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Locost EV conversion log

10295 Views 86 Replies 11 Participants Last post by  Wayne Alexander
Hi everyone,

I am still in the early planning stages here, but I thought I may as well start a thread to document some of what I'm working on/planning, both for information and feedback.

The car:
-Lotus 7 replica (AKA 'Locost'), built by myself to original book dimensions.
-Currently powered by a 2004 Yamaha R1 motorcycle engine.
-Current weight is about 1200 lbs without driver and half a tank of gas
-Primary use is Autocross, though it is street legal

-Safe and well-built EV conversion
-Torque at low speeds but still possible to cruise on the freeway
-Keep reasonably close to the same weight
-Range isn't a huge concern but 30 miles or so would be nice
-Keep costs under $10k CAD?

Here's a photo from last season:

Tire Wheel Vehicle Car Sky

So the car is obviously a front engine RWD layout (I guess, technically the engine is 'mid' since no part of it is forward of the front axle). The rear axle is a narrowed Ford 7.5" out of a Ranger. From what I can tell, it looks like I can get up to a 4.56 gearset for it. It is currently geared to 3.45, which means the engine is running at 6-7000 rpm on the highway (not good) and still doesn't have as much low end torque as I would like. Also, due to the open intake and limited space for a muffler, I am constantly within a decibel or so of going over the noise limit when I get near redline at WOT. The car will do over 60 MPH in first gear, so I rarely even hit second on the courses we typically run here.

Since I will be left without a transmission after the bike engine is out, and to keep weight down, I am planning to go directly from motor to axle with a two-piece driveshaft. I thought long and hard about a DC motor, but decided against it due to a few reasons, including choice of controllers and availability of suitable motors locally.

So I am looking at either a Leaf motor or a Hyper 9. It would be nice to use a Leaf motor for the lower cost, and bonus if I can get the charger to work with whatever batteries I choose. Right now I can't seem to find much info on using the charger, and even less info on using it with a different battery voltage.

And speaking of batteries, I'm leaning towards second gen Chevy Volt. It looks like I could fit four modules in the car - two under the hood and two in the back where the gas tank is now. The Volt battery comes with three 16s modules and four 12s modules, so I can think of two possibilities:
1. Use two of each module size to get 28s2p (actually 4p since each module is internally 2p) (Good for Hyper 9)
2. Use all three 16s modules plus one 12s in series for 60s (Leaf motor would need at least this much voltage)

Since I designed all the suspension and everything except for the basic layout of the 'book chassis', I already had a CAD model to start with. Here's what it looks like with the Hyper 9 and a Spicer PTO u-joint adapter:
Wheel Tire Vehicle Automotive parking light Motor vehicle

And finally, some of my comparisons. I had to go and wrap my head around how synchronous motors work, so hopefully this is right. I would really appreciate some feedback on this! To get the Leaf numbers, I first took the commonly available graph for the 80kW motor and scaled it up to 110kW at the same RPM. I then scaled the RPM for a given torque value, based on going from 360v to 240v.
Sky Rectangle Astronomical object Slope Parallel

If I did that all correct, it looks like the Leaf motor would give me better torque up to about 40 mph and then the Hyper 9 takes over. The MPH line assumes 4.56 gears. On one hand, I like the added low end power, but on the other hand, power is only useful if you can put it to the ground. Maybe as a next step, I should make another graph that compares wheel torque instead of driveshaft torque, and play around with different combinations of motors and rear end gears.

And that's about where I am at now. Any input is appreciated. My timeline for buying parts is basically most of the summer, then once this season is over I plan to tear the car down and start the conversion as a winter project.

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If it was mine I would cut the back out and put the entire Leaf unit in there -
making up a rear suspension setup would be relatively easy and it should be better than a live axle
I have a subaru rear diff with MacPherson struts - easy to make and works well
That would also give you the entire front engine bay AND the old transmission tunnel area for batteries

Your acceleration times are a bit slow - I'm doing 95 mph in just under 8 seconds
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If going for a new car I would make one comment
Battery space is limited - the space between the wheels on each side is completely wasted on a 7
I would be going for a car without that wasted space
You could either put batteries down each "sill" - or move the driver and passenger further apart and put batteries down the middle

Something like this
Wheel Tire Vehicle Car Hood
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Yeah, the new car path would most likely be a full body, and hopefully more aerodynamic. Not sure how much I would want to be climbing over a huge sill, so I'd probably prefer batteries down the center.

After some more thought, I'm going to go ahead with a front engine conversion for the time being, and make the new build be a future goal. For the next couple years I would prefer not to start a whole new build due to other life factors, but I can at least start designing something new, completely from the ground up.

Meantime, I phoned a nearby salvage yard and asked about both a Leaf motor and Volt battery they had. Waiting to hear back on whether they can include some of the extra wiring I asked for.
With the motor in the front you will not have anywhere near enough room for the batteries
My motor is situated where the gearbox should be - and I managed to fit only 14 kWh of the 16 kWh Volt battery in the engine bay
That is with the battery on the floor - which gives a wonderfully low C of G
You could probably fit a 9 inch diameter motor in your "tunnel" - remember you can lose the clutch pedal so you can afford to move the tubes over a bit
Hmm, yeah it does look like I could offset the tubes but it would be pretty extensive. The inside dimension of the tunnel is 4" now do I guess I'd have to do 2.5" per side if I wanted to keep it centered.

Unfortunately the Leaf motor looks to be a good bit wider, but on the bright side it's also shorter than the Hyper 9. I'm not planning to use all of the cells from the Volt, but it looks like I could put two 16s modules right up front, one 12s beside the motor, and one each of 16s and 12s modules in the back, for a total of 72s without having to mod the tunnel. That arrangement would leave me with about 30 pounds added behind the rear axle compared with a full tank of gas. If I really had to I could move a 12s or a 16s to the back as well, but that would be a last resort.

As far as frame mods go, I did have to offset the motor to the right, so I'll have to model up a driveshaft and see what the u-joint angles look like. Probably going to have to mod the steering rack mounts, definitely relocate the 12v battery, etc. But I think it looks promising?

View attachment 128788
The trouble is the Leaf motor is a high revving low torque unit - it really wants about an 8:1 final drive

The advantage of the DC forklift motors is that they are low revving high torque units

Your two tubes are at floor level - right?
So the motor is above that level - at floor level the motor is effectively zero thickness

The tubes on my car are too close together to lower the motor into place - so I have to drop the motor into the passengers footwell and then shift it sideways into place

On my car the floor is completely flat with the batteries and motor as low as I can get them
This means that I'm running with soft springs (70 lb/in front 100 lb/in rear) and no roll bars at all and it still only just rolls a tiny amount
Hi Zeig
The relationship is simple
Torque is proportional to Current
So lets took at your Warp 9 - 500 amps = 100 ft lbs - with a 4:1 final drive that gives 400 ft lbs at the back wheels -
Your wheel will be about 2 ft in diameter - so - 400 ft lbs will give you 400 lbs of push
Is not enough for blistering acceleration
My car is 900 kg (with me in it) and has 55% on the rear = 495 kg - 1089 lbs
So 1200 amps would give - 960 lbs
But my 11 inch motor gives me more torque per amp - So I am getting more "push" than that

Your car is almost certainly going to be lighter so maybe 1000 amps would be enough

You need more Volts to drive the torque as the revs rise
The dreaded back EMF
The Motor diagrams are not much use as they have a controller in the circuit -
they feed say 72v to the controller and the controller reduces that voltage to the voltage that the motor needs to to get the desired current

The way I got the "numbers" for my motor was that I used too low a battery voltage
At 120 volt I accelerated to 100 kph and 200 amps and stayed there - the controller had maxed out and the battery was only supplying enough volts for 200 amps

So knew that 120 volts and 200 amps and 3600 rpm was one of the points on the graph
Back EMF is proportional to current and to rpm
It takes about 10 volt per 1000 amps to overcome the resistive loads in the motor
And the battery sags a bit

So I ended up with about 100 volts Back EMF = 200 amps x 3600 rpm

With my current setup - nominally 340 volt - call it 320 volt
I start off with 1200 amps
At about 1900 rpm (55 kph) my controller is maxed out
By 3800 rpm (110 kph) - I am down to 600 amps
By 5700 rpm (160 kph) - I am down to 400 amps

That is for my 11 inch Hitachi motor
Your 9 inch motor will almost certainly give LESS back EMF per rpm/amp
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Yeah I was just editing all that, haha. Think I got it now.

Also found out that the stock fuse in a 2018 Volt is apparently rated for 350 amps. I'm trying to verify this but it SEEMS like it can pass 200% of its rated current for a period of time. If that means Chevy thinks the cells could support that much current for short periods, I may be okay running just two in parallel. Around here our autox courses are about 60-70 seconds. Probably around 15 minutes between runs.
I'm using the early Volt modules in series - AND the std Vollt fuse -

The cells appear to sag by 20% with 1200 amps - the fuse has not blown!!!
(My thread has me testing this at a drag race a couple of years ago)

To Brian's point about track use verses drag racing - I use my car for both

Yesterday was a triple event - 3 1/4 mile runs - Autokhana on grass - Autocross on tarmac - I was down near empty after the third Autocross run

The drag runs were for a much shorter time - but at constant max power - the track runs were for longer but at lower power levels
I'm planning a similar build with a 1972 Datsun 1200 and 4.56 gears. I had really wanted to do a Nissan Leaf motor with a Chevy Volt battery, but realized it would be much heavier than a Hyper 9 setup when all was said and done. Your locost may be better off with a Hyper 9 and Tesla modules. 5 to 7 tesla modules (120v) and a direct drive Hyper 9 in the transmission tunnel. 250lb or so for 26kwh battery vs over 400lbs for a 18kwh Chevy Volt pack - the Tesla modules are much lighter for what you get. The motor is lighter too it seems. And a yoke for a driveshaft can be bought from evwest for the Hyper 9 shaft. You can't easily do that on a Leaf motor. Vibrations are more likely spinning at that high an rpm anyway. So you'd need to couple to a transmission or torquebox if mounting longitudinally...

Check out the miata zeroev built with a direct-drive hyper 9 on YouTube. It has a torquebox (1.6:1 i think) to a 4.1 rear end but its also got bigger diameter tires (at least bigger than the 13s on my 1200) and more weight. They don't say how well it accelerates though!
That would give you a very low voltage setup - and Volt batteries are not that heavy when you throw away the junk - about 160 kg
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Okay, battery has arrived! I measured the cell voltage at 3.5v, and I think their optimal storage voltage is something like 3.85v. I have a hobby lipo charger for my airplane/drone batteries that can charge up to 6s @ 5 amps. Thinking about disconnecting the BMS and everything, then using the charger to bring the cells up to storage voltage. Any issued with doing that?

I also posted in my other thread about the Leaf transmission conversion, going to be pulling the bearings off the jackshaft as soon as I can get my hands on a suitable puller, then start CADDing up a new housing.
View attachment 129466
Yes that is exactly what I would do
First remove all of the bolt on interconnects - which gets you down to the modules
Take the covers off (one at a time) and measure every cell (every pair? of cells in parallel)
Then charge them up to your "storage voltage"
If its like he two Volt batteries I have worked on they will all be within about 12 millivolts
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At 3.5V, I'd say it's "close enough", if they were mine - I would not futz with them.
3.5 V is about the lower limit as far as I'm concerned - but I would definately check every cell make sure they are all the same
For storage?

This is your reference point for these cells -
you should measure and write down the voltage of each cell so that you can compare that to the numbers that you will get when you re-check in X months when you get around to finally using them
From my experience they will not have changed - but its good to know that for your actual battery
So, buy a Sharpie, number the cells, measure each, put them away.

I don't understand why the risk and complexity of charging is needed at this point when all he plans to do is hoard a Volt battery 😉
Its exactly what I did when I got my second battery (its not mine its for a maniac who wants to build a drag bike)

I checked the voltages - decided that they were too close to the 3.5 volts that I use as a minimum and charged them up to 3.9 volts

I feel much happier about that battery being stored with a middle ish charge

I did have a power supply with a large enough voltage that I could simply charge each module individually
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Hey Duncan: why do you consider 3.9 to be middle charge? I must be missing something. My middle for storage is 3.6, about a volt over my calculated low warning under heavy operation.
Hi Piotrsko
I don't have a "low under load" number - I always back off before looking at the voltage
Just had a look at my videos of the 1/8th mile drags - the peak "sag" was about 20% which would give me a voltage under full load of 3.1 volts

With the cells just sitting I am treating 3.5 volts as empty and 4.1 volts as full -
Yabert provided the actual curve I'm using
Somewhere in the middle of this thread
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I tried to use the large contactors from the battery - but my "abuse" made them stick shut !! - so I bought a pair of big contactors

I'm still using one of the large contactors for my pre-charge circuit

Pre-charge resister --- I use an old kettle element
I am a maverick about suspension design - as far as I am concerned the "roll centers" are complete bollocks

What you need to look for is the angle recovery so that if the car rolls 5 degrees the tyres are still vertical to the road

Saying that my car with the battery on the floor simply does not roll much at all

When you know the final corner weights then you can calculate the required spring rates - I'm using 70 lb/in on the front and 100 lb/in on the back
and I expect your car to be considerably lighter
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