Six months ago we received a request for a quick list of parts to make a B&B Manufacturing Turnkey Minus 66 Cobra kit car into an electric drive vehicle. We responded that it wasn’t quite that easy. The components required kind of had to be worked out based on what you wanted the car to do and it required a bit of thought. We were more correct in this than we realized. And therein lies a tale.
I have enjoyed and endured a variety of experiences in my brief life. Racing automobiles never was one of them. Vehicles of my youth revolved around farm and construction equipment and we worked on them of necessity. They were not a lot of fun frankly. So I never got into the passion for automobiles and specifically not the race scene.
The 66 Cobra was produced specifically for racing. And it is one of the more popular and enduring replicas, usually available in kit form. An entire culture has grown up around this mythic beast. But I was learning Z80 assembly language with Rodney Zaks at the time all that was going on.
But a clean sheet design using someone else’s idea of what an electric car should do might be a challenge. Er, duh, yeah.
The topic has actually come up again with the same car. The answer is the same only more so.
This week we pretty much completed assembly of the battery pack for this vehicle. I think it illustrative of how one basic design decision/component choice drives three others, and those in turn three others until you are in a quagmire of components all deriving from the initial suppositions. Be careful what you ask for.
The basics go like this. There are three factors in an electric car conversion:
You can optimize for two. By way of example, the little spreadsheet I keep on component costs for the Cobra currently shows $42,519.40. No labor. A bit of outside fab. But by far and away just components. And it doesn’t include the original cost of the “Turnkey – Minus” at all.
The original concept involved a car that would do a true 120 mile range, 120 miles an hour, and a 0-60 time in six to eight seconds.
To do 120 miles reliably, we have to have a 100% discharged range somewhere in the neighborhood of 140-150 miles. That takes a LOT of battery to do in a conversion – which adds WEIGHT. Weight is the enemy of performance. To launch a 3000 lb vehicle to 60 mph in six to eight seconds requires a lot of POWER. In this case, I calculate about 250 kw of power.
To do this, we chose to go with a Netgain Warp 11HV motor. Why? Series DC motors are just very good at producing torque, which is what we need to launch this beast. But they are power limited by the voltage you can run them at before brush arcing occurs. The 11HV features Interpoles – windings between the field windings that are aligned so as to neutralize the induced voltages in the windings NOT connected to the brushes so when they DO make brush contact they do not arc. This allows higher voltages, and thus higher power. THe normal Warp 11 is traditionally limited to 170volts. This HV version let’s us go to a higher voltage, and so a smaller current for the same power.
But not much smaller. To get 250kw, at 231volts, we have to have over a thousand amps. Of course, when you take 1000 amps out of a 231 pack, it really is NOT 231 volts any more. It’s more like 175 volts. And so we’re back to needing more current.
We are going a bit out on a limb opting for the reallly still beta Netgain Controls Warp-Drive Industrial version. But it purports to do 260volts (actually higher) and 1400 amps (we hope). So even if our pack sags substantially, we can still be up around 250 kw total electrical power applied to the motor.
Of course, we don’t want to burn up the motor. Our best effort at avoiding that is the addition of a cooling air blower. Not all blowers are created equal. But we can get 450 cubic feet per minute from a converted Garrett Turbocharger. Unfortunately, it is $350.
We also have to cool the controller – rather substantially. So big glycol cooling system.
But the biggest wrestling match is the battery cells. We have to have a lot of them in series to get to 231volts (69). That points toward a smaller cell. But what about current? We’ve done 1000 amps from these basic types of cells using 180Ah cells. Could they do 1400 amps? Perhaps. For a few minutes. But 180 AH cells are kind of bulky. And as it turns out, a lot of our spaces are pretty constrained in this car.
So we opted to strap two 90Ah cells together. That gives us a very different dimension and granularity of positioning to the cells. But it also means 138 cells total.
And that drives us to a LOT of battery boxes – seven that have to be fabricated and assembled. We go to Southeast Fabricating to do the basic foldup/weldup. But then we have to add angle aluminum, attachments, lids, paint, terminals, etc. It’s a good bit of work with seven boxes. Two underneath only hold seven cells each. Another two hold eight cells. Etc. It turned into a wrestling match to fabricate, install, strap up, and wire all these cells more or less securely.
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Unfortunately, a LOT of these cells then pose accessibility issues. We have a two tier box in back. You have to completely disassemble, in the right order, the rear top box to access any of the cells underneath. The two rib boxes underneath are a little easier. We made little doors on the outside of the car you can remove with five bolts to expose the terminals of the cells. But many in front are likewise rather a procedure to access.
So we bottom balanced those 138 cells rather carefully,, many in the lab before installation. We tried to pair higher capacity cells with lower capacity cells to diminish the variation in capacity across the pack.
And it wound up being a LARGE pack for a car this size. It weighs in at 973 lbs for the cells, not counting boxes and straps, and provides 41,607 wH of energy storage.
But that gets us several things. First, 300 wH per mile for a 3000 pound car is about the right 10:1 ratio we see in all our cars. If you’ve followed the Spyder/Speedster fiasco you will now be much smarter that range is not always range and indeed it is a very ethereal thing. But we’ve found the 10:1 rule pretty good in practice.
The 300 wH and the 41,607 wH work out to 140 miles at max 100% discharge. 120 miles would then be 36000 wH or 86% DOD. That’s a little higher than we like but we’re making tradeoffs here and doing our best. It will safely deliver 120 mile range in a pretty good variety of conditions. We’re hoping he charges more often than that.
So you see the game we are playing.
Second, we get our higher voltage for the Netgain Warp 11HV. And finally we get our 1400 amps, at least for a few moments. We’re hoping for a full 250 kw on the dynamometer.
Speedster Redux got us 147 kw in 2385 lbs for 62 watts per pound and does a zero to sixty in 6.5 seconds. At 3000 lbs, that will require 186 kw. But frankly we are hoping for some upside at 250kw, if we can pull it off. And there’s a lot of slips between here and 1400 amps.
Of course, to handle that amperage we have to have a kind of massive high current relay. We took two fairly ordinary ones and coupled them together. In doing so, we hooked up quite a bit of copper plate to help dissipate the heat.
As to cable, the choice at these currents is between 2/0 and 4/0. The 4/0 would be better, but it is heavier. MOre to the point, it is harder to work with. With seven battery boxes, that’s a challenge. And since we don’t have any very long runs connecting all those boxes, we opted for the 2/0.
To keep the noise/EMI down, we have gone with Champlaign Cable Company shielded cable designed specifically for electric vehicles. Unfortunately, it’s also designed to fetch $8.57 in ducats for each linear foot of the cable.
To fuse all that current, we use a Ferraz Shawmutt A30QS800-4. This part number indicates a 300vdc rating and a 800 ampere rating. It’s not precisely a slow blow, but it will take 1400 amps for a few seconds.
As to a high current relay, we faced a bit of a problem. We used two of the Tyco Kilovac EV200 relays. The proper relay for this is probably their EV500 series, which will do 1600 amps for 10 seconds.
But the EV500, called “Bubba” is typically available for $1200 to $1400. Occasionally they come up on eBay at $350-$500 each. But the EV200, rated at 500 amps, are common as dirt at about $70 each. So we used two of those in parallel.
The current capability of the EV200 is kind of dependent on the size terminal lug connected to it. The terminal lug actually acts as a heat sink. So to do the advertised 500 amps, you need a pretty largish lug and cable. While we’re ok there, more is better. So to parallel them, we actually used a pretty hefty piece of copper bar stock. This should help dissipate heat and allow our pair to do 1400 amps briefly. We’ll see. At $140 instead of $1400, it’s worth a try.
On the other end, we spent a couple of grand on lighter wheels and low rolling resistance tires, which did save us 10 lbs each on the front and 16 lbs each on the rear for 52 lbs. We also spent $1120 on an aluminum third member for the differential. Again, the weight savings was substantial
The result is our car is 32% battery by weight. But the components to do it would total over $42,000.
All of this from a couple of basic assumptions regarding what kind of car we’d wind up with. Had we said 11 seconds 0-60, and 80 safe miles with 100 miles to 100% DOD, we would have a much lower weight and cost and a much easier time fitting battery cells.
My experience with these cells comes out the same way every time. They want to make a car with 100 max 80 safe miles. Yes, you can get them to do more, as we have in Redux and now Cobra. But it grows exponentially more difficult and more expensive.
The question you have to ask, is what do your REALLY need and want in an electric vehicle. Everything in design is a tradeoff. There are no little easy answers. I can’t give you a “kit list” until I’ve done the design and build, tested it, revised it probably several times, and so forth.
Will a go fast Cobra with a 120 mile range scratch the itch? Let’s hope so. But be careful what you ask for.