 Ah batteries: 100 x 200 = 20,000 Wh, or 20 kWh. Higher pack voltage reduces the current flow from the pack for a given motor power output, but the savings in energy loss isn't all that great since the resistances are so small - unless you have loose connections. The energy loss internally in the batteries will be larger than this if you use lead acid batteries.

You should size the motor to the power required to move your vehicle. Any motor you use has to deliver this power, and if you use a small one not designed to supply constant power at the required level it will overheat. Answers to your questions and more are in the wiki here:


http://www.diyelectriccar.com/forums/showthread.php?t=669




ginfaxi082 said:


> Is there a good way to estimate watt-hours per mile at a given speed? I'm on a budget, and my daily commute is kind of far, so I'm wondering if I can use a smaller motor with alot of batteries. I can make my daily commute without getting on the freeway, so a 40 to 45 mph top speed is all I really need. But I do need at least a 60 mile range.
> 
> So I was thinking of using a 48 or 72 volt motor, which would require less batteries in series to attain my required voltage. Then I could afford to wire more batteries in parallel giving me a longer range.
> 
> ...


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## tomofreno (Mar 3, 2009)

Ace_bridger said:


> ...The range of an EV is determined by how much power your pack has. Power in the pack = volts x Ah. So a 144V 100Ah pack has 144 x 100 = 14,400 Wh.


 Probably just a typo, but to clarify, the product of voltage and charge, V x Ah, is energy. The pack Wh determines the upper limit on the amount of work that can be done moving the vehicle. Power is energy per unit time, Watt, the rate of work done as the vehicle moves at constant speed for example.


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## tomofreno (Mar 3, 2009)

ginfaxi082 said:


> Anyway, I was going to convert an old beetle, but that deal fell through. Right now, I'm trying to secure a deal on a late 90's Saturn sedan. Has anyone used one of these as a donor for an EV conversion?


Do a search in the garage here (tab at upper right of your screen) and on www.evalbum.com. Careful about range estimates though, as some are just WAGs (wild ass guesses) made before the car was ever run. Follow up with the owner if possible to see how the estimate was determined. There are also ev calculators you can use to estimate range.

Asking questions is good, but you will learn a lot more if you use the resources here and ask follow up questions.


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## ginfaxi082 (Jan 13, 2012)

tomofreno said:


> Do a search in the garage here (tab at upper right of your screen) and on www.evalbum.com. Careful about range estimates though, as some are just WAGs (wild ass guesses) made before the car was ever run. Follow up with the owner if possible to see how the estimate was determined. There are also ev calculators you can use to estimate range.
> 
> Asking questions is good, but you will learn a lot more if you use the resources here and ask follow up questions.


Awesome link - thanks!!!


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## Ziggythewiz (May 16, 2010)

charliehorse55 said:


> Having to comprise to meet your primary goal is not the sign of a functional design.
> 
> It is easily possible to achieve that range while driving at highway speeds! While you do use more power travelling at a higher speed, there are many EVs using lithium-ion batteries to achieve 100+ mile ranges even while driving at high speeds.
> 
> If you set out to create a vehicle that can't fulfill your needs, you won't be satisfied with the result.


It's also good to consider the number of stops on your route. If you have to go out of your way to avoid the highway, and there are many stops, it's likely you'll use just as much (possible more) power at the lower speed.


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## ginfaxi082 (Jan 13, 2012)

Ziggythewiz said:


> It's also good to consider the number of stops on your route. If you have to go out of your way to avoid the highway, and there are many stops, it's likely you'll use just as much (possible more) power at the lower speed.


Actually, the shortest distance is the non-freeway route. Taking the freeway adds a couple miles. And I pass work on my way to school, so I never NEED to get on the freeway. So if I can save money and/or increase range by going slower, I definitely want to go that route!


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## Ziggythewiz (May 16, 2010)

ok, just checking. Your wording made it sound like it was only possible (not preferred) to avoid the highway.

You'll likely need something like 144V (45 cells) x 180 ah

Will be a little difficult in a small vehicle, but certainly doable. Check out corbin's bug.


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## Ace_bridger (Nov 22, 2011)

My Golf was measured at ~300 Wh/m over a combined trip. Both low speeds and faster routes.

Wh/mile is indeed energy and quickly that energy is used is power, sorry typo


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