# General Purpose range predictor



## coulombKid (Jan 10, 2009)

Range([email protected] mph) = 8.14 x (battery) Kw-hours/ Kilo-pounds (vehicle weight)
You may find that this equation pretty much nails it. It is for cars only. The constant would be reduced for pick-up trucks. Good for what ifing without the big disappointment later.


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## steven4601 (Nov 11, 2010)

Air resistance varies greatly per vehicle. However it does come very close to my expected range . Maybe weight & size and drag are related 

I am converting a tiny z3 roadster. (target is 2800lbs with driver) Mainly intended to be a 'rapid' (15.6s quarter mile) car around town. I calculated & simulated that at 72mph I would get 35miles range with a 11..12kWh pack. that is 90..95% dod though.


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## DavidDymaxion (Dec 1, 2008)

Gives the range I think I should have (currently I'm about 2/3 that, but think my homegrown controller is wasting too much power).

I like Bill Dube's idea, use the gas mileage and battery weight to estimate range. This can take into account your driving style. I forget his constant, but I found that something like about 750 lbs of lead acid batteries represents about 1 gallon of gas. Lithium would be something like 200 lbs of battery represents the energy in a gallon of gas.


coulombKid said:


> Range([email protected] mph) = 8.14 x (battery) Kw-hours/ Kilo-pounds (vehicle weight)
> You may find that this equation pretty much nails it. It is for cars only. The constant would be reduced for pick-up trucks. Good for what ifing without the big disappointment later.


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## dtbaker (Jan 5, 2008)

lead is going to give a way different constant than Li, especially dependent on how many starts/stops you do....

my little Swift got 40mpg on gas
40 mile max at 90%DOD on 96v worth of 8v FLA (almost 900#)
50 mile at 80%DOD on 120v worth of 100ah Thunderskies (about 250#)


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## Guest (May 30, 2011)

dtbaker said:


> lead is going to give a way different constant than Li, especially dependent on how many starts/stops you do....
> 
> my little Swift got 40mpg on gas
> 40 mile max at 90%DOD on 96v worth of 8v FLA (almost 900#)
> 50 mile at 80%DOD on 120v worth of 100ah Thunderskies (about 250#)


At what speeds did you get your 40 and 50 mile ranges? Surely not at 55 mph.


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## dtbaker (Jan 5, 2008)

gottdi said:


> At what speeds did you get your 40 and 50 mile ranges? Surely not at 55 mph.


average speed for my 'average day' is probably closer to 40mph, with a fair number of starts and stops in typical urban/suburban traffic in generally 'rolling' terrain. I tend not to be a super eco-modder and do accelerate up to speed limit hard enough to keep up with normal traffic.

The 40 miles on lead was for about the first year or so, then range regraded steadily. at 2 years (7500 miles) the pack was getting only about 25 mile range, so I ditched the pack at that time and upgraded to Lithium.

The 50 mile range on Li is estimated from the fact that I can go a good solid 25 miles and use right around 40ahr according to the Cycle Analyst. I have NOT drive my lithium pack below 80ahr, and don't intend to ever try since I have top-balanced I want to stay away from the bottom!


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## DavidDymaxion (Dec 1, 2008)

Thanks for the data point. From your numbers it would be about

900 lbs of lead to go as far as gallon of gas, and
208 lbs of Lithium.


dtbaker said:


> lead is going to give a way different constant than Li, especially dependent on how many starts/stops you do...
> 
> my little Swift got 40mpg on gas
> 40 mile max at 90%DOD on 96v worth of 8v FLA (almost 900#)
> 50 mile at 80%DOD on 120v worth of 100ah Thunderskies (about 250#)


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## dtbaker (Jan 5, 2008)

DavidDymaxion said:


> Thanks for the data point. From your numbers it would be about
> 
> 900 lbs of lead to go as far as gallon of gas, and
> 208 lbs of Lithium.


...something like that, at least for the large format prismatic cells... in 'urban/suburban' conditions with a 2000# car. At those speeds aero plays a smaller part than starts/stops I'd say.

But as we are all aware, other factors in sizing a lithium pack come into play... like working with a high voltage for motor efficiency and lower amps, and enough ah to reduce the need for constant pull over 2C or 3C at most.

personally I have come to the conclusion that 120v w/ 100ah cells is about the minimum even in a small car with 8" motor for decent performance and long life with large format (thundersky/CALB) cells. If you can bump the motor size up to 9", I'd go with 144v and 100ah minimum.


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## lowcrawler (Jun 27, 2011)

dtbaker said:


> If you can bump the motor size up to 9", I'd go with 144v and 100ah minimum.


Why?

If you had the same total kWh pack - what benefits are there to 144 other than 'top end driving performance'?

Lets do the math...
Lets say it takes me 12kWto drive the car on the freeway. 
Lets pretend we have a 200Ah pack @ 72v. This pack draws .83333C.
Same car, same 12kW requirement but now with a 100Ah @ 144V pack (same kWh size pack). It's the same C draw.

It's my understanding that the 'C' is the main determinant of the 'wear' on LiFePO4 batteries.

So, why is higher voltage (pack size kWh staying equal) better?


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## gojo (Feb 1, 2011)

How about 20 pounds of lead per mile? I know there are a lot of variables, but I have heard that rule of thumb. This would give me about 40 miles.

According to my my Cycle Analyst, I have been averaging 2.5 ah's per mile. This would give me a 37.5 mile range. 

I just have to make my 22 mile round trip to work, and either one of these works for that.


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## EVfun (Mar 14, 2010)

The common 9 inch motors (Advanced DC 9" FB-1 and the Netgain WarP 9") make more torque per amp but less rpm per volt compared to the smaller 7 and 8 inch motors. Your vehicles gearing isn't going to match very well, and you could be freeway power limited, when your motor makes peak power at around 3000 rpm. More voltage raises the rpm range of the power band. The gas engine in most cars makes peak power in the 5000 rpm range.


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## dtbaker (Jan 5, 2008)

lowcrawler said:


> So, why is higher voltage (pack size kWh staying equal) better?


you only got half the math. 

for the same given kW output to roll down the road... if you raise the voltage, the amps required go down, giving a lower C draw on the cells, which is 'better'.

The other distinction I was making is that while 120v/100ah system is fine for urban/suburban, you *should* have more like a 144v/160ah or 200ah system and probably a hefty controller like the Soliton if you plan to do much highway speed. The extended draws on the highway make a big difference.


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## lowcrawler (Jun 27, 2011)

dtbaker said:


> for the same given kW output to roll down the road... if you raise the voltage, the amps required go down, giving a lower C draw on the cells, which is 'better'.


Right, but if you raise the voltage of your pack while _keeping the kWh the same_, you lower the Ah proportionately at the same rate that the amp draw lowers ... meaning that you end up with the same C draw (unless you increase the kWh). No?


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## dtbaker (Jan 5, 2008)

lowcrawler said:


> Right, but if you raise the voltage of your pack while _keeping the kWh the same_, you lower the Ah proportionately at the same rate that the amp draw lowers ... meaning that you end up with the same C draw (unless you increase the kWh). No?


almost.

what I am saying is that for the same output rolling down the highway, you draw less amps with higher voltage.... so for ANY given size battery, the 'load' on it is easier with higher voltage. this is just the simple math part of W=VxA

The consideration for highway versus urban is that you have longer periods of steady draw, so it is more important to raise the voltage and reduce the amps to be kinder to batteries regardless of range requirements. If you want to maximize life of batteries and avoid too much internal heat you should consider going on the 'large' (160ah/200ah) side and at higher voltage (144/156).


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## rwaudio (May 22, 2008)

lowcrawler said:


> Right, but if you raise the voltage of your pack while _keeping the kWh the same_, you lower the Ah proportionately at the same rate that the amp draw lowers ... meaning that you end up with the same C draw (unless you increase the kWh). No?





dtbaker said:


> almost.
> 
> what I am saying is that for the same output rolling down the highway, you draw less amps with higher voltage.... so for ANY given size battery, the 'load' on it is easier with higher voltage. this is just the simple math part of W=VxA
> 
> The consideration for highway versus urban is that you have longer periods of steady draw, so it is more important to raise the voltage and reduce the amps to be kinder to batteries regardless of range requirements. If you want to maximize life of batteries and avoid too much internal heat you should consider going on the 'large' (160ah/200ah) side and at higher voltage (144/156).


You are both correct, but you aren't talking about the same thing.

lowcrawler, you are exactly correct, however there is a bit more to the equation since typically higher current means more heat, and larger wires to handle it. So even with the same size battery (kwh) you may get an extra couple miles out of the higher voltage version (if driven the same), but one thing you will get is higher top speed with the higher voltage pack. It's a tricky balancing act to get the the Voltage/AH/Price/Power/Current all to work together in the perfect system for your Wants/Needs/Budget.


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## Cvillalobos (Jun 16, 2011)

Thanks for the quick equation. That has really helped me understand that curb weight is a key factor. 

I have been looking at converting a Jetta which is around 2600lbs. Given a 12kw battery I would gain around 15 miles per charge if I went with a Geo Metro (1600lbs) instead.


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## dtbaker (Jan 5, 2008)

Cvillalobos said:


> Thanks for the quick equation. That has really helped me understand that curb weight is a key factor.
> 
> I have been looking at converting a Jetta which is around 2600lbs. Given a 12kw battery I would gain around 15 miles per charge if I went with a Geo Metro (1600lbs) instead.



curb weight is key if your driving has a lot of stops/starts or hills, won't change a thing once you are at speed. Also, be careful with sources for curb weight. I can tell you for sure that a stock ICE metro is going to run closer to 1900#-2000#. Mine converted, with 12kw of lithium on board weighs in at 2020#.


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## EVfun (Mar 14, 2010)

lowcrawler said:


> Right, but if you raise the voltage of your pack while _keeping the kWh the same_, you lower the Ah proportionately at the same rate that the amp draw lowers ... meaning that you end up with the same C draw (unless you increase the kWh). No?


Yes but...

It is true for that the pack doesn't usually care if the kWh is the same. There are some cases where there is a weakness in the higher amp hour batteries, like using the same size cell interconnects.

The cables between the batteries will need to be larger because the power will be in the form of more amps at less volts. The pack size will be the same (roughly) because the higher capacity cells will be fewer but larger.

The motor may need to be a little larger with a lower pack voltage (or you will need to select the next larger motor at a lower vehicle weight) because of the higher amps. Here is a catch, the larger motors we generally use make less rpm per volt. With a pack voltage of 96 volts or less it becomes a good idea to look into used forklift motors as these are often designed for lower voltage.


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## EVfun (Mar 14, 2010)

coulombKid said:


> Range([email protected] mph) = 8.14 x (battery) Kw-hours/ Kilo-pounds (vehicle weight)
> You may find that this equation pretty much nails it. It is for cars only. The constant would be reduced for pick-up trucks. Good for what ifing without the big disappointment later.


I think this one will be optimistic for light cars. I have a pack of 32, 60 amp hour TS cells in an 1100 lb. vehicle. I can't go anywhere near 60 miles per charge. Realistically, it is more like 200 watt hours per mile, so about a 30 mile range.


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