# Any 'rule of thumb' on pack size?



## Woodsmith (Jun 5, 2008)

I am trying to determine how much battery pack I may have to have in my EV so that I can determine what donor or scratch build I go for.

I have little to go on so I am looking for a sort of rule of thumb so that I don't work on way too much or too little volume and weight for the pack.

I want to have a fairly light car with single seat or 1+1 seating. It will need to have a range of 35 miles just to get to work and maybe 70 miles to get me back again if I can't recharge. 2/3 of that distance will need to be at 60+mph as I travel against the rush hour and so the roads are clear and other drivers are hitting 80-90mph. I don't what to be a rolling roadblock.
I will most likely be acquiring my motor from a forklift and getting an off the shelf controller.
I want to be able to make the investment in LiFepo4 as I know that LA just won't do it and would be a false economy as I will shorten their life.

I have no idea as to:
The wh/mile or what sort of range any figures would fall in.
The capacity of the pack required.
The voltage I should run.
The physical dimensions and weight of the cells I would have to use.

It means I don't know if I need a van load of cells or if I can squeeze the whole lot into a Caterham 7 type chassis.

Any pointers based on your group experiences?

Thank you.


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## dimitri (May 16, 2008)

Many of my posts here in past 2 months have covered everything you ask. Sorry, I don't have time to go into details that have been covered before, please search a little, its all there. Once you get some details and come up with few options, post here and we will help you narrow it down.

Use cell sizes and prices from EV Components Web site for size / price calculations.

Hint: assuming DC forklift motor conversion and small car, look at cells between 100AH and 180AH. Balance between voltage allowed by motor/controller and size/weight/cost of the cells. Then use rule of thumb of 250-300 Wh/mile and 80% DoD to figure out approx range you will get from a given pack.

Hope this helps.


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## Woodsmith (Jun 5, 2008)

OK, thanks for that. I guess if I work to 250-300wh/mile I shouldn't be too far off.

I've had a search around and found a battery supplier for LiFePo4 12v 120ah cells.
I have made up a quick spreadsheet that says:

Using 10 cells at 12v and 120ah
Gives 120v pack
With 14400wh capacity
Or *11520wh* capacity at 80% DoD

Assuming 300wh/mile
Range is 48 miles
Or *38.4* miles at 80% DoD

It implies that if I need a return trip on the one charge then I can have twice the number of cells but that means 240v.
Would that be too high and would I be better off wiring the pack series parallel to keep it at 120v?

10 cells would give a pack area of:
1820mm x 546mm at 245kg
20 cells would give a pack area of:
1820mm x 1092mm at 490kg
Both at 207mm high.

Does that seem reasonable?

Thanks.

Edit to add: Just looked at Optima LA for comparison. Essentially half the capacity for very slightly less weight per cell. so I would be halving my range or doubling the pack weight.


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## dimitri (May 16, 2008)

I'm not a fan of Lifebatt products, but then again, you are in UK so they are local to you, so its your call. Last I read they were pretty expensive compared to TS cells. They have integrated BMS, so its not a fair comparison, all depends on your skills and budget.

If you are doing DC conversion, then voltage is limited by motor/controller, so to increase pack capacity you have to parallel cells. Its better to parallel individual cells and then put them in series, rather than paralleling 2 series strings, for better balancing.

I prefer to deal with 3.2V cells and then design single series string of appropriate size cells.


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## Woodsmith (Jun 5, 2008)

Lifebatt was just plucked out of the ether as a starting point. Price will be the decider and I am sure that BMS design and available cell capacity would also be a major factor too along with the cost of importing from somewhere compared to a local supplier.

I was figuring on a pack of the yellow 3.2v cells being a similar size and weight to the 12v Lifebatt ones just to work out how much volume and weight I need to design for as I couldn't find the size and weight specification on them just now.
If my numbers seem reasonable as a starting point then I can fine tune the requirements and component suppliers and prices as I go.

I am having to make a lot of assumptions about the whole thing until at least one of the components comes home to stay. I don't even have a budget yet as I don't know what I will buy new and what I will source used.


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## Jordysport (Mar 22, 2009)

Woodsmith said:


> Lifebatt was just plucked out of the ether as a starting point. Price will be the decider and I am sure that BMS design and available cell capacity would also be a major factor too along with the cost of importing from somewhere compared to a local supplier.
> 
> I was figuring on a pack of the yellow 3.2v cells being a similar size and weight to the 12v Lifebatt ones just to work out how much volume and weight I need to design for as I couldn't find the size and weight specification on them just now.
> If my numbers seem reasonable as a starting point then I can fine tune the requirements and component suppliers and prices as I go.
> ...


Lifebatt are expensive, i am in the UK also and having trouble sourcing cells for a resonable price best thing i can find is Technologics ones at $1.1 /AH and ship them to the uk which would cost about £400 ontop of cell costs


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## dimitri (May 16, 2008)

I like to use www.evcomponents.com , they have cell dimensions, weight and price all in one page, easy to compare.

I guess you need to pick at least one major component to minimize variables. Say, pick motor/controller, which determines max voltage, which gives you number of cells, then work out cell size based on cost/weight/volume. Of course it helps to know what kind of car you will be converting


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## Jordysport (Mar 22, 2009)

dimitri said:


> I like to use www.evcomponents.com , they have cell dimensions, weight and price all in one page, easy to compare.
> 
> I guess you need to pick at least one major component to minimize variables. Say, pick motor/controller, which determines max voltage, which gives you number of cells, then work out cell size based on cost/weight/volume. Of course it helps to know what kind of car you will be converting


Was looking on there for cells and its the same price as Tech, what does it mean though when it says Online delivery / No shipping ??? 

thanks


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## Woodsmith (Jun 5, 2008)

dimitri said:


> I like to use www.evcomponents.com , they have cell dimensions, weight and price all in one page, easy to compare.
> 
> I guess you need to pick at least one major component to minimize variables. Say, pick motor/controller, which determines max voltage, which gives you number of cells, then work out cell size based on cost/weight/volume. Of course it helps to know what kind of car you will be converting



I want to get a motor first as I can recondition that in house while I am plotting the rest of my scheme. However, I may have the option of getting a Freecycle 1989 Daihatsu Charade for nothing, a 1997 VW Polo for scrap value or a 2000 Nissan Micra for trade in value. I'd prefer something more like a Caterham 7 or MR2 but beggers can't be choosers as they say.
Currently I am realising that I need to reduce my wh/mile to keep the batteries reasonable. It seems to be the most significant variable in my spreadsheet.


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## Woodsmith (Jun 5, 2008)

Jordysport said:


> Lifebatt are expensive, i am in the UK also and having trouble sourcing cells for a resonable price best thing i can find is Technologics ones at $1.1 /AH and ship them to the uk which would cost about £400 ontop of cell costs


Whereabouts are you, Jordysport? We may need to do a group purchase to reduce battery costs and generally share ideas.


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## Jordysport (Mar 22, 2009)

Woodsmith said:


> Whereabouts are you, Jordysport? We may need to do a group purchase to reduce battery costs and generally share ideas.


Devon lol miles away but closer than the US


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## Drew (Jul 26, 2009)

I just did some back of the envelope calcs for you, based on your numbers I've just worked out a trip of 1 hour at 112km/h (70 miles per hour) and assumed a Crr of .03 and Cd of .6 for the clubman example and .35 for a normal car type shell and the answers are;

Clubman = approx 45kWh

Conventional = approx 27kWh

The Crr is quite high, but only contributing 9kW at that speed and I've also assumed that you're travelling in 0 degree C air, which contributes about 1kW over 20 degree C air  This is also based on a frontal area of 3m which might be a little high. But on the other side of things this is the mechanical power expended to maintain 70mph for 1 hour continuous, not the battery power required to keep a car moving at that speed, I've got no idea about the electrical side of things, but for the mecanical side of things speed of travel has a massive effect.

Also note tis doesn't include the effects of any change in grade.


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## Woodsmith (Jun 5, 2008)

Drew said:


> I just did some back of the envelope calcs for you, based on your numbers I've just worked out a trip of 1 hour at 112km/h (70 miles per hour) and assumed a Crr of .03 and Cd of .6 for the clubman example and .35 for a normal car type shell and the answers are;
> 
> Clubman = approx 45kWh
> 
> ...


That's an interesting finding. What's the formular that you use to find that?

So your figures offer a full 70 mile round trip at 70mph using 27kwh, 13.5kwh each way. My battery figures give 11.5kwh at 80%DoD each way.
I guess my slower sections of the drive will help but the up hills will not. Perhaps I will need the full 70 mile range to average out for a return drive.

My journey to work consists of getting up to 60+mph within 100yards of my front door. Maintaining that speed for about 22 miles mostly up hill and then down to 30mph up hill for 5 miles and then 50 mph for 5 miles up hill and then 30mph for about 3 miles urban stop start. That gives the 35 miles.
The return trip is easier as it becomes mostly down hill.

I have asked about recharging at work but the faculty manager hasn't a clue what I am talking about.

My diesel commute gives me about 58mpg going to work and 65-68mpg return so that shows the difference.


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## Drew (Jul 26, 2009)

I just used the standard aero drag and rolling resistance formulae, I estimated Cd based for a clubman and a standard car, and used an approximation of rolling resistance based on standard car tyres (not economy tyres, I think they're dangerous).

The forumlae are;

Power Consumption (aero) = 1/2 x Air density(Rho) x velocity^3 x Frontal Area x Cd

Power Consumption (rolling) = Crr x Vehicle mass (in newtons) x velocity

Cd I guestimated to be .35 for a standard car (most are between .3-.35 AFAIK)

Crr I guestimated to be .03 which is pretty high, but road dependant so I went conservative.

The reason the clubman chassis would use so much more power is because of their awful Cd, something like .6 or worse.

If you want and you don't mind your car looking a bit whacky its pretty easy to reduce aerodynamic drag by doing things like closing off the radiator grill (you won't need it), making a dam for infront of the windscreen wipers (most current cars have this anyway) and covering off the rear wheel arches. 

If you're running something with exposed front tyres (clubman or reverse trike type vehicle) then building a shroud that covers as much of the wheels as possible (right to the ground if at all possible) will give you a major benefit, you'll need to duct in a bit of air for brakes though 

BTW I forgot to mention above that I used 1.29 for Rho as thats air density at 0 degrees C which I also figured is worst case.

Also, remember that all of the above is only applicable with SI units.


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## Woodsmith (Jun 5, 2008)

Drew said:


> Also, remember that all of the above is only applicable with SI units.


Thanks for that, I will make up a spreadsheet to that effect.

I grew up with SI units so I am struggling with Bob Brant's 'Build Your Own Electric Vehicle' book.


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