Weber and Coulomb's MX-5

Post up a thread for your EV. Progress pics, description and assorted alliteration
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Post by weber » Wed, 30 Mar 2011, 19:58

antiscab wrote:How long are those bolts supplied with the SE cells?

Hi Matt,
If you mean the terminal bolts, they are M6 x 12 mm. If you mean the threaded rod (M5), most are 390 mm (8 cells), four are 665 mm (14 cells), four are 205 mm (4 cells). Approximately n*46 + 20. We just had to specify what block sizes (in cells) our total cell order was to be broken into. We didn't really know at the time, so now we're cutting and welding them to make other lengths.

BTW Matt, could you please take a look at the problem described here
viewtopic.php?t=2553
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Post by coulomb » Thu, 31 Mar 2011, 04:24

antiscab wrote: How long are those bolts supplied with the SE cells

The threaded rods come in various sizes; you might be able to make out three sizes in the photo.

At the time we ordered our first and largest batch of cells, through EV Components before it shut down, we were asked to supply the number of cells in each string, i.e. we got to choose in advance the length of the rods.

At the time, we weren't sure, so we guessed some, and asked for the vast majority to be right for 8 cells. Eight seemed like a useful number, and at the time we really thought we'd be throwing away the rods and using steel strapping or similar. Fortunately, we made an effort to guess some of the lengths.

So for our strings of 23 cells, we've welded 3 rods designed for 8 cells, and cut off about 100 mm from the result. I doubt that they would have supplied threaded rod correct for 23 cells anyway; it would have been a nightmare to transport.

We'll probably have to buy some more rod at some point, although the technique of sharing rods between columns may save us from that. It's all M5 stainless steel, I believe.

I can't help thinking that the copper links and stainless steel rods, nuts, and bolts must cost an appreciable fraction of smaller cells like our 40 Ah cells. Charging the same amount per Ah no doubt simplifies things, but I suspect that purchasers of 200 Ah cells must be subsidising buyers of 20 Ah cells.
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Post by weber » Wed, 06 Apr 2011, 06:29

Here's how we plan to run the return cable inside the battery box in those cases where we have a single row of cells in a box (or in an ELV segment within a box).

This method lets us see the LEDs on the BMUs and lets us clamp the cells down with lengths of black polyethylene between the lid and the shoulders of the cells (shown on one side only). Thanks to mcudogs for the polyethylene.

The cable is 16 mm^2 105°C 0.6/1 kV from EVWorks and will run back to an Anderson connector on the outside of the box.

Image
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Post by weber » Thu, 07 Apr 2011, 01:48

Electric heater/demister and power steering


We have a problem with how to do a heater demister, because of our high pack voltage. We have two half-packs of around 380 volts (416 V on full charge) which will initially be in parallel, but hopefully later in series giving 760 volts (830 V on full charge).

The two basic paths are wet vs dry. i.e. Either keep the existing wet heater core and pump electrically heated water through it or replace it with an electrical heating element that heats the air directly.

Initially the wet path seemed attractive because of the notorious problem of dashboard removal (with inevitable breakage) which is admirably documented here by Zeva.
http://www.zeva.com.au/conversion_blog. ... e=1&post=5

EVWorks have a pump/heater solution for $245, but it's designed for 240 V.
http://www.evworks.com.au/index.php?pro ... -TH240V2KW
And if a higher voltage element was made, or multiple elements connected in series, it would still be a bit scary having 760 V anywhere near water.

Today I had a phone call from Newton (Jeff Owen) telling me that it was possible to remove the wet heater core from the MX-5 without removing the dash. These guys describe how to do it, for a left-hand drive vehicle.
http://forum.miata.net/vb/showthread.php?t=257800
It turns out to be even easier for right-hand drive MX-5s since there is no need to cut any copper pipe. Both pipes already have rubber connections.

So the wet core is out of the vehicle now, and it would be relatively easy to make a dry core "cartridge" that would slot back into the same space.

Newton's suggestion was to buy some cheap ($30) low-powered 240 Vac PTC ceramic space heaters, like these
http://www.myshopping.com.au/PT--341_He ... er&Sort=PS
remove their elements and connect the elements in series. He suggested it was even possible that those with a half power setting might have centre-tapped elements and thereby be usable with the two parts in series at 480 V by ignoring the centre-tap. So two centre-tapped elements could be used, one across each half-pack.

It turns out that this is the only way it can work. If you connected separate elements in series, one of them would "turn off" first and get the full voltage across itself. It is only when they are intimately thermally coupled that they can be connected in series and have any semblance of voltage sharing. This is explained more here.
http://www.specsensors.com/ptc-engineering.asp
I've ordered one to experiment with.

Tritium James, how hard would it be for you to add a CAN bus command to the WaveSculptor controller to tell it how much power you want it to waste as heat, or what minimum temperature you want the heatsink to be at? Then we could just pipe the controller's coolant through the wet core inside the car before it goes to the radiator. I understand there are dissimilar-metals issues, but I'd be willing to replace the core inside the car with an all-aluminium job if necessary.

Newton also mentioned the possibility of an electric power-steering pump from a Holden Astra. About $1000 new, maybe $250 from a wrecker's. Picture here.
http://www.ultimatepowersteering.com.au ... den-astra/

EVWorks and Canadian Electric Vehicles have solutions for this too. $450 from EVWorks, no price on the Canadian.
http://www.evworks.com.au/index.php?category=9
http://www.canev.com/KitsComp/Component ... ngKit.html
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Post by weber » Thu, 07 Apr 2011, 03:52

Newton also pointed me to this video of a Holden Astra electric power-steering pump running in one of the CERES Berlingo conversions. Yikes! What a noise.



BTW, Newton says they are in the '98 TS model Astra, and maybe the AH.
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Post by coulomb » Thu, 07 Apr 2011, 06:00

weber wrote: Yikes! What a noise.

Hopefully it's not that noisy from the cabin. Also, maybe it is only as loud as than when moving very slowly, or as in the video, actually stopped. It might prevent surprising people in the car park Image
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Post by Johny » Thu, 07 Apr 2011, 06:17

weber wrote:Newton's suggestion was to buy some cheap ($30) low-powered 240 Vac PTC ceramic space heaters.......
That's kind of what I did - except a cheap fan heater. If you check my blog and use the labels to the right about Heater you'll see some details.

Essentially I found that there were 4 elements in the 1500W heater I used. Two of about 500W and 2 of about 250W. I wired one 500 and 250 in parallel and the other 500 and 250 in parallel then that set of 2 in series. From 600V they still drew too much - upwards of 5 Amps (and climbing without air-flow - what happened to self limiting that I read about???).

So I built a PWM controller using a TL492 PWM controller and a 50 Amp IGBT (small TO92 case). That gives me continuous control and it works fine. My test run with the Vogue blower and current at around 4 amps (2400W) appeared to hold the elements at a reasonable temperature - I haven't done final installed test because it's not in yet (all talk, no action - well not quite). I have the IGBT on it's little heatsink and controller board inside the heater box upstream of the core so it gets force cooled.

I should blog the controller but I've been waiting to finish the whole heater bit so I can do the pictures which in turn depends on getting the firewall sound proofing in etc.....

I hope this helps.

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Post by Tritium_James » Thu, 07 Apr 2011, 15:29

Although using the motor controller waste heat for cooling is a pretty elegant concept (and we've been considering it in the past!) it's not really something we're interested in doing. Basically the hotter that waterblock is, the shorter period of time you can put out high currents for before it has to start backing itself off. So really the best thing from the controller's point of view is to keep as cool as possible.

My thought on a neat concept for cabin heating is to tie it in to high-powered bypass resistors on the BMS. This gives you a nice high bypass current for the BMS, and also lots of cabin heating when you command all the resistors to turn on. But it does complicate the battery pack design somewhat when you have plumbing through it...

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Post by weber » Thu, 07 Apr 2011, 15:45

Thanks Johny, that is very helpful. And TJ, if you can figure out how to use that BMS plumbing for cheap comms as well ...
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Post by Johny » Thu, 07 Apr 2011, 15:51

weber wrote:....And TJ, if you can figure out how to use that BMS plumbing for cheap comms as well ...
Ha ha - permaculture in the BMS connection system. I was going to suggest just blowing air over the battery packs into the cabin (noxious gases notwithstanding) but your MX5 has so many cells in the cabin already it'll be a sauna as it is. Image

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Post by weber » Thu, 07 Apr 2011, 17:13

Johny wrote:Essentially I found that there were 4 elements in the 1500W heater I used. Two of about 500W and 2 of about 250W. I wired one 500 and 250 in parallel and the other 500 and 250 in parallel then that set of 2 in series. From 600V they still drew too much - upwards of 5 Amps (and climbing without air-flow - what happened to self limiting that I read about???).

They are very non-linear devices, as you can read in one of the links I posted above. They are a doped semiconducting ceramic. I expect the problem was you were giving them too much voltage.

Why didn't you parallel the two 250 W sections, then connect this composite 500 W section in series with the other two 500 W sections, thereby averaging 200 V per section instead of 300 V per section? You may have been able to do without the PWM controller in that case.

Can you tell us how they were connected originally. It looks like the 4 sections are all connected in series, with terminals at the 2 ends and the 3 junctions. Is that correct? If so, does it go
-500-250-250-500- or -250-500-500-250- or something else?

Can each section take 240 V or were they always in series pairs in normal operation?
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Post by PlanB » Thu, 07 Apr 2011, 17:36

Just curious for peoples thoughts on reverse cycle air con? Given that you need air con in Oz for cooling couldn't you just swap the role of condenser & evaporator for heating?

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Post by Johny » Thu, 07 Apr 2011, 17:51

weber wrote:Why didn't you parallel the two 250 W sections, then connect this composite 500 W section in series with the other two 500 W sections, thereby averaging 200 V per section instead of 300 V per section? You may have been able to do without the PWM controller in that case.
Because the 500W, 250W were informed guesses. It's REALLY hard to measure what power they really are. As you said they are so non-linear that a change from 250 to 300V totally changed the characteristics. I went for the most stable way I could connect them. The PWM was my fastest way to do it - plus after reading about EV and inadequate heaters I wanted to be able to get LOTS of power if I needed it. A linear control also lent itself well to the Vogue controls.
weber wrote:Can you tell us how they were connected originally. It looks like the 4 sections are all connected in series, with terminals at the 2 ends and the 3 junctions. Is that correct? If so, does it go
-500-250-250-500- or -250-500-500-250- or something else?
Image
The fins with white stuff that has gaps are the 250W. The fins with white stuff with no gaps I call the 500W. The way it came originally was each of the 500W elements had a 250W in parallel.
Note the Blue wires connecting the outsides (500W elements) together and connecting the centre of the 250W elements thereby paralleling them with the 500s (that's Neutral). Then they feed Active into one of the brown wires for LOW and both of them for HIGH.

I hacksawed the whole thing down the middle and connected the two sides end to end - bringing out a connection to the centre so I could test it on 240 VAC. You can't see my connection in the centre (vertically and horizontally) on the Blog as it's buried under the RTV Silicone.

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Post by Johny » Thu, 07 Apr 2011, 17:54

PlanB wrote: Just curious for peoples thoughts on reverse cycle air con? Given that you need air con in Oz for cooling couldn't you just swap the role of condenser & evaporator for heating?
Yes. EVWorks even sell the parts to do it. It's been talked about a lot on this forum if you do a search.

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Post by coulomb » Sat, 09 Apr 2011, 06:05

coulomb wrote: We hope to fix this [BMU failure damaging a cell] by inserting a diode or two (using a dual diode, with one diode shorted for now) in series with the gate of the bypass transistor. ... The idea is to make the BMU "over-discharge fail-safe".

We tested the almost fail-safe bypass circuit today:

Image

It seemed to work well, at least for this particular combination of components. At a "cell" (power supply) voltage of 3.1 V, the MOSFET transistor turned on pretty hard, dropping only about 60 mV.

As the cell voltage falls to 2.2 V, the bypass current cuts back to 0.3 A; at 2.1 down to 0.2 A, and at around 1.9 V or so, the current is essentially zero. So a cell might get heavily discharged if the processor goes crazy, but it should not get badly damaged.

The highest power dissipation for MOSFET Q1 is about the 2.2 V and 0.3 A point, where the power is less than 250 mW. It didn't seem to get excessively hot at this point; our non-contact IR thermometer couldn't even pick up the tiny MOSFET against the general warmth (38C) from the bypass resistors.

With the cell voltage at 3.1 V, and the processor power supply regulated to 2.50 V, the processor output pin drooped a little to 2.45 V, and the voltage at the gate of Q1 dropped to some 1.27 V. So the diodes were dropping about 0.59 V each. We needed to drop R15 to a lowish one kilo-ohm so that the diodes would drop as much as that (at very low currents, they can drop as little as 0.4 V). The BAV99 is a dual diode device in an SOT-23 package (same as Q1), so the two diodes don't take up a great deal of printed circuit real estate.

[ Edit: changed circuit to show Si2312 (was Si2302, with twice the on resistance); transistor -> MOSFET ]
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Post by weber » Wed, 13 Apr 2011, 04:11

Thanks to Johny's PTC ceramic heater info, I bought a different cheap heater that has two separate half-width elements, which avoids the need to hacksaw the element in half. And each element has two equal-powered parts. Each part is 500 W at 240 Vac. So the plan is to use each element with its two parts in series, and connect one element across each of the two 380 Vdc half-packs with appropriate DC-rated fuses and contactors. Whether the two half-packs of the battery are wired in parallel or in series will make no difference to this heater arrangement.

The intact heater.

Image

Opened up to show the elements.

Image

Showing size in relation to the original wet core.

Image

[Edit: Jump to this post to see what we did with it.]
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Post by coulomb » Sat, 16 Apr 2011, 20:36

High Tensions

In email, NevilleH mentioned something about Bill Dube (the Killacycle guy) and 1 mm per volt clearance. A little research (thanks Weber) turned up this thread:

SRJC student's Focus-conversion-EV burned to its metal frame

There is an overview of the vehicle here, but (as yet) no information about the fire.

The actual reasons for this fire aren't clear yet. It will be interesting to read what caused it, but in the mean time, it sparked some other discussions, like these, where it was recommended that there be 1mm separation between conductors for every volt of potential difference between them:

(single posts now)
http://www.diyelectriccar.com/forums/sh ... hp?t=56648
http://www.diyelectriccar.com/forums/sh ... hp?t=56743
http://www.diyelectriccar.com/forums/sh ... hp?t=56758
http://www.diyelectriccar.com/forums/sh ... hp?t=56759

This next one introduces the scary concept of the "plasma daughters" and how "plasma boy" got his name:

http://www.diyelectriccar.com/forums/sh ... hp?t=56775

I actually read the full Plasma Boy article; it's long but interesting (not all of it is relevant to the plasma subject, however. If you do skim it, make sure you don't skip the bit about the marshmallows; hilarious).

Ok, well, where does that leave us?

First of all, although they mention these plasmas being fed by metal vapour, I'm not at all certain that there would be enough to sustain a plasma for minutes. So surely the safe limit would be closer to the sparc in air limit, which is about 30 kV per centimetre (3000 V per millimetre). That's a huge difference, and I'm nowhere near confident enough about the physics to say all those experienced guys must be wrong.

So for now, let's just consider 1 mm per volt and see where it goes in a high voltage conversion like the MX-5. We have 228 cells in series, for a total of some 228 * 3.6 = just over 820 V across the whole pack (if and when we get to series half-pack (Low Voltage, LV) mode; maybe we'll stay with parallel half-packs at "only" 410 V). That requires 820 mm (410 mm for the LV mode) separation at worst case.

We have a creepage distance of 5 mm on our BMS boards; with this figure it's good for 5 V (TTL signals)!! We do have notches in the PCB under the opto isolators, but in some cases, these notches are required to keep the creepage distance as large as 5 mm! (In other words, the air gap is treated as a perfect insulator. But if a plasma starts, air is way worse than most insulators. This creepage distance is mainly to avoid dirt and moisture tracks that can become conductive).

Suddenly, our whole BMS design is looking unsafe. Granted, it would be rare that the full 820 V would appear across a 5 mm gap, but it's almost certain to happen at one point or another, if the pack ever develops a leakage to chassis. So we can't use arguments like that; worst case has to be totally safe. The last thing we want to do is to set back high voltage AC conversions by a decade because we didn't do our research correctly. Plus, we'd rather have the MX-5 to drive than to have it used as a bad example for future conversions.

So what can we possibly do about it? Resourceful as ever, Weber came up with the following "industrial fibre" solution:
Weber email wrote: Transmitter is an IR LED with lens and coupling
http://i-fiberoptics.com/pdf/IFE91A.pdf
Footprint 25x9 mm. Height12 mm.
$3.82 from Digikey

Receiver is a photo transistor with lens and coupling
http://i-fiberoptics.com/pdf/IFD92.pdf
Footprint 25x9 mm. Height12 mm.
$4.01 from Digikey

The "cable" is $19 for a 10 m roll. It's cheaper than the TOSLink audio stuff and is "connectorless". Just cut the cable with a sharp knife at right angles, poke it into the transmitter or receiver on the PCB and tighten the knurled knob.
Image

Believe it or not, Weber had two lengths of this cable in hand; part of a tubular guitar project from another life. It's actually plastic, and can be bent to a minimum radius of 25 mm. Not totally convenient, but certainly doable.

It means that we can eliminate any proximity of high voltage components with chassis, on the BMU boards. (The current design uses 12 V and ground from the auxiliary battery to power the end-of-row circuitry. It happens to dovetail with our existing experiments (thanks, Neville!) with infra red optical connection between BMUs. We've shelved that idea for the time being, as it seems to be too sensitive to sunlight; convertibles are liable to see a lot of sunlight at times!

We will likely have to drive the transmitter end with significant current, either 20, 50, or even 100 mA. This does increase current consumption, but the current will be on for only a quite low duty cycle, except when performing a download, and these are infrequent (and never happen on the road). So we may consider evening out the current load, and drive the optos (which we will still use for the 95% of cases where cells are next to each other) with more than the present ~ 2 mA, so we can use cheaper optos. In fact, they can also be smaller optos, since they will be isolating only 3.6 V at most, so there is no need for a large package with a PCB notch under it. [ Edit: we just realised that if a cell goes open circuit, there could be pack voltage across the optos, so actually the long format optos and the slot will remain. But we can still go to cheaper optical isolators. ] Any time that communications have to leave the immediate vicinity of the BMU, we will use the fibre connection.

Another place where we have to consider clearances in battery boxes where a series string of cells turns around (does a U-turn, if you like). One box for example has two strings of 23 cells (and another string). The string of 23 enough that we will have a contactor separating the two strings, so if a plasma event starts for some reason, at least those two strings will be separated by the contactors dropping out. We believe that the EV200 contactors can interrupt the full pack voltage with full short circuit current (if only a few times in their lives; probably we'd replace them after any such event).

Other boxes however have the limit of 28 cells arranged in two strings of 14 which turn around at one end. The initial plan was to simply cable to two strings of 14 cells. But that means that we end up with 28 * 3.6 = 101 V maximum, separated by less than 100 mm. So we considered wiring the strings with a "long link" the length of the string, so that if you dropped a spanner across the gap between strings, you'd see a constant 50 V plasma event (dangerous enough as it is), as opposed to a 0 - 100 V plasma, depending on where the spanner was dropped. The average case is the same (50 V), but the worst case is better (50 V verses 101 V). But then we have an extra cable in the battery cage, running past all the other terminals on the way. If a plasma event were to start, surely one could not rely on the (double) insulation of the cable to last, so that introduces many more places for the plasma to go, all of them spawning daughter plasmas... shiver.

At one point, we considered putting Andersons at the end(s) of the cable, so that the unfused plasma can be broken [ edit: or prevented, when working inside the cage] . But that doesn't help when the plasma short circuit burns off the cable insulation. We considered yet another insulation barrier over the terminal posts. 25 mm cable conduit (the type with rectangular cross section and a snap-on lid being one side of the rectangle), and it looked promising for a while. However, that didn't leave enough space for the return cable. We also tried sawing round 20 mm conduit in half, making two semicircular sections. These fitted pretty nicely over the terminals, and we may end up doing this if we can think of a quick way of cutting the conduit without a really wide cut (or there would not be enough material left).

In the end, we decided against the "long link", and instead decided it would be best to put "half anderson" connectors (single pole; they don't seem to be available in finger safe versions) at the turn arounds. So this is the "medium link" option Image .

[ Edit: The main point of the andersons at the turn arounds is that the battery can be made spanner-drop proof, or much more so, by disconnecting the half-way link before working inside the box. In the event of a plasma, I think it would take too long to get to the connector; better to yank at or chop the cable. ]

Image

This is where a lot of our time goes, instead of actually building the car: figuring out how to make it safe. I guess that's one of the costs of being (almost) pioneers in this area (nods to acmotor and a4x4kiwi, and thanks guys for blazing the trail).

We may well have many other places where we'll have to consider how we could deal with a possible plasma event.

[ Edit: It's occurred to me that this 1 mm per volt thing only applies where it is impractical to fuse the conductors. So with many contactors in series, and a pack fuse, we don't need to worry about clearance inside the controller, for example. Similarly, if a cell opens, the high voltage that appears across a tiny printed circuit board is protected by contactors and the fuses, so a "plama boy" style event should never occur, or at least should blow itself out in a second or so, and not cause a significant fire hazard. ]
Last edited by coulomb on Sat, 16 Apr 2011, 13:59, edited 1 time in total.
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Post by Tritium_James » Sat, 16 Apr 2011, 21:35

Completely over the top, guys.

Per the IPC-2221 and IPC-2222 PCB layout guidelines, electrical clearance distances should exceed the following:
- Internal conductors (inside multi-layer PCBs): 0.25mm for the first 500V, plus 0.0025mm/V above this
- External conductors, conformal/soldermask coated: 0.8mm for the first 500V, plus 0.00305mm/V above this
- External conductors, uncoated, up to 10000' altitude: 2.5mm for the first 500V, plus 0.005mm/V above this
- Component lead/termination, uncoated: 1.5mm for the first 500V, plus 0.00305mm/V above this

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Post by PlanB » Sat, 16 Apr 2011, 21:38

Maybe that's why Dale Vince's 'Nemesis' has the big red fire extinguisher button for the battery compartment?
Amazing stuff Plasma. When the arc strikes it heats the air which tends to make it rise. If the voltage is sufficient to maintain a current this gives you a longer arc that can digest more low temperature matter & turn it into plasma too.jacob's ladder

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Post by coulomb » Sat, 16 Apr 2011, 21:53

Tritium_James wrote: Completely over the top, guys. ...

Agreed, TJ, and thanks for the details, for circuits that are fused and/or can't sustain a plasma in air because they can't sustain massive currents.

But I believe that unfused batteries, capable of 1000 A or in many cases much more, are a different situation, which demands much greater separation.

In Plasma Boy's case, the plasma was sustained for many minutes, long enough for the fire service to arrive and chop out the plasma with an industrial mask, breathing gear, and a fire axe. These were 16 Ah (!) lead acid batteries.

I'm sure he would have had much more than the 2.5 mm (for up to 500 V) separation for "external conductors, uncoated". (Ok, that might apply only to PCBs, but still.)

BTW, 0.8 mm seems way too close for 500 V, even for conformal/soldermask coated PCB tracks. [ Edit: Oh, is that the thickness of fibreglass separating tracks, or tracks where air would contact (if it wasn't for the solder mask and/or conformal coating)? ]
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Post by Tritium_James » Sat, 16 Apr 2011, 23:04

That is the distance between tracks, on the same layer. Don't forget your 3kV/mm figure for air. The only reason PCB is worse is because of surface contamination, which is the reason conformal/soldermask covered tracks are rated higher than bare ones.

If you're looking at mains rated equipment, and the large track spacings usually evident there, that's because they're usually cheap nasty PCBs with no soldermask or conformal, and consequently require the large space. But for a decent product, 0.8mm for 500V will be perfectly OK. Having said that, the spacing on our stuff is a fair bit more than this!

Clearance distance through the PCB has even less requirements. FR4 is rated for almost 12kV/mm, even after aging.

John Wayland's story, and the posts on the list you mentioned previously, all have one thing in common, and it's not small clearances: something/someone screwed up and started the arc in the first place.

I think you are thinking about this stuff the wrong way. You should be considering what you have to do to not start arcs, not avoiding having your car burn down once they do. If you've got an arc in your pack, you're pretty much screwed anyway. Put your time and effort into making sure they never happen in the first place: crimps/bolts don't come loose, wires don't rub through, you don't have to work in the pack with metal tools every 2nd weekend, etc, etc.

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Post by weber » Tue, 19 Apr 2011, 05:47

I think someone mentioned 32 Vdc above. The lower limit of "Hazardous Voltage" has been raised to 60 Vdc in the latest NCOP14. See
http://www.infrastructure.gov.au/roads/ ... an2011.pdf
Tritium_James wrote:John Wayland's story, and the posts on the list you mentioned previously, all have one thing in common, and it's not small clearances: something/someone screwed up and started the arc in the first place.

I think you are thinking about this stuff the wrong way. You should be considering what you have to do to not start arcs, not avoiding having your car burn down once they do. If you've got an arc in your pack, you're pretty much screwed anyway. Put your time and effort into making sure they never happen in the first place: crimps/bolts don't come loose, wires don't rub through, you don't have to work in the pack with metal tools every 2nd weekend, etc, etc.

We have been thinking about not starting arcs, right from the beginning of this project. But the fact is humans do have to work inside the battery boxes from time to time, and humans do make mistakes. John Wayland is just a typical human. I regularly teach electricians to design and install grid-connected PV system with DC voltages up to 600 V and in every class of 12 there are one or two with stories of a dropped tool or some other mistake that causes a major arc. And this is only with AC, which goes through zero 100 times a second and so is much easier to extinguish, and it always has fuses or circuit breakers somewhere*, so the consequence is usually limited to a loud bang and a bright flash and skin burns and eye damage if no protection is being worn.

* Or nearly always. Check out this explosion at a substation in Florida. http://video.google.com/videoplay?docid ... 7702093783#

The consequences of the same tool-drop accident when working on unfused DC above about 50 V, as found inside traction battery boxes, can be a long lasting arc, or ball of plasma as described by Wayland. Wayland makes it sound like it is metal vapour that continues to make the conductive path, but we know air itself works just fine once you get it started.

Image

http://en.wikipedia.org/wiki/Electric_arc

Sure you have to go down to a tiny fraction of a millimeter to start an arc with 400 Vdc (3kV/mm). But once it gets started accidentally and is pulling more than about 90 A, you really do have to widen the gap to about 400 mm (1 V/mm) before the arc will stop.

There are some fascinating graphs and formulae in this presentation by Professor P K Sen that confirm this.
DOE_Elec_Safety_Workshop_2008/docs/12-P_K_Sen_DC_Arc_Presentation_DOE_EFCOG_2008_Workshop.pdf
Note particularly pages 11 thru 14.
The full paper is here, but costs $30.
http://ieeexplore.ieee.org/articleSale/ ... er=5297174

There are some well written explanations in this article by Matthew Krolak. http://www.myelectricengine.com/info/pl ... ysics.html
But please note that he wants to make stable arcs because he's designing electric rocket engines.

So if we can possibly design the inside of a battery box so that there are no easily bridgeable gaps with more than 1 V/mm (unfused) across them, then surely we should do so.

I figure the half-Anderson connector shown above at the turnaround betweeen rows, really needs to be replaced by a fuse. And my recent realisation, and correct me if you think I'm wrong, is that this fuse doesn't have to have the full 1000 Vdc rating of the whole pack, only the voltage of the battery box it is on, provided that it has a higher trip current than the actual 1000 Vdc fuse which appears elsewhere (near the middle of the pack). This is good because fuse price seems to go up exponentially with DC voltage. The 1000 Vdc fuse being around $130.

And now that we have found optic fibre connectors at less than $15 per battery box, using non-conducting fibre to get signals in and out of battery boxes just seems like The Right Thing (TM). And there's a bonus of much greater noise immunity.

[Edit: "california" -> "Florida", and some spelling]
[Edit2: "more than about 20 A" -> "more than about 90 A"]
[Edit3: Fixed P K Sen link]
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Post by Tritium_James » Tue, 19 Apr 2011, 16:19

I don't think your extra fuses will help much, especially if you have already designed the pack with the high clearance distances. This is because an arc, and even more so for a long arc, has a reasonably high resistance, probably on the order of a few ohms. So across your 450V pack, the arc is dissipating 10's of kW, which explains the melting things and general destruction. But note that this is less power than your drivetrain will draw during normal operation - a fuse won't help put the arc out, it may stop it starting, but only if the fuse blows before the arc gets started.

This is where my emphasis on not starting one in the first place comes from. With DC, basically the only way that arc is going out is if it gets longer than the voltage can support. While your high pack clearance help with this, something (eg a spanner) had to start the arc in the first place, and that has obviously managed to get two HV points close together. So until the spanner melts away completely, those points are still close together, and the arc keeps going. The fuse won't help because the arc is drawing less current than your motor does normally. Basically you're screwed. So design your pack so that you CAN'T drop that spanner in the first place.

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Post by weber » Wed, 04 May 2011, 22:00

Here's what happens at switch-on when you try to run two 12 V 55 W headlamps in series from 26 V. One gets more voltage than it bargained for. The same one every time, for about the first 0.8 second.

Image
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Post by woody » Wed, 04 May 2011, 22:54

weber wrote: Here's what happens at switch-on when you try to run two 12 V 55 W headlamps in series from 26 V. One gets more voltage than it bargained for. The same one every time, for about the first 0.8 second.
Temperature variable resistance?
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