Battery Calculations and Range Expectations
One thing I am quickly learning about EV projects is its very chicken and egg.. You sort of need to roughly have every decision made as one thing effects the other so dramatically.
Take the battery pack, forget the size, shape and weight briefly. The voltage of the pack is determined by the needs of the motor. Then the amps give the range, well its actually the watts that give the range but that's how I think of it.
So remember amps x volts = watts
You may have heard EV cars battery packs being referred to as kWh, so we need to consider the voltage of the pack needed for the motor so we can then start looking for available cells.
That's a key part of this build, there are lots of start ups out there doing battery packs and motors, however I'm keen to try and only use off the shelf parts to hopefully make this easier to replicate if I ever need to do another. You never know what the future brings.
So before I can look at batteries I need to look at motors....
Is your mind hurting yet, mine is.
Now currently at this point in time (I imagine in a year 90% of this blog will be outdated) a second hand tesla performance rear drive unit is the best bang for your buck.
However there are 2 issues, I'm making a kit car, so it will have to go though an IVA test. Without going into too much detail here, you need all new parts or proof where the old parts came from or you can end up with a Q reg. Also I'm building a GD with a Euro chassis, the whole point of this is it's a new car not a hash of other written of vehicles "cough, Jag based kits, cough." So not only are there a few implications to consider using a second hand motor but also it doesn't sit right with me to be using a second hand motor in a new car.
Issue 2, yes a tesla can do phoneominal standing starts however that's only peak output of 636hp, its continuous output is only 61hp. I'm getting these figures from googling various sources so happy to be corrected.
To be credible it needs to be able to do a good track day stint without loosing power.
So I need to find a supplier that can provide a new motor with half decent continuous output. The search begins watch this space....
Battery pack voltages
From my research I have seen 400V to be the sweet spot between availability of parts and power. Pack voltage effects battery heat, charging times, and many more attributes of the EV system.
Now I have a voltage set I can start on pack calculations.
We are all familiar with MPG being the measure of economy for IC vehicles. Well, the EV equivalent is "Wh/Mile" watt hours per mile. How many watts of energy were used to move the car a mile. Unlike MPG where the higher the number the better, we are looking for lower numbers here. I have completely guessed at 250wh/m this is an educated guess looking at other OEM cars and other conversions, the Cobra isn't the most aerodynamic, however it will be light.
Lead acid isn't even a consideration due to its low power density, think of this as watts per KG. Lithium it is then, let the search begin.
Quickly remember a battery pack comprises of 3 parts. cells, modules and then the pack. The pack is made of x number of modules, the modules are made up of x number of cells. Also remember series and parallel, quickly, series increases voltage, parallel increase amperage.
I'll cover this more later, but the calculation in short:
Nominal Pack voltage (V) = Nominal Module Voltage x number of modules in series
Pack size (kWH) = ((Nominal pack V x Module ah)/1000) x number of modules in parallel
Now usable pack size is 80% the pack size as you shouldn't drain lithium down to 0% (Although the display will be configured to read 20% as 0%, so you will think you are going to 0%)
Range (Miles) = (Usable pack size / wh per mile) x 1000
Here was a mock up on the size of an LS3 and T56 transmission vs battery packs in the front and where the fuel tank is, then batteries down the tunnel. That was enough for me to believe it might be possible and lets go for it. This was looking at twin hyper 9 motors, they seem to be fairly easy to source and lots of documentation about them is available.
Tune in next time were I will properly start considering options out there on the market. I'm late writing this so I will also be able to let you know what I have actually put a deposit on!!
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