# I need help to figure range before I buy LiFePO4

6193 Views 14 Replies 6 Participants Last post by  Crash
Hello All

I need some expert help, I have been doing some calculating and want to see if my numbers are accurate, I used the methods out lined in "Sizing Your Battery Pack" http://www.diyelectriccar.com/forums/showthread.php?t=11709 . I would like to have a pack that can handle my 70 KM trip to work and back round trip 140km. Below my calculations is for 160 KM for some a buffer zone.

1 Mile = 1.61 KM
400 W/Mile (power to move my 1984 Fiero) = 644 W/KM
644 W/KM x 160 = 103,040 W (103.04 KWH)
103,040 / 72 volts (motor and controller) = 1431.11 Ah
1131.11 x 1.25 (adjustment for leaving 20% left in batteries) = 1788.88 Ah
1788.88 x 1.05 (to compensate as in step 3 of "Make Allowances For Your Battery Type") = 1867.83 Ah
1867.83 x 1.32 for Lithium("compensating for the characteristics of the batteries we choose") = 2465.63 Ah

So does this math seem correct? It is very difficult to get Lithium batteries in Canada so I am hoping to buy and pay for shipping once.

Any good links for different companies for Lithium?

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#### MN Driver

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"1 Mile = 1.61 KM
400 W/Mile = 644 W/KM"

Not quite...
1 mile is a bigger unit than a kilometer. 100km=62mi
So if we have 400wh/mile and a kilometer is a smaller unit, your math was done wrong.
400*.62=248wh/km

160 km is 100 miles, which makes math easier.
100*400=40kwh 40,000/72=556Ah * 1.2(20% buffer) = 667Ah

I'm not sure what the extra .05 and .32 are about though.
...but keep in mind that you took your 140km and buffered it to 160km and then added 20% to get 192km or a 37% buffer.
I'm also curious where you got your 400wh/mile figure. Are you measuring this from your current setup, if so, how? Your projection for a battery seems huge. ...maybe another Fiero owner could share their wh/mile figure.

#### Duncan

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Hi Steve
MN Driver has the correct numbers

160 Km = 100miles

400wh/mile (seems high - I have been planning on 200 - 300)

400 x 100 = 40,000wh = 40 Kwhrs - discharge to 80%

40 / 0.8 = 50 Kwhrs thats what you need for a battery

72 volts is too low - you should be thinking 150v or above

50,000 wh / 150v = 300 Ah
50,000wh / 200v = 250 Ah

If you plan with 300W/mile
you need 300 x 100 = 30,000 Wh, 30Kwhrs - at 80% discharge 30 / 0.8 = 37.5 Kwhrs

37.5 Kwhrs at 150v = 250 Ah

With cells at 3.2 v you will need 150v/3.2v = 47 Cells
So you are looking at 47 off 250Ah cells

Is there any chance of charging at work?

a 50 mile pack would be half the price

#### Crash

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I agree with the above two posts... 400w/mi is super inefficient for a fiero. That's more or less the w/mi you could see for a pick up truck and still a bit high.

You should look at some of the other fiero projects done on this forum. I'm sure they could chime in as to what kind of w/mi they're getting. I'm planning on 320-350w/mi for my project which is going to be a bit bigger than a fiero would be. (If it ever gets off the ground. LOL)

#### rillip3

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To elaborate more on the voltage, you can see the required Ah are quite high. I don't believe there is a 300+ Ah lithium-ion battery available right now, so assuming 200 ah cells is the largest you can easily get, you'd be buying four cells to get to 12v, then eight more cells to get 600 Ah (assuming 200 is the largest you can get; that's typically the top of what I've seen). And that's still 67 Ah short of your target, which will cost you 4.8kwh, or 12 miles using your 400 wh/mi (and I agree that this sounds too high unless you've done a coasting test or talked with someone who has a similar setup). That's 72 cells and you're not meeting your goal.

By contrast, at 120v (assuming you picked a low voltage because of controller/motor concerns; most have a top rating of 120v), you'd be looking at four cells to get to 12v, then four more cells to get the Ah you need. That's 80 cells and you hit your Ah needed exactly.

If your efficiency numbers are off, and it's more like 300 than 400, you could do the same 120v setup, but get 150 Ah batteries instead of 200.

#### MN Driver

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I think you can use the Zapi and current motor but the low voltage is a big challenge since anything bigger than a ThunderSky 160Ah or a SkyEnergy 180Ah gets tough to fit into the car due to the size of the cells, of course you will only fit so many of them before you are out of room too. I'd look into trying to find the max voltage for that controller and seeing how many cells you can actually fit inside.

#### Crash

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To elaborate more on the voltage, you can see the required Ah are quite high. I don't believe there is a 300+ Ah lithium-ion battery available right now, so assuming 200 ah cells is the largest you can easily get, you'd be buying four cells to get to 12v, then eight more cells to get 600 Ah (assuming 200 is the largest you can get; that's typically the top of what I've seen). And that's still 67 Ah short of your target, which will cost you 4.8kwh, or 12 miles using your 400 wh/mi (and I agree that this sounds too high unless you've done a coasting test or talked with someone who has a similar setup). That's 72 cells and you're not meeting your goal.

By contrast, at 120v (assuming you picked a low voltage because of controller/motor concerns; most have a top rating of 120v), you'd be looking at four cells to get to 12v, then four more cells to get the Ah you need. That's 80 cells and you hit your Ah needed exactly.

If your efficiency numbers are off, and it's more like 300 than 400, you could do the same 120v setup, but get 150 Ah batteries instead of 200.
Actually, thunder sky makes some rather large batteries. 400AH 600AH, 800AH, 1000AH and they even have a 9000AH battery. Crazy, huh?

#### JRP3

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I average around 300 wh/mi in mixed driving, including hills, with my 2500lb Fiero. Running 36 100ah SE cells for 115V nominal, 11.5 KWH pack. The cells are actually 110ah and above so 12.65 KWH and I've gotten 50 miles from the pack in mixed driving which would be about 250 wh/mi. but that was near 100% DOD in warm weather. Obviously your speed and terrain will play a large factor in your wh/mi numbers, as will temperature. Using 250 wh/mi times 100 miles equals a 25 KWH pack, 300 wh/mi time 100 miles equals a 30 KWH pack, etc. 30,000 WH divided by 72 volts equals 417 ah. 46 200 ah cells split in two parallel strings would give you 400 ah at 72V.

#### eco steve

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Thank you everyone for their ideas.

Using 250 wh/mi times 100 miles equals a 25 KWH pack, 300 wh/mi time 100 miles equals a 30 KWH pack, etc. 30,000 WH divided by 72 volts equals 417 ah. 46 200 ah cells split in two parallel strings would give you 400 ah at 72V.
I think I follow all the great input except the last calculation from JRP3 how did you calculate the quantity of 46 200 ah cells

#### rillip3

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Thank you everyone for their ideas.

I think I follow all the great input except the last calculation from JRP3 how did you calculate the quantity of 46 200 ah cells
23 x 3.2 = ~73v. So doubling that (put another 23 in parallel) is 46 cells. Using 200 Ah cells gives you 400 Ah. Same math I did at the end of my post.

#### Crash

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2 Parallel strings means that you'll double the amp-hour capacity of the batteries you're using while getting half the voltage of the sum of all the batteries:

46 / 2 = 23..

So your pack would be represented as 23s2p (23 batteries per series, 2 parallel strings)

The math is as follows:
Voltage: 72v (3.13V per cell)
Amp Hours: (2 parallel strings of 200AH batteries) = 400AH
Total Pack Watt Hours: 400 * 72 = 28800Wh.

Although, nominal voltage with the same pack would be:
Voltage: 85v (~3.7V per cell)
Amp Hours: (2 parallel strings of 200AH batteries) = 400AH
Total Pack Watt Hours: 400 * 85 = 34000Wh.

Driving distance would be figured out by your average w/mi.
Assuming that you're averaging 300w/mi with a max 80% DOD:
72V pack: 28800/300 = 96 | 96 * 0.80 = 76.8 miles
85V pack: 34000/300 = 113 | 113 * 0.80 = 90.4 miles

Some people have noticed that the Thunder Sky batteries have more capacity than Thunder Sky claims. 40AH batteries sometimes discharge 46AH for example. With 200AH Thunder Sky batteries, you'll probably get a bit more than 200AH and you'll likely exceed 100 miles before 80% DOD.

#### JRP3

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Although, nominal voltage with the same pack would be:
Voltage: 85v (~3.7V per cell)
Amp Hours: (2 parallel strings of 200AH batteries) = 400AH
Total Pack Watt Hours: 400 * 85 = 34000Wh.
Not sure where you get nominal voltage of 3.7, nominal voltage is 3.2 for LiFePO4. As soon as you put a load on a fully charged cell it will drop to 3.3 or lower. Just as 12V is nominal for a lead acid battery which fully charged will sit at 12.75V until loaded.

#### Crash

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Thunder Sky cells say their nominal voltage is 3.7V. 4.25V max charge. Meaning, if you're charging that pack to 3.7V/cell (85V) you'll get more out of the pack. Am I incorrect?

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#### Crash

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Ah. Gotcha...

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