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Thanks,

Kurt

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Joined

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60 Posts

Thanks,

Kurt

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7,829 Posts

I liked this one. There may be more recent editions or others by now. Basic physics texts can help. And there may be schools in your area.I want a book with equations and explanations.

Build Your Own Electric Vehicle by Bob Brant

Also, you can find a lot here http://www.diyelectriccar.com/forums/showthread.php?t=669

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I just ordered the 2007 or 2008 (?) edition of that book.I liked this one. There may be more recent editions or others by now. Basic physics texts can help. And there may be schools in your area.

Build Your Own Electric Vehicle by Bob Brant

Also, you can find a lot here http://www.diyelectriccar.com/forums/showthread.php?t=669

The EV info has two guidelines for the first two steps of sizing your battery pack:

Step 1:

"

Step 2:

"As a general guide motor bikes will generally use 75-150Wh/mile, cars may use around 200-400 and pick-ups and heavy vehicles around 400-600Wh/mile ..."

I'd like to calculate those values instead of estimating them.

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Dig a little deeper. http://www.diyelectriccar.com/forums/showthread.php?t=15508 That's not everything you need to know (by a long shot), but is the basic stuff.I'd like to calculate those values instead of estimating them.

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Doh! I had glossed over that one night- the 35kW/1000kg was what I'd focused on, and I'd forgotten about the other info.Dig a little deeper. http://www.diyelectriccar.com/forums/showthread.php?t=15508 That's not everything you need to know (by a long shot), but is the basic stuff.

Thanks,

Kurt

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4,838 Posts

Everytime I found something that might be useful I made a note and put it in my spreadsheet until I had enough to get something useable.

I included little tables of air density at various temperatures, for aerodynamic drag, and for rolling resistance of different tyres on different surfaces.

So...

Power consumption of aerodynamic drag

= 1/2 x air density x V³ x frontal area x Cd [notice that that is a V cubed in there]

Power consumption of rolling resistance

= Crr x Mv x V

Where:

air density (rho) = 1.29 at 0ºC

Mv = Mass of vehicle (kg)

V = Velocity (metres/second)

Crr = Coefficient of rolling resistance, 0.03 for car tyre on asphalt

Cd Coefficient of drag, around 0.6 for an open top car and 0.3-0.35 for average modern car.

The combination of these two will tell you how much power, in Watts, your car needs to keep moving at a specific velocity on level ground in still air at 0ºC with ordinary car tyres on asphalt.

Dividing your W(power) by your mph gives you Wh/mile

Energy consumption = Wh/mile

Using your required range you can determine the battery capacity.

Energy Consumption x Range

eg: 200Wh/m x 40mile range = 8000Wh or 8kWh

However you only have an 80% Depth of Discharge (DoD) so you require a 10kWh pack.

Pack capacity

Cell voltage x ah rating x number of cells

eg:

3.2v x 100ah x 30

9600Wh @ 96v

You can vary the number and size of cells until you get the power and range you require.

You can also look up the mass, in kg, of the range of cells and include that in the overall mass of the vehicle.

This is what my spread sheet looked like when I did a quick calculation for my tractor.

Using LiFeP04 cells:

Using Lead Acid:

I have since extended the spreadsheet to include gear ratio calculations (that are not linked to the above sheets) and weight distribution (that is linked) to determine front rear weight and centre of gravity.

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1000 kg of Optima lead acid batteries ~= 7 kWhr of usable energy

10 mpg vehicle => 7 kWh / 10 mi = 700 wh/mi

15 mpg vehicle => 7 kWh / 15 mi = 470 wh/mi

20 mpg vehicle => 7 kWhr / 20 m = 350 Wh/mi

30 mpg vehicle => 7 kWhr / 30 m = 230 Wh/mi

40 mpg vehicle => 7 kWhr / 40 m = 180 Wh/mi

50 mpg vehicle => 7 kWhr / 50 m = 140 Wh/mi

70 mpg vehicle => 7 kWhr / 70 m = 100 Wh/mi

100 mpg vehicle => 7 kWhr / 100 m = 70 Wh/mi

Of course, YMMV.

... "As a general guide motor bikes will generally use 75-150Wh/mile, cars may use around 200-400 and pick-ups and heavy vehicles around 400-600Wh/mile ..."

I'd like to calculate those values instead of estimating them.

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Build Your Own Electric Vehicle by Bob Brant

I bought the Second Edition of this, and can't recommend it. The content may be good, but unfortunately the editing is extremely poor. For example, on one page I found the same paragraph in two different sections (but only relevant to the second). Also (it may have been the same page) there's a diagram showing a basic setup for switching between AC and DC drive-trains, and the diagram has the AC and DC motors switched so it doesn't match the text (or make sense) at all.

These are not the only things I found wrong, they're just the ones I remember...

But it does have lots of formulae

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