# Voltage Vs Top Speed



## Ace_bridger (Nov 22, 2011)

I was just wondering whether anyone had pack voltage Vs top speed information which would be appropriate for my VW Golf.

I'm going through inner turmoil as I consider battery options. I built an electric bike and spec'd the batteries to match my modest commute requirements and as such it is a great success.

I want a useful and enjoyable EV but again I have modest performance and range requirements. I live in a built up area and am unlikely ever to use the car on the freeway/motorway. I have been thinking that I would replecate the pack that was in the conversion before I bought it, that being 120V Trojan, but I only need the car to do 40 mph so was considering lowering my pack voltage such that I can access cheaper batteries and controllers whilst extending my range (E=MC^2 and all that!). I really don't need a vehicle that does 70 mph and would hate to think of my poor batteries trying to release enough energy to overcome the drag...not efficient EV motoring, what?

If anyone could estimate data relating top speed to pack voltage I would be your friend for quite a long time 

Cheers,

Adam


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## Guest (Jan 22, 2012)

My experience says stay with the 120 volt system in place. Remember you do not need to drive faster than 40 mph even if your car can go 70 mph. Just stay off the throttle. But if you ever found you needed to get out of the way of some idiot driver you would have a vehicle that could respond much better than say one with only 72 volts. Keeping the 120 volt system would allow you to retain the current standards already in the vehicle saving you money. I do hope your considering lithium batteries. A good pack of 160 ah or 200 ah lithium cells will give modest performance and long run times between charges. And at speeds of 40 mph or less you could have a very long range car and very long life batteries.


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## Ziggythewiz (May 16, 2010)

120V gives me a top speed of 45mph in 2nd gear, so if you only want 40mph that's a great voltage to be at. If you went with lower voltage, you would likely need to shift around alot to still get to 40.


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## mora (Nov 11, 2009)

96V and 9" Kostov (220V version) in my Golf give me a top speed of 100 km/h (roughly 60 mph). Amps are around 150-175 then. When going 80 km/h (~50 mph), 100-125A. Slower than that and amps stay well under 100. If voltage was higher amps would be lower at those speeds.

Do you have electric heater in your Golf? Plan to add it later if there is none right now? Go for 120V setup then (or higher). I bought ceramic elements rated for 120V AC and at 96V DC those aren't producing enought heat for me. It is fine at 96V until temperatures drop below 0C. I wired a temporary setup, 128V, and tried those elements again. Huge difference. Same applies if you get/have 220V heating elements. 120V doesn't cut it then.


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## Salty9 (Jul 13, 2009)

Ace_bridger said:


> . . . . . (E=MC^2 and all that!). . . .


F= ma might be more appropriate.


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## Ace_bridger (Nov 22, 2011)

Thanks guys, all valid points. I know I should be buying lithium but I may have found a good source of cheap 6V Trojans and am concerned about weight of a full 120V 225Ah pack. (I know, lithium would solve this problem too but I'll struggle stretch to a decent pack of lithium in the short term). I could pick up a relatively inexpensive set of Trojans and then save for a full lithium pack in 2 to 3 years. A new drain on my resources is due in May, not sure if it's a boy or a girl one as yet!!

I could just stick 72V in and then add as required as I'll be using a 144V controller and will ensure the rest of the conversion allows for this (wiring, fuses, etc). That would cost me about GB£240 and get me moving and then all the money I save on gas can go towards the lithium pack that I have on a pedestal at the moment....uuummmm....lithium!!! 

Ahem Salty? Force does indeed equal mass x acceleration but the energy of a body also equals it's mass x velocity squared...hence the energy to keep a body moving equals it's mass x velocity squared


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## Salty9 (Jul 13, 2009)

E = mc^2 is used in the conversion of mass and energy. Usually on the nuclear scale. Kinetic energy is E = 1/2mv^2. http://en.wikipedia.org/wiki/Kinetic_energy


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## Ace_bridger (Nov 22, 2011)

You're right, I realised this morning in the shower...I should have said that E is proprtional to mc^2 

Either way, the faster I go the more energy I use and it's a square function.

So, perhaps I'll drive at 10 mph and have one of those men with a red flag in front!! 



Salty9 said:


> E = mc^2 is used in the conversion of mass and energy. Usually on the nuclear scale. Kinetic energy is E = 1/2mv^2. http://en.wikipedia.org/wiki/Kinetic_energy


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## Ziggythewiz (May 16, 2010)

Pretty sure if you're going 10 mph you'll want the flagger behind you.


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## Ace_bridger (Nov 22, 2011)

True, he'd be road kill unless he can run fast.

So, nobody has any data on Voltage Vs Speed then?


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## madmike8 (Jun 16, 2011)

Well... Yabert kinda does...

http://www.diyelectriccar.com/forums/showthread.php/72v-systems-small-car-68596.html


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## Yabert (Feb 7, 2010)

Ace_bridger said:


> So, nobody has any data on Voltage Vs Speed then?


Hi Ace

Speed is related to power, not about voltage. But power is related to Volt x Amps = Watts 

So theoretically, if a car need 10 kw (on battery side) to maintain 40 mph there aren't difference if the battery pack provide 24v and 417A or 120v and 83A.
But in reality, it can be hard to put only 24v and 417 amps on a motor because you can't controlled the motor RPM.

Anyway, most DC EV motor are rated around 200A for one hour, so they need more voltage to create more power (turning faster).

In your case, if you have the 144v controler, go for 144v systems.
If you have other reason to think than a lower voltage systems can be good for you, the link above is for you.


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## Ace_bridger (Nov 22, 2011)

Cheers Yabert, I'll follow your thread with interest.

I think I'll get some batteries around me and do some research. I know some of you will take a sharp intake of breath, make the sign of the cross and suggest I say some Hell Marys but...I'm going to find secondhand Trojan lead to get the car going while I save for a lithium pack.

As I find them I'll add them and see the effect on speed and driveability. I've heard of 72V cars, 96V cars, 120V, etc and they all have comments regarding performance, of course.

Given a free unrestricted budget I would go for 144V 100Ah+ lithium but impending additional new drains on my resources (children as some people call them!) are causing delays in getting my wish list!! 

I'll post results in a few months.

Cheers for all the comments and advice.

Ace


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## spdas (Nov 28, 2009)

What I have observed in my small car conversion. (Yaris 2650#, Warp9, Zilla, Lithium). This is only using 1-1.5 C and using about 2/3 of the cell amp capacity. Of course you could push it a lot more.

40 x 180 ah cells =128v gives you a comfortable freeway cruise speed

44 x 180 ah cells = 141v give you crisp freeway speeds and intown great take-off power

I am using 52 x 180 ah cells = 166v and I get great freeway speed and jackrabbit in town power, but weight is becoming an issue. But have 150mile range,

I think ideally using 56 to 60 x 130ah cells as I rarely need more than 70 miles range, but like the power. Possibly even using 100ah.

francis


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## Ace_bridger (Nov 22, 2011)

Great range Francis and many thanks for the data. I'm heavier and have a greater cross-sectional area with my Golf and mixed with a possible lead pack I think I've some work to do to collect sufficient batteries to achieve a reasonably spec'd car...but a good spec'd car I will finally get, I have no doubt.

Thinking, thinking, think.....great fun!!!


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## DavidDymaxion (Dec 1, 2008)

Remember power = Volts * Amps. As things size and get hot more with Amps, usually you'll want to go higher Voltage rather than higher in Amps. Most converters go as high in Voltage as they can, until they hit space limits, or equipment limits.

I was able to go 55 mph on 48V, and 59 mph on 96V (I think I can go faster, but cooked a controller one time and brushes the other time). My next stop is 144V, I'd go higher but I have a 144V controller.


Ace_bridger said:


> True, he'd be road kill unless he can run fast.
> 
> So, nobody has any data on Voltage Vs Speed then?


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## Woodsmith (Jun 5, 2008)

Ace_bridger said:


> A new drain on my resources is due in May, not sure if it's a boy or a girl one as yet!!


Congratulations!

Start planning for one of those Toylander things to encourage the EV grin from an early age.


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## dougingraham (Jul 26, 2011)

Ace_bridger said:


> So, nobody has any data on Voltage Vs Speed then?


It is dependent on a lot of unknowns. The specific motor characteristics , the final drive ratio, the wheel diameter, road grade, air temperature, humidity, rolling resistance, vehicle mass and what you consider to be a tolerable acceleration. An example would be a vehicle which on paper has a top speed of 40mph but takes a minute to get there on level ground vs one that could reach that speed in a few seconds. The former would not be a fun or particularly safe car to drive on city streets but it would get to 40 eventually. In order to make a car that would be usable you can't design for a top speed. It is going to have to be able to go faster than the speed limit or you won't be happy with the acceleration.


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## Ace_bridger (Nov 22, 2011)

Yeah, with heat loss being I^2xR the higher the voltage the low current an vice versa.

I wasn't so much involving myself with designing for a given top speed rather trying establish an estimation for speed for a given pack voltage.

The answer is, as you've contributed, is to go as high as is feasible considering space, weight and price.

My battery selection dilema continues.

Woodsmith: thanks for the congratulatory message and I hope your recovery continues.


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