Weber and Coulomb's MX-5

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Post by zeva » Sat, 12 Jan 2013, 20:52

Take out some of the batteries from behind the rear axle? Since they add more than their own weight to the rear axle load, and actually reduce front axle load.

The weight balance isn't terrible (about 48:52 front/rear, ~5% rearward shift) but that rear axle loading sure has gone up a lot. I'd be very surprised if any licensing centre would pass that without engineering sign-off on chassis & suspension integrity, and a 51% increase would probably make most engineers nervous about putting their name on the line..

Can you run a smaller pack (even temporarily) to get the weight down, or do you need the full voltage?

If you're at the weighing stage, it must be just about test drive time!?
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Post by weber » Sat, 12 Jan 2013, 20:53

Renard wrote: Two of my -- fortunately much smaller -- under-the-rear-seat boxes also have to go in from underneath the car. So I'm curious to know -- what I can't work out from the photos -- whether you raised the box or lowered the car or both?

Hi Renard. We raised the battery box.

Four of us lifted it from a 4 wheel trolley onto the aluminum plank which is wider than the car. The plank was on some timbers on the ground and the battery box was on some timbers on the plank. We slid the plank forward along the timbers then levered the plank upward on one side and then the other, putting more timber under until we could get the trolley jack under the middle of the plank. Then we jacked it up while adding more timbers under the ends as we went, for safety.
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Post by acmotor » Sat, 12 Jan 2013, 22:27

Darn those heavier than air batteries.

So I guess you have weighed and made detailed spreadsheet of removed and added weight. Can we see it so we can help ponder the situation. It seems you may need to find more than 150kg of savings ? What is the GVM on the MX-5 compliance plate ?
Any chance there is an error in the (creative) weighing system ?
Can you borrow a load cell and check, also check L/R balance.
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Post by acmotor » Sat, 12 Jan 2013, 22:35

weber wrote:

Image


Can you simply remove the blue battery packs 60+28 that would be 150kg less to get things going ?
If I follow correctly the Wavscuplter only requires the 375V ?



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Post by BigMouse » Sun, 13 Jan 2013, 00:09

As long as the car is below the maximum gross weight with two passengers, the engineer should be fine with it. I expect that most of the "payload" which would bring a car from kerb-with-occupants to maximum gross would go in the boot anyway. Is there really any difference between what you've got now and driving around with a couple bags of concrete in the boot all the time? Both should be legal (assuming the headlights are aimed properly Image)

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Post by weber » Sun, 13 Jan 2013, 00:13

Some corrections of arithmetic/transcription errors:
Front 658 kg, Rear 710 kg, Total 1368 kg.

I don't think there is much value in the left/right measurements as the concrete slopes down a little from left to right, for runnoff, and there could easily be some twisting of the car due to slight differences in height of the stacks of wood blocks under the wheels not being weighed. Wood blocks are approx same height as scales plus steel tube.

front left 316
front right 342
rear left   365
rear right 345

That drawing is good to refer to, acmotor. Thanks. The cells are 1.5 kg each.

The NA model MX-5 does not have a GVM. Back then, GVMs were only issued to commercial vehicles. Our engineer says we will be applying for a new GVM and will have to do braking distance and brake-fade tests. I have emailed this latest info to our engineer, who will hopefully reply Monday.

Spreadsheet used for estimating total mass.
EV - MX5 Masses               
               
Original tare      983
               
Added mass               
batteries          342          
batt cages          60          
Motor mass          95          
rollbar             20          
motor controller    12          
vac pump             3          
DC-DC converters     2          

Removed mass               
IC engine          160     
Fuel and tank       35     
Exhaust              9     
IC engine rad       10     
               
Total add          534          
Total subtract     214     
               
New Tare          1303
               
Passengers (X2)    136          
luggage             27          
               
payload            163          
               
GVM               1466
Dunno why the estimated tare is 65 kg too low. Maybe the estimated mass of the ICE is way too high.

I sure hope we don't have to remove 150 kg. Engineer previously said he thought 1290 kg tare would be OK (assuming well distributed). So most we should have to remove is 78 kg.

Mark and I decided the first thing to do, like zeva said, is to remove the row of 14 cells from the rear of the rearmost box (we've just been and done this). We would then remove a matching 14 from the rollbar box (which would only have 5 left in it). This would remove 42 kg from the car and might remove 48 kg from the rear and add 6 kg to the front. That would give
Front 664 kg, Rear 662 kg, Total 1326 kg.

[Edit: Aligned numbers from spreadsheet]
Last edited by weber on Mon, 24 Jul 2017, 04:47, edited 1 time in total.
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Post by acmotor » Sun, 13 Jan 2013, 00:46

The andersons, 16mmsq cable, silicon, cable lugs, BMS system, fuses, conduit, juntion boxes etc (assuming they were in the weight) all add up as well.

Have you also done a check to see if there is anything ICE left to remove ? Fuel lines, fuel tank neck, exhaust hangers, muffler heat shields, hidden fuel expansion tanks. (road wheels Image )
Replace steel number plates with ali ??

I know you've thought of all these. Just hoping you'll find a body in the boot or something !
Image
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Post by KDRYAN » Sun, 13 Jan 2013, 01:36

The GVM in South Africa is 1375kg, it would be the same or very close here in Australia.
In your ownwes manual under specifications is there a maximum axle weight front & rear. This is usually a lot more than the GVH. You can go up to these figures but as you stated above you will need to have a brake performance test carried out. And from what my certifing engineer said it is a pretty daunting affair.

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Post by weber » Sun, 13 Jan 2013, 03:35

acmotor wrote: The andersons, 16mmsq cable, silicon, cable lugs, BMS system, fuses, conduit, juntion boxes etc (assuming they were in the weight) all add up as well.
Good point, acmotor. I came up with the following estimates
Cable              10 kg
Contactors          9 kg
Power steering pump 7 kg
Conduit             6 kg
Various brackets    5 kg
Contactor boxes     5 kg
BMS                 3 kg
Fuses               1 kg
Andersons           0.8 kg
Silicone            0.6 kg
Lugs                0.4 kg

Total additional   48 kg
Have you also done a check to see if there is anything ICE left to remove ? Fuel lines, fuel tank neck, exhaust hangers, muffler heat shields, hidden fuel expansion tanks. (road wheels Image )
Replace steel number plates with ali ??
Definitely none of those left (except the road wheels Image).
I know you've thought of all these. Just hoping you'll find a body in the boot or something !
Image
Sadly, no. Image There is the original-equipment-type lead-acid battery in the boot, however.
KDRYAN wrote:The GVM in South Africa is 1375kg, it would be the same or very close here in Australia.
Thanks KDRYAN. I'm guessing this is for a model later than our NA 1.6 (1989 to 1993). But in any case, we will need to apply for a higher GVM. More like 1490 kg.
In your owners manual under specifications is there a maximum axle weight front & rear. This is usually a lot more than the GVH.
The only thing remotely like that, that I can find, is a statement that the tyres must have a load rating of 475 kg. So that would be 950 kg per axle and a total of 1900 kg. I'm guessing there's not much point insisting that the tyres must take that if the axles can't.

This thread is useful.
http://www.diyelectriccar.com/forums/sh ... 79515.html
It lists kerb weights of 10 converted MX-5's. Half of them are over 1300 kg. The highest is 1496 kg (that's supposedly kerb, not GVM)!

And folks do commonly put V8's in these things. So I'm hoping the removal of 42 kg of cells (28) from the rear will be sufficient.

[Edit: Spelling]
Last edited by weber on Sat, 12 Jan 2013, 16:37, edited 1 time in total.
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Post by Renard » Sun, 13 Jan 2013, 13:55

Are the chargers hard-wired in? If they were detachable, you could claim that you were only going to charge at home, and then the charger weight could be subtracted.

Edit: I now remember you have just the one charger, but it weighs a lot. (Can't find the data offhand.)
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Post by coulomb » Sun, 13 Jan 2013, 14:43

Renard wrote: Are the chargers hard-wired in?

Not yet, but it was the intention to hard-wire two 2.X kW TCchargers. We might consider only bolting in one charger, and leaving the other one out of the vehicle.

Maybe we should leave them both out and install a CHAdeMO connector instead of two 240 VAC sockets. After all, Tritium will be selling quick DC charging stations, so they might be as common as AC pay-chargers in Brisbane soon. But that would only save 2 x 7 kg (each charger weighs 7.07 kg, per this page).

Acmotor asked if there was still any ICE remnants to pull out; there is still the original, complete wiring loom. However, I think that pulling out selected wires would be a huge pain, and might only save a few kg.

It's such a pity that we didn't consider weighing even one of the battery boxes over the 3+ years that we had them lying around unfilled. If one of the battery boxes has to come out again, let's weigh it! In fact, the rear battery box surely came out to take out the 14 cells from it, and surely you would have thought to weigh it. So what was it?   Image

In fact, a filled box would have been more useful to weigh, because it gives us the total weight, including BMS, links, cables, Andersons, bolts, packing, silicone, and paint. From the weight of one, we can accurately figure out the weight without cells, and do a pretty good estimate the complete weight of the other boxes.
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Post by coulomb » Sun, 13 Jan 2013, 15:07

coulomb wrote: However, I think that pulling out selected wires would be a huge pain, and might only save a few kg.
Then again, if a typical wiring harness looks like this:

Image

then maybe it really is worth it.

From this page: http://hull-murtaya.blogspot.com.au/, second last image. Even the last image is scary.

At least half of that might be under the dash, and I don't think that much of what is under the dash can go. It would only be under the bonnet that could be removed, I think, plus most of the wires to the engine ECU. We're using some of those for our DCUs (Driver Controls Units), but we don't need the ~50 that are there now.

Does anyone know the weight of a wiring harness?
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Post by Nevilleh » Sun, 13 Jan 2013, 15:46

I remember pulling the wiring harness out of a Mitsubishi some years ago and i could pick the whole thing up with one hand, so it wouldn't have been 10 kg. I doubt you could save much by removing a few wires.
You might do away with power steering, I find it quite unnecessary in the BMW. Also the 12 V battery. they weigh about 12 kgs and you could us 4 Li cells instead (6 kg).
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Post by acmotor » Sun, 13 Jan 2013, 17:17

Guys, my take FWIW.
Go with one battery pack at the max voltage the controller will take.
Remove the highest, most front / rear overhang cells.
Even if you can get present axle weight and GVM accepted, the performance and handling will suffer with the present vehicle weight.
You were chacing sporty performance.... and we needed you to get there !

You will get good range with the present battery capacity however it is getting back to the old lead sled EV days in weight.

This may be cruel, but perhaps the battery technology has moved fast even in the few years since the SE cells were in favour. Their specific power is a limitation.
Like with many ? EV conversions, the platform is good and you may need to move on with the battery types.

Get the MX-5 finished and on the road. It still breaks plenty of new ground with its systems but I hope your technical skills, creativity and open diary of progress (that helps us all) can be directed to your next battery pack.

Check out the tesla model S battery pack. The cell technology is Panasonic and the tesla pack build is smart and innovative but not beyond the skills of capable EV convertors.
Image

Edit: and don't be affraid to 10C the SE cells. If they last 12 months and you have fun then at least you have some spares while you work on the next battery pack. After all, if they last 10 years so what ? I think you will move on to other cells long before that.
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Post by jonescg » Sun, 13 Jan 2013, 18:08

I have to agree; battery chemistry has advanced faster than your build.

If you future-proof your build you can install the next best chemistry and know that you have plenty of headroom on the maximum system voltage front.
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Post by BigMouse » Sun, 13 Jan 2013, 18:57

jonescg wrote: I have to agree; battery chemistry has advanced faster than your build.
This is why I always wonder why people buy their batteries right when they start a conversion, then they sit for years, warranty and technology expiring while the rest of the build takes place. Luckily, the SE cells are still very nice cells and more than capable.

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Post by acmotor » Sun, 13 Jan 2013, 20:37

BigMouse wrote: ....
This is why I always wonder why people buy their batteries right when they start a conversion, then they sit for years, warranty and technology expiring while the rest of the build takes place....


Often the case. Image Image Image

In (our) defence, if it wasn't for the eternal optimism of some then the world would be a static place. I tips me lid the Elon Musk !

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Post by weber » Sun, 13 Jan 2013, 21:09

Thanks for all the discussion.

There's absolutely no point in trying to fool anyone by asking for a GVM that does not include the mass of all the things/people that might sometimes be in the car at the same time. If you have an accident and you're so much as one gram over the GVM that you've applied for, been tested for, and had stamped on your mod plate, then you can kiss your insurance good-bye, and maybe a fine or your license as well.

Folks talking about taking out power steering, or unnecessary wires in the loom, seem to be missing the fact that the problem is excessive weight in the rear. The front is OK. But replacing the lead-acid accessory battery in the boot, with lithium, is definitely of interest now.

And yes, we're aware that battery chemistry has advanced in the 3+ years we've been working on our BMS (oh yes, and its test platform Image). But it hasn't advanced as much as I thought it would. Basically we can have 6C now for about the same price we could have 4C when we started, but they don't seem to have got any lighter or more compact or less expensive for the same amp-hours.

Actually, we could have had 6C for about the same price back then, but it wasn't from a company we trusted, and we would have had to take a 25% capacity hit.

Some reasons we bought the cells only 6 months into the project:
1. We had no idea it was going to take us more than 1 year.
2. It gets very tedious making fake cells and trying to use them to figure out how many cells we can fit where. We needed 4 separate battery boxes just to make use of the fuel tank space -- the prime real-estate nearest the center-of-gravity of the MX-5.
3. To hedge against unfavourable exchange rates.
Remove the highest, most front / rear overhang cells.
Will do. At least in the rear, where the problem is.
Even if you can get present axle weight and GVM accepted, the performance and handling will suffer with the present vehicle weight.
You were chasing sporty performance.... and we needed you to get there !
I agree that handling suffers for every kg increase over the original kerb weight. But straight-line performance improves with every additional cell we can shoe-horn in (assuming the same cell life). Halving the number of cells will halve the power, but will only decrease the mass by about 12%
You will get good range with the present battery capacity however it is getting back to the old lead sled EV days in weight.
Yes, but with a lot more power.
Edit: and don't be affraid to 10C the SE cells. If they last 12 months and you have fun then at least you have some spares while you work on the next battery pack. After all, if they last 10 years so what ? I think you will move on to other cells long before that.
I'm afraid I can't come at that idea. It just seems too wasteful. Yes, we'll move on to other cells, but with a later model MX-5 (or a completely different vehicle). There's no reason why someone shouldn't enjoy at least another 4 years out of these SE cells while driving this MX-5.

Yes, the under-boot battery box is out of the car, but I haven't weighed it yet. The scales are still (usefully) under the front wheels and I have to do other things for the next couple of days (paying work -- eek!). I haven't touched any other boxes.

Jeff Owen, who prefers to phone rather than post, has suggested we remove 26 cells from the under-boot box and change the connection to the frontmost box of 13 so it becomes part of the "B" (blue) half-pack. The effect is to remove 13 cells from each half-pack while getting more weight off the rear axle than would happen with Mark's and my earlier suggestion to remove 14 from the under-boot box and 14 from the rollbar box.

Jeff suggests removing the rear row of 14 completely (already done) and removing 3 from each end of the other two rows (leaving 17 in each of the two rows). The battery box could then be correspondingly reduced in size to save more weight and eliminate the tight squeeze we have at the sides.

I like this suggestion and propose to remove the 26 cells from the rear box and reinstall it, re-weigh the two axles, and propose the result to the engineer.
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Post by acmotor » Sun, 13 Jan 2013, 21:50

weber wrote: ....
There's absolutely no point in trying to fool anyone by asking for a GVM that does not include the mass of all the things/people that might sometimes be in the car at the same time....


Just to clarify, deception was not my suggestion. I was thinking that the battery pack weight needed to be reduced to a more workable value until greater Wh/kg and W/kg battery pack was fitted in the future at equivalent or lower weight.

1/4 mile times come from lower weight and higher discharge rates.

Sounds like you have a plan anyway.
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Post by weber » Sun, 13 Jan 2013, 22:29

acmotor wrote:Just to clarify, deception was not my suggestion.

Rest assured, that was not in response to anything you wrote.

I understand your suggestion, but I don't have money that would allow me to "burn through" the SE cells I have, by having them in a pack that's too small for the motor and inverter, just so I can replace them with something better.

So, given that I'm going to use the SE cells that I already have, I assume you agree that best straight-line accelleration is obtained by stuffing as many cells as I can into the vehicle (while not overloading any axle), and eliminating as much non-cell weight as possible.
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Post by acmotor » Mon, 14 Jan 2013, 04:55

So do we start a tab on the prospective 1/4 mile time ? Image
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Post by Johny » Mon, 14 Jan 2013, 14:45

weber wrote:That rear increase is a bit scary. Any suggestions?
Yes, accelerate carefully around corners in the wet. Great work guys. I can't believe that great big battery box fit in that space.

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Post by Johny » Mon, 14 Jan 2013, 16:00

Guys. If it helps, my engineer had me do a spreadsheet where he had me measure and position everything removed and added with respect to the front axle.
It's primitive but very useful. I have placed a copy (for a week or so) at:
https://dl.dropbox.com/u/46309508/Vogue ... 20CALC.xls

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Post by weber » Mon, 14 Jan 2013, 18:18

Thanks Johnny. We did a spreadsheet like that in the early days. We even had heights for everything, to estimate the increase in height of the centre of gravity. But the design has had many changes since then and we didn't keep it it up to date.

And now that we have actual measurements with everything in, and our only real option is to remove cells*, it seems easier to just calculate the effect of those removals relative to the latest measurements.

* As Jeff pointed out in another phone call, there is a tiny amount of space left under the bonnet where we could fit a small 12 V battery (we might have to stagger contactor closures to get them all closed) and we could thereby eliminate the lead-acid battery in the boot.

How's this for a GVM argument?

We can calculate the maximum transfer of weight between axles due to extreme drive torque or braking, by using the highest coefficient of friction between tyre and road, which several sources give as 1.0.

The MX-5's wheelbase is 2266 mm and its wheel radius (axle height) is 572 mm. So with a friction coefficient of 1.0, the most that can be transferred is 1.0*572/2666 = 0.215 = 21.5% of the final weight on the braking front wheels, or driven rear wheels.

So if the final weight is allowed to be 950 kg per axle (based on the 475 kg per tyre specification I found recently, thanks to the urging of KDRYAN) then the initial weight can be 950 * (1-0.215) = 746 kg per axle, for a total GVM of 1492 kg.
[Edit: This calculation is completely wrong. See viewtopic.php?title=weber-and-coulombs- ... 907#p40907]
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Post by woody » Mon, 14 Jan 2013, 18:23

Is there a "height of centre of gravity" missing from that argument?
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