Weber and Coulomb's MX-5

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Johny
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Post by Johny » Mon, 14 Jan 2013, 18:41

weber wrote:..... (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 many contactors do you have all up? The DIMEC 9AH AGM I use puts out about 11.7 VDC with a 30A load. Thats 10 of the EV200s. Size 191 x 65 x 95mm (LWH) and weighs 2.55kg. There would have to be some car taken with your 12V accessories though. Seems to me getting rid of the big Lead Acid in the boot would be a good move. Some heavy cable might go too!

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Post by coulomb » Mon, 14 Jan 2013, 19:29

Johny wrote: How many contactors do you have all up?
With the controller pre-charge and all the ELV segmentation contactors, 13 come on at once with the key-switch.
The DIMEC 9AH AGM I use puts out about 11.7 VDC with a 30A load. Thats 10 of the EV200s.
The datasheet says 3.8 A max inrush. So 30 A is really only 8 EV200s; our 13 could peak at just under 50 A, plus all the other things that probably take a surge of current at key-switch on time.

Have you measured the typical inrush current as more like 3.0 A each?

But it might still all work, and we might be able to fit a 17 Ah battery. My concern is whether it would run the statutory 20 minutes of hazard lights (blinkers). I suppose we could replace the hazard lights with LEDs.
Some heavy cable might go too!

Indeed. Though one of our design points is that the DC/DCs are far away from the auxiliary battery, so the long leads will allow the DC/DCs to share the load better.

However, we might never get this MX-5 past 375 V (or 333 V now), i.e. the half-packs would always be paralleled, so balance of the DC/DCs would not then be as important.
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Post by Johny » Mon, 14 Jan 2013, 19:42

coulomb wrote:My concern is whether it would run the statutory 20 minutes of hazard lights (blinkers). I suppose we could replace the hazard lights with LEDs.
At 24W per bulb and assuming ON all the time (so I don't have to take into account lights on the side guards), that's 96W or around 8 A for four lights. The DIAMEC 9AH curves indicate 9AH for 30 minutes down to 10.8 volts.

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Post by coulomb » Mon, 14 Jan 2013, 19:50

Johny wrote: The DIMEC 9AH AGM I use puts out about 11.7 VDC with a 30A load.

I guess that would be this one:

Image

From http://www.jaycar.com.au/productView.asp?ID=SB2487

$39.95 qty 1. They seem to be a lot cheaper in the USA (big surprise there).

[ Edit: the "initial charge current < 2.7 A" seems a bit of a worry, though. ]
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Post by Nevilleh » Mon, 14 Jan 2013, 21:21

I think you should scrap the MX-5 and shift everything into a bigger car! That way you could spend another four years happily puddling around with the project. Obviously the journey is far more important than the destination. Image

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Post by acmotor » Mon, 14 Jan 2013, 21:47

Ouch ! Image

Weather's nice at South Pole.

Geez you have to have a thick skin to be an EV enthusiast !
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Post by weber » Tue, 15 Jan 2013, 04:34

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

Thanks Woody! Duh. That's what I needed to use. Not the wheel radius. But I totally screwed up the numbers anyway. 572 mm is approximately the wheel diameter not radius, and somehow the wheelbase grew from 2266 to 2666 mm when I punched it into the calculator.

As shown here the formula is

weight_transferred = GVM * deceleration_in_gees * CoG_height / wheelbase

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Post by weber » Tue, 15 Jan 2013, 15:56

Jeff Owen points out that weight transfer and tyre rating can't possibly be definitive of GVM, since weight transfer on braking and acceleration is not as severe as when "cornering on two wheels". For cornering, the wheelbase in the formula is replaced by the track.

Track is 1418 mm. Original CoG height is 430 mm. So if we were limited to the 475 kg tyre rating then 1G cornering (friction coefficient of 1) would limit the MX-5 to a GVM of 1182 kg -- so even the original vehicle would fail.

And shock loads from bumpy roads could easily lead to short term loadings of more than 475 kg per tyre even with the original vehicle. So tyre load ratings are apparently closer to continuous ratings, with higher peaks allowed.

BTW, Braking tests are described here http://www.comlaw.gov.au/Details/F2010C00150. Scroll to Annex 3, about 40% of the way through.
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Post by acmotor » Tue, 15 Jan 2013, 16:20

For an EV, do braking tests include the use of regen braking ? Regen alone can lock the drive wheels if fully applied. As will DC injection braking. Extra weight over drive wheels in MX-e will help there !

Or to put it another way, does regen have to be turned off for the braking tests ?
If it is built into the drive system then usually it can't be disabled (except dynamically by ABS,TCS or ESC in the iMiEV)
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Post by weber » Tue, 15 Jan 2013, 20:04

acmotor wrote: For an EV, do braking tests include the use of regen braking ? Regen alone can lock the drive wheels if fully applied. As will DC injection braking. Extra weight over drive wheels in MX-e will help there !

Or to put it another way, does regen have to be turned off for the braking tests ?
If it is built into the drive system then usually it can't be disabled (except dynamically by ABS,TCS or ESC in the iMiEV)

The procedure linked above specifically mentions electric vehicles in many places. But it's pretty complicated with many caveats so I can't really summarise it in a few words. But it seems that basically, if you can't rely on the regen always being there (e.g. if the battery was fully charged, or a regen strength control happened to be turned down to minimum) then you can't use it in the test.

I can now answer coulomb's question. The under-boot battery box weighs approximately 112.5 kg (with its full complement of 60 cells) and it seems that about 15 kg of that is the box. Bare cells are 1.54 kg, and cells with BMS, links, cables, andersons, clamp bars and rods average 1.63 kg each.
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Post by Richo » Tue, 15 Jan 2013, 20:33

From what I have seen GVM = Curb weight + 81.6 x passengers + ~50-60kg
Curb weight being ready to drive with full tank.

You don't need a bigger car just a Handi Image
It's lighter than an MX-5 and takes hundereds of extra kg of batteries!
So the short answer is NO but the long answer is YES.
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Post by weber » Wed, 16 Jan 2013, 04:02

Neville and Richo, you've inspired me! I've got it! Outriggers!

Image
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Post by weber » Wed, 16 Jan 2013, 04:52

Jeff kindly came over today and we removed the lead-acid from the boot, simulated a small 12 V battery under the bonnet, removed 26 cells from the under-boot box, reinstalled it and attempted to reweigh the axles.

We first checked calibration of the 6 bathroom scales by weighing 10 cells on each one. The cells had previously been established to average 1.54 kg each. We got readings of 15, 15.5 and 16 kg. Not too bad.

Then we weighed the front axle and got 618 kg, which was very odd since this was 40 kg less than we measured on saturday! It should have been a little more. So then we measured the rear and noticed that one of the scales was sticking beyond a certain point, and therefore under-reading. So we weighed the two sides separately and got 689 kg which was only 21 kg less than saturday's measurement when we had in fact removed about 50 kg. So clearly these measurements are useless. We'll have to try to borrow a load cell scale as acmotor suggested.
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Post by acmotor » Wed, 16 Jan 2013, 05:11

weber wrote: .... But it seems that basically, if you can't rely on the regen always being there (e.g. if the battery was fully charged, or a regen strength control happened to be turned down to minimum) then you can't use it in the test....


Someone correct me but I don't think any mass produced commercial EVs or hybrids have regen strength control. Let me clarify that... There is usually the option to adjust how much regen is assigned to the accelerator pedal and the balance to the brake pedal but net regen total is the same. iMiEV, Leaf, Volt, Prius that I know of.
Regen on rare full battery seems to be dumped as heat. I'd like more info in that.
If regen was not allowed in EV brake test then you could draw a parallel to ICE that a braking test should be performed without vac boost brakes and power steering, since the ICE may well be stopped and vac reserve used up. Probably more likely than the emotor system stopped !
EV conversions do present a special case and thus the regulatory authority feels a need to test the brakes when GVMs are pushed. Rather a historical test perhaps. Afterall the leather straps might brake or something ! Image
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Post by KDRYAN » Wed, 16 Jan 2013, 11:54

You may already be aware of this, but when weighing the vehicle you need to have a level line marked on the vehicle using a spirit level. This line must remain level when weighing the front and rear axles, with scales at exactly the same position in relation to the axle centre.
I had my ev weighed using a aircraft weight cell set. Both front and rear were weighed at the same time.

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Post by BigMouse » Wed, 16 Jan 2013, 13:45

I agree, the mechanical scales are going to be your biggest problem. Any load applied that's not directly vertical (side load from scales being used side-by-side or even from one tyre to another) could cause binding problems. You could replace your scales with digital ones. They use load cells/strain gauges and are much more repeatable. Try to get a scale with a strain gauge on each foot rather than one that uses a linkage internally to reduce the force on the cell, a cost-cutting measure.

If I recall from another post, one of you works at a university, no? Does your uni have a FSAE team? If so, they might have a proper racecar scale set you may be able to borrow. I know the team at UOW has a very nice kit. Any FSAE team should have one as it's critical to setting up suspension. May be worth checking.

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Post by Richo » Wed, 16 Jan 2013, 20:54

I made a "H" frame that takes 4 scales and fits the wheel in the middle.
The other wheels are chocked up to the same height.
The el-cheapo scales seemed accurate as they all sad I was fat Image
So the short answer is NO but the long answer is YES.
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Post by coulomb » Sat, 19 Jan 2013, 17:12

weber wrote: Dunno why the estimated tare is 65 kg too low. Maybe the estimated mass of the ICE is way too high.

Yes, I think that's where a big part of it came from.

From this thread, they seem to be saying 272-275 lbs, which is 123-126 kg, call it 125 kg. So that was an overestimate by some 35 kg, more than half of the discrepancy. Maybe we originally included the transmission, and forgot to remove the transmission weight when we finally decided against direct drive.

I think a bit more comes from the rest of the weight in a battery box that isn't the cells or the box itself: BMUs, links, foam, a share of battery cables, and so on.

We also didn't seem to include cabling in general, contactor boxes, fuses, etc. Unfortunately, this is one of those errors that only gets caught right at the end of the project, unless I suppose you are considering weight all the time, and weigh the partly completed car after every major installation step.
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Post by weber » Sat, 19 Jan 2013, 18:26

ACMotor,

From my reading of the braking tests they seem to be based on common sense. There are two tests. One with the engine/motor engaged and one without. They have different decelleration/stopping-distance requirements. If it's not possible to disengage the engine/motor (no clutch and no neutral) then you don't have to do that test.

If there is a state-of-charge or a switch or lever setting that reduces the amount of braking in either of those two cases (motor engaged/disengaged) then they would have to be set to their worst case settings for each case, as they might happen to be in an emergency.

They don't require you to simulate equipment failures, only worst case settings or conditions that can occur in normal operation, hence I don't see the correspondence with vac reservoir depletion following engine failure while travelling at high speed.

If the regen braking can be guaranteed to always be there in normal operation then it can be there for the test.

I haven't heard of an electric car with resistive braking but I have heard of warning lights that say full regen is not available, and EVs that discourage 100% SoC.

The test will be conducted on a level track and so will consume more energy in getting to the required speed than will be recovered during the subsequent braking, so the state-of-charge thing shouldn't be a major problem.

As I say, it's complicated, and I'm no expert, so please read the test procedure yourself and/or consult an expert, e.g. your gov't approved engineer.
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Post by acmotor » Sat, 19 Jan 2013, 19:53

....Or stay within the OEM GVM as a lot more expertise was put into that by vehicle designers / manufacturers in the first place. Image
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Post by weber » Sat, 19 Jan 2013, 21:06

acmotor wrote: ....Or stay within the OEM GVM as a lot more expertise was put into that by vehicle designers / manufacturers in the first place. Image

And how many kilograms would that be [Edit: in the case of the NA series MX-5? And what is your source for that figure?]
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Post by weber » Sat, 19 Jan 2013, 22:48

We finally heard from our engineer. It turns out he had a "mishap" last weekend and ended up in hospital for most of last week. He hopes to be back to work on Monday.

So far we've had no luck finding suitable scales to beg borrow or steal, however it was suggested at the Brisbane monthly AEVA meeting on Wednesday that it might be good for the association to own such scales and loan or rent them out. So we'd love to have suggestions of what exactly we should buy.

Coulomb wasn't up to an EV day on Friday, but Jeff Owen brought his laser level and we levelled up all four wheel-platforms to within about 3 mm.

I found out what was wrong with the sticking bathroom scale and repaired it. It had a rivet on the inside of the top that had never been flattened, which would scrape against the rotating circle with the numbers on it, when you went over a certain weight (about 80 or 90 kg). So all earlier measurements should be considered invalid.

We only got to re-weigh the rear axle and the result was 700 kg +-2%. That was with only 34 cells in the under-boot box (26 removed). That implies that when we weighed it as 710 kg last Saturday it was really more like 740 kg!

We did not get to weigh the front, but think it is probably fine. Using the laser level I measured the road clearance of the chassis rails (the ones that run under the cabin) as 75 to 80 mm at the rear and 85 to 90 mm at the front. The lower number in each case is on the drivers side -- presumed due to greater spring sag on that side due to a history of being driven with only the driver. But the main thing to take away from that is that the rear is 10 mm lower than the front so the front probably has a lot less weight on it. Standard MX-5 road clearance is 115 mm and minimum legal in Queensland is 100 mm fully loaded.

So it looks like we still need to remove a lot more weight from the rear. In fact it looks like we should just remove the under-boot box completely (so removing all 60 cells and their 15 kg box).

We found we could put 12 more cells at the front, in two boxes of 6, in the mudguards, just in front of, and just inboard of, the front wheels. These could be wired in series with the frontmost box of 13 as the combination would still be ELV. But this would be something of an act of desperation. We will wait until we get to discuss the situation with our engineer.
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Post by acmotor » Sun, 20 Jan 2013, 01:42

Perhaps check with Police Licensing or Mazda Australia for a GVM. One or the other MUST have it. I don't think any vehicle would get to be on Oz roads without Police Licensing having a GVM on their files. An approving engineer would need to find the number.

It would be nice to know before taking gas axe to vehicle though I think your feeling that weight is high on the rear is correct. Image
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Post by BigMouse » Sun, 20 Jan 2013, 03:01

With all the cells you're pulling from your car, I wonder if it might be worth making a "long-range" battery trailer. You could use it for DC fast charging at home (with appropriate charger), or hook it up for when you need to double your range.

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Post by coulomb » Sun, 20 Jan 2013, 03:38

BigMouse wrote: I wonder if it might be worth making a "long-range" battery trailer.

That's an interesting idea.

But I think if it's safe to drive the MX-5 with the weight of the trailer, assuming it doesn't have separate brakes, then it's safe to drive it with them in the car all the time.

Plus of course it's nice to be able to uncouple the weight of the extra cells when they're not needed, so you get better power to weight ratio for those trips where you don't need the longer range.
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