# Battery calculation



## Jan (Oct 5, 2009)

300Wh per mile is a conservative estimation. 

Your motor must not be to much underpowerd. If it gets hot or to hot, there is some of the energy wasted.

And nobody owns English. Not even the grammar nazis.


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## PhantomPholly (Aug 20, 2008)

Watt Hours per mile is completely vehicle and driver dependent, just like mpg. 300Watt-Hrs per mile is approximately equal to a 30mpg car being driven by the sort of person who actually gets 30mpg from it.

The easiest number to wrap your head around is that 8KwHrs is approximately equal to 1 gallon of gas for an equivalent vehicle of equivalent gross weight (sort of). Since batteries are very heavy, just adding more is self-limiting because of prohibitive weight. Making your battery too small will also cause problems, because high discharge rates necessary for acceleration from a small pack cause "sag" that reduces the total amount of effective power you can use from the pack.

The size of the motor is only important in two ways:
1. If it is undersized, you will have miserable performance
2. If it is over sized, it will eat into the weight penalty of the vehicle for efficiency.

Other than that, most electric motors run between 80-90% efficient, so there is not a whole lot of difference.

Finally, I'm about the least EV-knowledgeable person on the forum. I've reduced everything I've read to these rules of thumb because I'm either too lazy or mentally slow to learn the precise math for all of these things.

Hope that helps!


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## ovi_gm (Nov 7, 2011)

Thx, that helped a lot.

So by now I made my decision to use a 20HP and a 96V battery pack system.

I am thinking of converting a small to medium car like 1990 opel kadett or something like that.

Would you assume that 10kwh is enough for aprox 50km range or should i get more?

as i understand i have to keep in mind the battery dod to be not less than 50%.

does that mean i would in fact need 20kwh of power?


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## muffildy (Oct 11, 2011)

the DoD you can go to depends on the chemistry of the battery you intend to use. Lithium iron phosphate batteries have the greatest available energy per pound i think.

a 20 hp motor will accelerate the stock opel cadet from 0-60 in around 28 seconds. 

I think a 20hp motor would overheat for long distances in that heavy of a car unless it was a liquid cooled motor.

If you use the popular 3C discharge batteries it would cost 50$ or so for 3.2v 40ah
so 96/3.2 = 30 cells = 1500 + BMS, this is a 3.84kwh system
your max amps would be 120 at 96v thats 11.5kw which is not quite the 20hp the motor needs. So, at a minimum you would need 2x this system or 60 cells and 7.68kwh for 3k + bms
Assuming 300wh/mi thats 7680/300 = 25.6 mi range if 100% dod
Im pretty sure lithium shouldnt be discharged more than to 20%, so at most you would be getting 20.48 miles. Thats assuming 100% efficiency though - most likely you will only achieve 70-90% system efficiency. So then your range would be 14 miles to 18.4 miles.
but you asked in kilometers...so
22.5km to 29.6km.
Which would mean to get the range you want you would in fact need that 15kwh pack costing around 6000 + BMS (battery management system) BMS probably run another 3k.

*edited for h's...*


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## Sunking (Aug 10, 2009)

ovi_gm said:


> as i understand i have to keep in mind the battery dod to be not less than 50%.
> 
> does that mean i would in fact need 20kwh of power?


The DOD will depend on which battery chemistry you choose. For lead acid batteries you do not want to go past 50%, but IMO that is still too deep of a discharge for lead acid because it will significantly limit the number of charge/discharge cycles.

With Lithium you can go safely down 80% DOD without any significant cycle loss of life, plus it has twice the energy density so we are talking significant weight reduction. 

Lastly IMO your estimate of 200 wh / Km is too optimistic. 250-275 wh/Km is probable more realistic for DIY purposes.


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## ovi_gm (Nov 7, 2011)

Thank you all for your answer. Well, doing the math this whole conversion to electric thing might not be so interesting at all.

I know that lead acid batteries have about 2-3 years of life so that means that every 3 years I have to buy another pack.

That means the costs would be approximately the same I am paying now for gasoline. A little less but the difference is too small anyway.

So, does it really worth it in the end?


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## PhantomPholly (Aug 20, 2008)

No hobby should be pursued under the mistaken impression that it will save you money. Boats, airplanes, and electric vehicles come to mind.


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## Ziggythewiz (May 16, 2010)

ovi_gm said:


> That means the costs would be approximately the same I am paying now for gasoline. A little less but the difference is too small anyway.
> 
> So, does it really worth it in the end?


I'd rather spread my money on innovative technologies to various vendors (hopefully mostly american) than just give it all to Opexxon.


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## muffildy (Oct 11, 2011)

lithium iron phosphate cells (which are the kind i costed out for you) will last longer than 3 years. No one knows for sure the exact figure yet, but 8 years to 80% of the original capacity is fairly likely.
that being said, it will likely cost about the same as just using gas until the gas becomes more expensive or the batteries come down in price.


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## ovi_gm (Nov 7, 2011)

I understand now. Thank you for your answers. Topic can be closed.


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## Ziggythewiz (May 16, 2010)

ovi_gm said:


> I understand now. Thank you for your answers. Topic can be closed.


You must be new here. We don't close topics, we like to drag them on forever.


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## EVfun (Mar 14, 2010)

Ziggythewiz said:


> You must be new here. We don't close topics, we like to drag them on forever.


LOL -- we are flogging this one for all it's worth right now! 

Welcome to the DIY EV forums, ovi_gm.


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## muffildy (Oct 11, 2011)

sooo bored!, now if that one guy in the other thread would just tell me what unit conversions i missed i might learn something.


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## Sunking (Aug 10, 2009)

ovi_gm said:


> That means the costs would be approximately the same I am paying now for gasoline. A little less but the difference is too small anyway.
> 
> So, does it really worth it in the end?


If you are doing this in hopes of saving money, you are going to be very disappointed and learn an expensive lesson. It will actually cost you quite a bit more over an ICE vehicle. You also forgot about electricity cost. Based on 400 wh/mile and 12-cents per Kwh electric fuel cost will be $0.048/mile. Still better than gasoline with a 30 mpg vehicle @ $3.20/gal of $0.106/mile.

As Philly indicated this would be a hobby or labor of love venture.


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## dladd (Jun 1, 2011)

while I agree that I am not really saving much if any money, there is also value in deciding where my money goes. I will be spending money on energy one way or the other, but I like being able to decide who I pay for my energy, and where that energy is coming from. And the oil/gasoline companies are not where I want my $ going. you know the saying, be the change you want to see in the world.


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