# Sizing a battery pack



## MN Driver (Sep 29, 2009)

Have you decided on your motor and controller yet? Knowing which motor you are using will help us with giving advice over the voltage that the system can allow and whether or not it would be good going to 160v for better performance. If you are going with a Warp 9 for example, I'd go with the higher 160v since that motor can handle 160v with no issues and you'll want extra performance to pass if you need to. I'm hoping someone with a VW can chime in on what sort of performance you would get for 65-70mph at 144 or 160v. Depending on which gearing you have in top gear you might still be spinning a bit fast to eek out a comfortable passing performance. I've been in a VW bug that was geared to where I'm not even sure it would survive 70mph driving with its normal gas engine installed. I might be wrong since I don't own a VW or know what RPM to expect at 70mph, how fast would your motor be spinning at 70mph?


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## Notthatbutton! (Jan 6, 2012)

I am probably looking at a warp 9 motor, no idea on controller yet. I am basing most of my numbers on _Cruisin_'s VW conversion. He claims to have a top end of 90mph at 130v. I don't know what the acceleration would be, but with a 160v system, I would think it would be considerable.


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## Sunking (Aug 10, 2009)

Notthatbutton! said:


> Estimated car efficiency : 250wh/mile


Me thinks you will find yourself on the side of the road waiting for a tow truck to take you home. IMHO 250wh/mile is being way to optimistic. Very few commercial production vehicles can do that, and I can only think of one that can. Me thinks 400 wh/mile is more realistic but still difficult for DIY


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## Notthatbutton! (Jan 6, 2012)

Okay, I will reconfigure the math at 400wh/m.

400wh/m X 115 miles = 46,000w
46,000w / 160v = 287.5ah
287.5 X 1.05 = 301.88ah

If my requirements are 301.88ah, how do I build my battery pack? Is it even possible with off the shelf batteries?


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## dimitri (May 16, 2008)

Notthatbutton! said:


> Okay, I will reconfigure the math at 400wh/m.
> 
> 400wh/m X 115 miles = 46,000w
> 46,000w / 160v = 287.5ah
> ...


And that is why you don't see many DIY EVs with 100 mile range.....

You need to reduce your range goal and increase voltage, which requires good controller like Soliton Jr. This will let you use smaller cells, like 100AH or 120AH or 160AH, but more of them, so you can stick them in every nook and cranny of your beetle.

As you are discovering, building EV is all about tradeoffs


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## Notthatbutton! (Jan 6, 2012)

Unfortunately the only way I would attempt the effort of starting an EV conversion and drop the 15 to 20 grand in a vehicle would be to use it as a daily commuter. I have to be missing something, I have seen other conversions that claim the 100 mile range, I just don't understand how they do it. Any ideas at all?


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## dimitri (May 16, 2008)

Notthatbutton! said:


> I have seen other conversions that claim the 100 mile range, I just don't understand how they do it. Any ideas at all?


Easy, they type "100 mile range" into the computer, that's how they claim it 

Seriously though, do you really drive 100 mile every day and have no way to charge during the day? If so, perhaps VW bug is not the best choice, get an S10 and fill the bed with cells.

40kWh pack is no joke, it takes an effort to put that in a sedan, especially small one. You need to use smaller cells and put them EVERYWHERE. You need to plan for higher voltage and good controller that takes such voltage. Gross weight and weight distribution become an issue too.


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## Notthatbutton! (Jan 6, 2012)

I do actually drive 94 miles a day, and I could theoretically charge at work, but I don't want to rely on a work day charge, since it would be a 110v power source. I was under the impression that the smaller and lighter the better, however, it seems that maybe Diyguy's "good ohm'n" would be the way to go.


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## EVfun (Mar 14, 2010)

If you want a 100 mile range in an old Bug you will need a substantial pack. I love the cars, but they suck wind. The VW Bus is more aerodynamic.

I would figure on 300 watt hours per mile to maintain 60 mph. So for a 115 mile range pack you would need 34.5 kWh. By dividing by 3.2 (volts per cell) I get 10,781 amp hours. Of course, 10k+ amp hours at 3.2 volts isn't a viable path, so you get to figure how many cells of what size you want in series to get that many amp hours on board. If you target a 60 cell pack then you would need 180 amp hour cells. You could also choose 54, 200 amp hour cells, or 83, 130 amp hour cells.


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## Notthatbutton! (Jan 6, 2012)

Ok, so 60 180ah calb's would get me the 115 mile range, that came out as 10 more than I would have hoped, that comes to about $14,000 in batteries. Figuring on a decent vehicle, such as an S-10 pickup, controller, motor, dc converter, etc, looks like about a $20-25k build. Still cheaper than that crappy chevy volt, and hell, could haul stuff too. 
Thanks for the break down, I think I may start checking craigslist this weekend for a donor truck. If I do go with a warp 9 motor, what controller would you recommend?


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## EVfun (Mar 14, 2010)

I think there are only 2 choices for high voltage DC motor controllers. Others may be good, but don't have the installed based to really know for sure. I would choose either a Zilla Z1k-HV from Manzanita Micro or the Soliton 1 from Evnetics.


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## charliehorse55 (Sep 23, 2011)

There is no way you will get 300 Wh/mile in a truck at 60 mph. Based on some rough calculations you are looking at more like 400 to 450 Wh/mile. Pick-up trucks have incredibly bad aerodynamics, and thus high speed travel is really energy intensive. You need almost 30kW continuous just to maintain 60 mph!

I think the same plan WILL work in a mid-size sedan however. Something like a honda civic or (for more class) a BMW 3 series will be able to get 300-350 Wh/mile at 60 mph. A 36 kWh battery pack will weigh about 350kg (750 lbs) and should be able to fit inside the car with a little work. 

Assumptions for above calculations:

-Drivetrain efficiency (battery to wheel) 75%
-Air temperature of 15C (60 deg F)
-Rolling resistance of 0.012 (common for normal consumer tires)


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## Sunking (Aug 10, 2009)

Notthatbutton what you are running into is the Wall and Law of Physics.

OK yu first think 400 wh/mile, and to go 100 miles you need 40 Kwh battery. Well first problem is you cannot use 40 Kwh in a 40 Kwh pack, only about 32 to 35 is usable.

But let's just assume you can use all of it. How much volume does it take and how much does it weigh. Well that depends on which chemistry you use. LFP you are looking around 90 to 100 wh/Kg, That makes a 40 Kwh battery weighing in around 400 Kg or 880 pounds. Factor that back in efficiency and you find out now it will take 450 wh/mile to move that much weight. Up the battery again and now you are at 1000 pounds which hits the efficiency again. You have hit the Wall or the physical limits.


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## Ziggythewiz (May 16, 2010)

You're trying to apply average usage rules to long distance highway use. The Wh/m is not going to go up much with the weight, because you are not starting and stopping.

And I don't think the aero can be that bad. My bug cruises at 60 at around 12kW, I can't believe a small truck would be nearly 3x that bad, maybe at real highway speeds, like 70-80, but not at 60.


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## charliehorse55 (Sep 23, 2011)

Ziggythewiz said:


> You're trying to apply average usage rules to long distance highway use. The Wh/m is not going to go up much with the weight, because you are not starting and stopping.
> 
> And I don't think the aero can be that bad. My bug cruises at 60 at around 12kW, I can't believe a small truck would be nearly 3x that bad, maybe at real highway speeds, like 70-80, but not at 60.


I rechecked my calculations, I'm pretty sure they are fairly accurate. Remember, a beetle weighs about half (820kg vs. 1600kg), and is equipped with tires with good rolling resistance. The truck has tires with higher rolling resistance and a much larger drag aera. The Drag coefficient's are basically the same however, 0.48 for the bug vs. 0.49 for an S-10. 

My calculator gets 12.6 kW to cruise in the beetle at 60mph. 

The exact value for the truck is 26.5kW.


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## Ziggythewiz (May 16, 2010)

Thanks, didn't realize the S-10 was that big


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## PhantomPholly (Aug 20, 2008)

The simplest rule of thumb is 8KWh approximately = 1 gallon of gas for propulsion calculations. More batteries = more weight, so a 5,000 lb car will get worse "mileage" than a 2,000 lb car. Thus, just adding batteries reaches an upper limit on range. 

Whatever mileage a particular car gets at a particular weight will tell you how many "gallons" you need. Note: Heating consumes more battery power (affects mileage) more than cooling (reverse of an ICE), and reduction in pack remaining power does nothing to reduce weight (while ICE cars get lighter as they burn gas).


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## Notthatbutton! (Jan 6, 2012)

Just a quick question Ziggy, you have a bug with 10 105ah batteries, with a range of 26 Miles. At what speed can you get that 26 miles? Since the math seems to suggest 60 180ah batteries for 115 mile range, it seems almost overkill. Five times your current range would be 26 X 5 = 130 Miles. Thus 50 100ah batteries should give me the range I need if I could cram them in a bug. Unless the added weight would affect the math, but that shouldn't be an issue since you are using heavy lead acid and I would use Calb lithiums. Any thoughts?


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## Ziggythewiz (May 16, 2010)

Had 10 125 ah batteries (12v ea). 26 miles was when batteries were new at ~45mph to 80% DOD. 

Mine were 12 volters. You will be using lithium, so 3.2V ea. 

50 100AH cells will give you around 50 miles. 

Corbin has a ~100 mile bug, you should check it out.

He only really needs around 80, I think, so yours would need to be bigger.

He has 48 200AH cells, so you'd probably need around 60 to be safe. I think it would be possible to stuff all that in a bug, with lots of work.


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## Notthatbutton! (Jan 6, 2012)

No matter which way I turn I will end up with a $14k battery pack, yowzers!  At that rate I could buy 4,000 gallons of gas. Which comes to 128,000 miles at 32 miles to the gallon for small car. I just don't think it's worth it for a daily commuter. Too bad, would have been a fun project. Maybe in a few years battery prices will drop.


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## Ziggythewiz (May 16, 2010)

It really is an investment, of time and money. These batteries should last you 10 years. In that time you'll drive 250,000 miles, and spend $30,000 on gas.

That's assuming gas doesn't go up, but it will. Oh how it will...


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## Mr. Fixit (Feb 25, 2012)

Heynotthatbutton, Just saw this posting on craigslist if you go with a S10 maybe this setup could do it for you. Spoke to the seller he says these are from a public works department project are used but some are as good as new.
Just thinking outside of the box. 

Mr. Fixit for the family
Chris 

http://portland.craigslist.org/mlt/pts/2821431273.html
http://www.azuredynamics.com/products/force-drive/documents/AC55_DMOC445ProductSheet.pdf


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## electric85 (Apr 10, 2008)

dimitri said:


> And that is why you don't see many DIY EVs with 100 mile range.....
> 
> You need to reduce your range goal and increase voltage, which requires good controller like Soliton Jr. This will let you use smaller cells, like 100AH or 120AH or 160AH, but more of them, so you can stick them in every nook and cranny of your beetle.
> 
> As you are discovering, building EV is all about tradeoffs


it is doable...

http://www.elitepowersolutions.com/electric_vehicle.html 

they used thundersy 200ah that they used to carry, and the thundersky bms they used to carry. i spoke with them and was going to order the same for my fiero...prices were still too high then, and due to time and money i'm still not finished the restoration/conversion


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## lowcrawler (Jun 27, 2011)

Notthatbutton! said:


> No matter which way I turn I will end up with a $14k battery pack, yowzers!  At that rate I could buy 4,000 gallons of gas. Which comes to 128,000 miles at 32 miles to the gallon for small car. I just don't think it's worth it for a daily commuter. Too bad, would have been a fun project. Maybe in a few years battery prices will drop.


Much of what I've read is that replacing a 30+mpg car with a electric is about the cut-off for it to make purely financial sense when figuring 3.50 per gallon gasoline. ... assuming you did nothing with that extra money other than invest it and use it for gasoline purchases.

That said, as we head for $5 per gallon gas... Things are going to move in the EV's favor.

15k for your batteries might sound like a lot (and it is), factor another 3-5k for the 'other stuff' and then whatever your donor car costs and it's not a cheap initial investment. But if you are looking at the long-term, it does still come out ahead. Especially the longer you drive it.

Also remember that after you've driven your 250k with your pack and it's dropped to 80% of it's original capacity... it's still worth a lot to people and can be sold to recoup much of your original investment.


As for it all fitting in a bug? I don't see a problem with that. I've got a 15.3kwh pack in my bug and it fits completely behind the back seat. I could easily drop 3 of those in the cabin if I didn't mind losing the back seat (which I'm not sure why I wanted to keep it... but I digress). That'd bring me to my max cargo weight of around 1k... and give me a pack that could take me ~150 miles. (with no buffer, of course)

Take a look at Corbins (already linked, but it's worth linking again) http://www.corbinstreehouse.com/blog/plug-bug/ to get an idea of what can be done. His conversion is fantastic.


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## Sunking (Aug 10, 2009)

lowcrawler said:


> Also remember that after you've driven your 250k with your pack and it's dropped to 80% of it's original capacity...


That is not even remotely realistic expectation.


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## lowcrawler (Jun 27, 2011)

Obviously none of us has real-world on this... but...

If, in theory, he puts in a 35-40kwh pack.

Drives 100 miles per day, he's taking it to roughly 70-80% DoD.

Depending on the cells, the manufacturers claim 2-4k cycles at 70% D0D before falling to 80% capacity. (note, not 'dead', just 80% capacity)

Doing the math gets us 200-400k miles. Those 2-4k cycles are measured at .3-.5C, and he'd be pulling 1-2C instead... So we can probably assume it'd be on the lower end of that range. In other words, 250k or so to get to 80% remaining capacity or so... give or take 10% on both numbers.

That's where I get my number from -- just some back-of-the napkin math -- where do you get you "not even remotely realistic" statement from?


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## Ziggythewiz (May 16, 2010)

lowcrawler said:


> where do you get you "not even remotely realistic" statement from?


Just his general outlook on life and the ideas of others.


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## Sunking (Aug 10, 2009)

lowcrawler said:


> Obviously none of us has real-world on this... but...


That is my point. Then winter comes around and all that theory goes out the window.

The EV has the exact same issues it had over 100 years ago, when electric vehicles ruled the road, the battery is the major problem. Th eEV will not become mainstream and widely accepted by the public until consumers are confident they can get 300+ miles between recharges, with all the Bells and Whistles, 10 year battery life, 15 minute recharges, and that cost the same as an ICE vehicle. To do that will require a battery with a specific energy density of 400+ wh/Kg and a charge rate that exceeds C5. To my knowledge there is not even a battery on the drawing board that can do ALL of that.


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## Ziggythewiz (May 16, 2010)

Sunking said:


> That is my point. Then winter comes around and all that theory goes out the window.
> 
> The EV has the exact same issues it had over 100 years ago, when electric vehicles ruled the road, the battery is the major problem. Th eEV will not become mainstream and widely accepted by the public until consumers are confident they can get 300+ miles between recharges, with all the Bells and Whistles, 10 year battery life, 15 minute recharges, and that cost the same as an ICE vehicle. To do that will require a battery with a specific energy density of 400+ wh/Kg and a charge rate that exceeds C5. To my knowledge there is not even a battery on the drawing board that can do ALL of that.


And yet we DIY anyway because we don't care what is mainstream or accepted. In fact, if it were there would probably be far fewer of us here. The world can go on waiting for the tech to keep getting better and better, and we'll build awesome stuff that works now, and upgrade as even better stuff becomes available.


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## jwalin (May 7, 2008)

Sunking said:


> The EV has the exact same issues it had over 100 years ago, when electric vehicles ruled the road, the battery is the major problem.


and, 100 years ago when the economy of Big Oil pushed them off the road. Gas was C_H_E_A_P then. Electric cars were preferred. Think about it....there was no reason to need the higher power density of gasoline back then, people didn't travel far at all. But, it was MUCH cheaper to use gas.




Sunking said:


> The EV will not become mainstream and widely accepted by the public until consumers are confident they can get 300+ miles between recharges, with all the Bells and Whistles, 10 year battery life, 15 minute recharges, and that cost the same as an ICE vehicle.


I have a little more faith in people than that. I used to work in the AVTI research lab at IUPUI in the late 90's, we had a formula lightning. There was huge interest in it from everyone who saw it. Every race track we went to there were many racers looking the car over. Interest was BIG. Look to the EV1's history of owners, ask people you know if they would commute to work in an ECONOMICAL EV.

We don't need EV's to match what an ICEV will do. We need the *economy of scale* that the ICEV has benefitted from for nearly 100 years, that is all.

jeff


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## lowcrawler (Jun 27, 2011)

Sunking said:


> Th eEV will not become mainstream and widely accepted by the public until...


I dunno. People accept quite a bit of really crappy stuff about ICE vehicles -- destroying the planet, constant wars, having to go somewhere to 'fill' rather than just doing it at night, overwhelming costs, etc, etc, etc...


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## charliehorse55 (Sep 23, 2011)

lowcrawler said:


> I dunno. People accept quite a bit of really crappy stuff about ICE vehicles -- destroying the planet, constant wars, having to go somewhere to 'fill' rather than just doing it at night, overwhelming costs, etc, etc, etc...


Other than having to use a gas station to re-fill, all of the reasons listed are external costs that may not be immediately obvious to a consumer. They are perfectly legitimate flaws, but they do not weigh as heavily on the mind of a consumer. 

http://en.wikipedia.org/wiki/Cost_externalizing


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## MN Driver (Sep 29, 2009)

Sunking said:


> That is my point. Then winter comes around and all that theory goes out the window.
> 
> The EV has the exact same issues it had over 100 years ago, when electric vehicles ruled the road, the battery is the major problem. Th eEV will not become mainstream and widely accepted by the public until consumers are confident they can get 300+ miles between recharges, with all the Bells and Whistles, 10 year battery life, 15 minute recharges, and that cost the same as an ICE vehicle. To do that will require a battery with a specific energy density of 400+ wh/Kg and a charge rate that exceeds C5. To my knowledge there is not even a battery on the drawing board that can do ALL of that.


That doesn't seem to be the issue that people talk about when I discuss electric cars with them. Most people aren't willing to foot the bill for the higher up-front cost of an EV. A great deal of people who are financially able to buy a new car are in a family with more than one vehicle. Since they have more than one car, as long as they aren't attached to only driving their car and can work together with their spouse or family to plan ahead when they need to take a car farther than 50 miles, its not that difficult. I've talked to a few people who think that a 300 mile range and a recharging infrastructure is needed but after I talk to them about their husband or wife having another car or how often they drive a long distance, they realize they've been fed a line by Bob Lutz about the '300 mile gold standard'. Once they think past some rhetoric fed to them by the auto industry, the oil industry, and whatever news network they heard it from usually range isn't really an issue. FWIW, the Tesla Model S with a 85kwh pack is sold as a 300 mile car, not cheap but its not an econo box either, its a lux car and priced accordingly. I'm thinking that past the currently developing generation of the S and X that they will produce a car that is priced for some real sales numbers.

I'm really thinking its the price premium and the 'I want to wait for the kinks to get worked out'. Think about it, the Prius wasn't popular until a few years after the 2nd gen was released and even at its price premium, there was enough demand for a car this car that they were unable to keep them on the lots even though production was ramped up. Once the 'is this technology proven' factor and price comes down, I think that is when these will really take off. Right now its early adopters buying them because they have the research and drive to have one, the average person will catch on in about 8-10 years or possibly less depending on how gas prices pressure them but even then it will likely be at least 5 years since the original release of the Leaf before any 'real' sales numbers come up. I'm willing to buy a used production EV but I'll most likely build my own before then.


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## rhea2011 (Feb 29, 2012)

*battery pack AH ,V*

Dear Sir,

Battery pack, need to know the AH and total Voltage 

AH= Current A X Hours
Current A= W/Voltage 
Hours = Running distance km / Average speed km/h 

Could you inform your electric motor power W, and estimate your e-car running hours?


HIPOWER LiFePO4 battery : cell normal voltage 3.2v ​Prismatic single cells. 20AH~500AH ​Normally 20Ah, 30Ah, 40Ah, 50Ah, 60Ah, 100Ah, 160Ah, 180Ah, 200Ah......​ 
Best Regards,
Alice


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