Weber and Coulomb's MX-5

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Post by coulomb » Wed, 26 May 2010, 15:09

Lots of goodies arrived recently: the cooling gear, the charger, the BMU boards (for 34 BMUs), and parts for these. Those parts that we ordered from Farnell arrived in about 5 packages, all in the same delivery. I think they could save some postage there somehow.

Image     Image

The radiator can take 4 120 mm fans. [Edit: was 150 mm ?] There is a smaller model [edit: with room for 3 fans], but not much cheaper. It's so big, we can probably get away with no fans at all. We have space for a wide radiator in front of the air conditioning radiator, so that's no problem.

Image     Image

We chose the combined reservoir and pump arrangement, which makes for one less thing to mount, and one less connection to make. We had to remove the top from the pump, and replace it with a different top (and uses different screws, provided), and that top adapts to the reservoir. We decided that we had to test it out with clear water. Alas, a 12 V @ 1A plug pack didn't seem to want to start it. When we looked up the specs, the pump needs 20 W, no wonder it didn't start. However, a larger plug pack (65 W) also failed to start it. This is getting silly. We checked the web for connections; it seems all we needed was ground on the black wire and +12 V on the yellow wire. The blue wire seems to be for the tacho feedback, even though on PC fans, blue is supposed to mean PWM input.

Ok, we have a few lithium cells with some energy in them; Weber connected 4 of them and at the same time tested the spacing of the new BMU boards. The first shock was that the links were about 5% too long. It appears that all the links that came with the original shipment of cells are like that. We'll likely have to re-kink them or drill new holes. Sigh. Fortunately, the links for the last 16 cells (which we got from EV Works) are the right size. We figured that 4 cells would provide 13.2 V, which is at the upper end of the voltage that the pump should work on.

Image

No pumpa da wata.   Image

In desperation, we pulled the pump apart. Sure enough, the PCB tracks to black and yellow are thick; the track to blue is thin. Surely black would not be +12 V. We decided to risk running the motor with no load off half the pack. No go. Well, that's below the minimum voltage; we tried 3 cells. Still no go.

But wait! With three cells, after a 4 second delay, the rotor did spin! Yay! Back to 4 cells: no spin. When we put the pump back together, and put in some water, it pumped on 3 cells but not on 4.

When I got the pump home, I put it on my small power supply, and found that it would start on voltages below 12.98 (I didn't check how low it would go), and it would definitely not start on 13.0 V. I guess with PC mother boards, 13.0 is considered out of spec. Yet the pump specifications state that it will run on up to 13.2 V. In fact, it happily runs to over 14 V once started, but won't start above 12.98 V (according to my cheap Dick Smith meter).

Since this is awkward for running in a vehicle, I set the power supply to 13.8 V (the highest we're likely to see) and tried starting with one diode from a power bridge. No go. But with two diodes, it started every time. The quiescent current must be enough to cause a little over 0.8 V to be dropped across the two diodes, which is enough to allow it to start. Sheesh. But that's reasonably practical. We'll have to keep an eye on it though; the last thing we need is for the cooling to not start and our controller to overheat. Well, I'm sure it won't die, just cripple itself with low output.

Edit: added photo of cells and boards.
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Post by coulomb » Wed, 26 May 2010, 15:23

The charger also arrived recently:

Image     Image


Image     Image

While it came with an Australian-looking plug, it has wide blades (like the earth blade of a 15 A plug) on all 3 blades. So maybe it's a standard Australian 20 A plug, which are of course almost never seen. [Edit: no it's not; see Questions on Elcon Chargers]. Fortunately, it's a standard IEC cord, so we can just use a beefy computer cord instead. Or just chop off the plug and wire it into the car with a waterproof male socket.

The supplier did mention something about the cord; I guess I should have paid more attention and specified a 10 A plug. [Edit: we did specify that we wanted to connect it to an Australian 240 V 10 A outlet.] No biggie.

We won't have cells to properly test the charger for some time. We intend to attempt testing with a pair of 240 V 500 W lamps in series, paying particular attention to insulation. However, battery boxes were the higher priority. I welded up a pair of gusseted brackets, and Weber and I found a solution for the third mounting point.
Last edited by coulomb on Sun, 02 Sep 2012, 10:44, edited 1 time in total.
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Post by coulomb » Wed, 26 May 2010, 15:41

The final set of things that arrived recently was the BMU boards, one panel worth (34 boards: 2x8 and 2x9 boards connected together with the squiggle bits). I was initially very sceptical that the three squiggle bits would be anywhere near flexible enough. Well, when on the cells, it's quite possible to wedge a screwdriver between the cells and move them a millimetre or two, which is just what we wanted. Also, the boards are quite unlikely to accidentally snap off. So that seems to have worked out rather well.

The big question with these boards is how to populate them with their SMD parts. We can only fit 5 in a row in our current oven/frying pan, and it will be less wiring to leave up to 9 of them connected with the squiggle bits. (Where we have to break them, it's just a matter of 3 wires, but these have to be cut to length, arranged, soldered, and siliconed).

A friend has suggested hand soldering, using a suitably small pair of tweezers that is available from Jaycar. I've got the solder paste that we've used for the baking, it should make hand soldering far less tricky, as only two hands will be needed (iron and tweezers, as opposed to iron, tweezers, and solder from a reel).

Image

Those tweezers look like they need to be registered as lethal weapons. I've had trouble with lesser tweezers before, so we'll see how these go. [ Edit: they're great. Highly recommended for any SMD work. ]
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Post by coulomb » Wed, 26 May 2010, 16:42

Weber asked me to find out the maximum voltage that the pump would start at with 0 and two diodes.

Well, with zero diodes, as before it's about 12.98 V. With 2 diodes, I found the threshold was just under 14.1 V.

Hmmm. So if you have an alternator putting out 14.4 V, you probably need three diodes. With three, the threshold was 14.7 V (the last diode happened to be from a 35 A bridge, maybe that accounts for a slightly different voltage drop; the other bridge was smaller, perhaps 20 A).

For completeness, the threshold with one diode was around 13.6 V.

In summary:
Diodes   Max voltage to start
  0      13.0 (just under)
  1      13.6
  2      14.1 (just under)
  3      14.7
Edit: 35 A voltage drop wasn't smaller
Last edited by coulomb on Wed, 26 May 2010, 08:00, edited 1 time in total.
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Post by weber » Thu, 27 May 2010, 05:11

At long last we have our proposed flywheel design drawn up in 3D, using the free Google Sketchup. [Yes Newton, it's available for Macs too]

Here are two views.

Image

Image

And if you install the free software linked above, you can look at it from any direction you like, and view sections, using this zipped file.
MX5_taperlock_flywheel.zip

Thanks go to mechanical engineer Ben Guymer of Ultramotive for confirming that the integral taperlock was a good solution. This had been suggested previously by Newton (Jeff Owen). But would I listen? Image

And many thanks to Pascal (Warrick Beatie) for precision measurement and working out the fine details and doing a full-size drawing on paper, with front and side views.

And to Coulomb for heroic efforts with Google Sketchup despite being laid low with a cold at the time.

Any suggestions or comments will be gratefully received.
All questions and puzzlements will be grudgingy reviewed. Image
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Post by 7circle » Thu, 27 May 2010, 07:58

coulomb wrote:
... We can only fit 5 in a row in our current oven/frying pan, and it will be less wiring to leave up to 9 of them connected with the squiggle bits.


Image All those packages.

Also have you thought about cutting off the sides of the electric frypan .. opps solder station Image so you can heat up sections of the BMU strip.

They look great. I look the text on the squiggles.

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Post by 7circle » Fri, 28 May 2010, 07:23

Had a look at the flywheel in sketchup.

When I viewed it in X-ray mode and I noticed the dowels don't show as passing though holes in the main disc.


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Post by weber » Fri, 28 May 2010, 12:43

7circle wrote: Had a look at the flywheel in sketchup.

When I viewed it in X-ray mode and I noticed the dowels don't show as passing though holes in the main disc.

Good point. Thanks for that. Will fix.
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Post by EV2Go » Mon, 31 May 2010, 01:59

Thanks 7circle I didn't know that function was there, I'm only a newbie on G-Sketch up 7, thought time had come to graduate from MS Paint. The ability to do 3D modeling has helped me so much in my chassis design work.
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Post by coulomb » Wed, 02 Jun 2010, 16:26

We've bought the aluminium for the adapter plate; just over $90 from Action Aluminium in Brisbane for 400 x 400 x 20 mm.

At this stage, the plan is to manually drill it, and probably manually cut it too.
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Post by coulomb » Wed, 02 Jun 2010, 16:41

I've almost completed a set of 8 BMU (cell top BMS unit) boards by hand. The consensus is that baking in the frypan is the better method: less labour, and a generally better result. The problem of accidentally disturbing already placed components is real, but nowhere near as bad as I assumed it would be.

Weber has assembled 13 boards using the frypan. He is able to paste up and load (ready for baking) 5 BMUs in about 1 hour 45 minutes; with practice, we'll no doubt get that down a lot further. Baking itself is about 2 minutes in the frypan (with the right amount of foil conducting from the unfortunately bowed frypan base to a flat stap of aluminium that the BMU boards sit on). It's perhaps 5 minutes baking with set-up and cool down, and perhaps 10 minutes maximum of inspection and touch-up afterwards. So that's about 2 hours for 5 boards, or about 24 minutes each, at present, not including testing and calibration. That's over 90 hours of assembly, over 11 days at 8 hours a day. With a second person getting the parts ready, it would be quicker again.

We've ordered a plastic stencil from Pololu Robotics and Electronics, which hopefully will speed up assembly a lot.

Testing and calibration has been excruciatingly slow so far. There is some weird problem whereby calibration on other than the first board in a string of boards is way off. Measuring cell voltage with Vcc = 3.62 V seems to be different to measuring at Vcc = 2.50 V. It doesn't seem to be the internal reference voltage changing, according to one experiment. I'm suspecting that the link voltage measuring circuit is pushing voltage into the voltage reference pin, and somehow external reference has been selected, or is at least the internal reference is being affected. I'll know more soon.

Edit: template -> stencil; added link to Pololu
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Post by EV2Go » Wed, 02 Jun 2010, 17:21

coulomb wrote: We've bought the aluminium for the adapter plate; just over $90 from Action Aluminium in Brisbane for 400 x 400 x 20 mm.

At this stage, the plan is to manually drill it, and probably manually cut it too.
Might I suggest laser cutting it... I have had a number of items laser cut (350 grade steel plate) here in Toowoomba and it is a ton cheaper than water cutting with just as good a finish.

Could save you a lot of time for minimal outlay.

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Post by weber » Wed, 02 Jun 2010, 17:26

EV2Go wrote:Might I suggest laser cutting it... I have had a number of items laser cut (350 grade steel plate) here in Toowoomba and it is a ton cheaper than water cutting with just as good a finish.

Can either of these methods be done by a human following a line (like we were taught in kindergarten) or does an NC machine have to be programmed, and hence we'd have to work out the radius and center of every arc?
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Post by EV2Go » Wed, 02 Jun 2010, 18:04

I just drew up and printed off this for them, and said please make me 2.

$30 later, they supplied material and made me 2 out of 350 grade 10mm plate.

Image

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Post by coulomb » Thu, 03 Jun 2010, 05:30

coulomb wrote: I'm suspecting that the link voltage measuring circuit is pushing voltage into the voltage reference pin, and somehow external reference has been selected, or is at least the internal reference is being affected.

I was close: the new link voltage measurement, when tested with BMUs in parallel instead of with actual links to measure the voltage of, is attempting to measure 3.6 V via a 1:1 voltage divider with 1.5 V at the bottom of it. Though the way I was configuring the processor, the 1.5 V wasn't appearing there, so really it was reading the 3.6 V directly via a resistor.

It specifically says in the data sheet that no analogue input can exceed analogue Vcc, else the conversion results will be affected.

So now, in test mode, I output 0V (as a digital output) to the pin that normally has 1.5 V (= Vref) on it, so then it sees 1.8 V, which is less than Vcc (= 2.5 V), and all is well.

[ Edit: Beware! Ten months later, we found that this was causing the internal reference to be shorted to ground, so all the readings are all ones. How something like this can fester for 10 months is beyond me. There is an easy fix: turn off the external reference (and hence the connection between internal reference and Vrefout pin) when doing this trick. ]

This link voltage measurement is new; we didn't anticipate any issues with it whatsoever. So we weren't immediately thinking that it must be that new circuit that we added since the last board that was tested.   Image
Last edited by coulomb on Sun, 24 Apr 2011, 16:12, edited 1 time in total.
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Post by Tritium_James » Thu, 03 Jun 2010, 17:53

There is a way around the diodes thing with the pump, but it involves pulling the pump apart and changing a resistor. I ripped one of ours apart yesterday and traced part of the circuit, and it has an ST ST7LITE3xx microcontroller on it, and one of its analog inputs measures the supply voltage through a resistor divider. Change this (I changed a 4.7k to 4.32k, a 6% shift), and it now operates perfectly up to 15V, and that's as far as I was game to push it. :)

But it does involve extensively gutting the pump, so the two external diodes thing might be the better way to do it...

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Post by weber » Thu, 03 Jun 2010, 18:10

Tritium_James wrote: There is a way around the diodes thing with the pump, but it involves pulling the pump apart and changing a resistor.
Great work James!

Is there enough headroom that we can just solder a 47k on top of the 4k7? Is it surface-mount? 0805 or 0603?
But it does involve extensively gutting the pump, so the two external diodes thing might be the better way to do it...

Nah. "Extensively gutting" is our middle name. Image
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Post by Tritium_James » Thu, 03 Jun 2010, 18:26

Well I didn't take photos as I went along, so a text description will have to do.

When you open up the pump, and lift out the back plastic piece, you can see the impeller, a block of plastic, and the PCB. The impeller just lifts out (it's held in by its magnets being attracted to the metal pole pieces inside the plastic bit).

Gently flex back the four posts on the plastic bit to let the PCB come out. But you also need to lift out the metal pole/winding former at the same time. In my pump, it was (sort of) stuck in with runoff transformer varnish. Some gentle persuasion and the whole thing lifts out of the plastic.

The PCB and the pole piece are soldered together, the wires from the windings come down onto tabs on the PCB. I unsoldered it (12 places) and lifted it off, because I was trying to trace the circuit. It might be possible to change the resistor without removing the windings, but I'm not sure.

Once you get this far, take a photo of the PCB and I'll point out the resistor :P    With the PCB oriented such that the micro is at the top, the one you need to change is just to the left of the micro. It's marked 4701 on mine.

Haha now I get to quote that infamous phrase: Reverse the steps above to reassemble the part! It did all go back together quite neatly. But the potential for screwups is quite high, I suspect I might have just been lucky.

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Post by weber » Thu, 03 Jun 2010, 20:38

Tritium_James wrote:In my pump, it was (sort of) stuck in with runoff transformer varnish. Some gentle persuasion and the whole thing lifts out of the plastic.
Sort of stuck in? Gentle persuasion? Runoff varnish? After it finally broke out, it looked to me like it was deliberately glued in, probably to stop noise and vibration. It's just lucky that no adhesive really sticks to PE or PP.

I did not fancy desoldering and resoldering those 12 pole wires, so I'm glad you did all that to find out enough so I don't have to. In fact I can now tell anyone contemplating this that you don't even need to prise the PCB and pole pieces off their plastic housing. A via comes through from that 4k7 resistor (R14) so that a through-hole resistor can be connected in parallel with it, between this via and a nearby GND via, on the readily accessible side of the PCB, with the resistor body off the side of the board so it has plenty of clearance in the housing.

I'll post a photo when I've done it. But I'm not yet convinced on what resistor value I should use.

You said it operated up to 15 V when you replaced the 4k7 with a 4k32, But operating up to 15 V (or even 17 V) was never a problem, as per coulomb's posts above. My question is, what was the highest voltage at which it would start, DOL? And what was its highest start voltage before the mod?

The high side of the voltage divider is a 10k resistor and when I do the numbers I get that this would only boost the start voltage from 13.0 V to 13.8 V. Paralleling a 47k should get me to 13.9 V but if I want it to start at up to 15 V I'll need to put a 22k in parallel (3k87 total). Is my maths wrong?

This also raises the question, why do they do this? Why worry about the start voltage, but once started not give a damn? Is it just cheap software design based on the (fairly safe) assumption that PC power supplies don't vary much in operation?

Should we be worried about frying the circuitry if we run it over its rated 13.2 V for hours?
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Post by Johny » Thu, 03 Jun 2010, 21:30

Considering how much in a PC would fry if the 12V supply went over voltage, it's surprising that they bother with any over voltage system at all. Maybe the manufacturer would respond to an email if you asked!
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Post by Tritium_James » Thu, 03 Jun 2010, 21:50

I've got no idea about long term reliability. The pump's rated for 50000 hours though (from what I can tell, originally designed as a hot-water circulation pump for houses) so there could be a significant reduction in life and you wouldn't care for an EV.

Weber, that's really weird about the varnish, mine had hardly in it at all. You shouldn't need to have it run much above 13.8V - oh, you're keeping your alternator, aren't you? Maybe you do need a bit more.

I'll go measure some startup voltages.

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Post by Johny » Thu, 03 Jun 2010, 22:01

Tritium_James wrote:... oh, you're keeping your alternator, aren't you?
Considering that you are running around 300 VDC why are you keeping the alternator guys? That would be a perfect voltage to feed any number of cheap 12-14 volt SMP supplies. Surely mounting and coupling the alternator is going to be a PITA?

Having had a generator on a car for 5 years and sat at traffic lights in the cold, rainy weather - at night - watching the 30 Amps current drain gradually slowing my heater blower down....
(Generators generate next to nil at low idle.)

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Post by Tritium_James » Thu, 03 Jun 2010, 22:10

OK, changing that resistor made no difference to the low end start up. It still (barely) started at 7V, and was running OK at 8V.

At the high end, it made the expected difference. An unmodified pump started at 13.15V, and the modified one started at 13.95. This is almost an exact match for the ratio of the resistor divider that I changed.

Having the 2nd one running now as well, I think the modified pump is running slower. I'm guessing it's running a speed control loop based on input voltage, and because I've modified what it thinks the voltage is, it's not running the motor as fast as previously. I haven't got the setup right now to measure the tacho output and confirm this directly.

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Post by weber » Thu, 03 Jun 2010, 22:15

Johny wrote:Considering that you are running around 300 VDC why are you keeping the alternator guys? That would be a perfect voltage to feed any number of cheap 12-14 volt SMP supplies. Surely mounting and coupling the alternator is going to be a PITA?

We're running 228 Sky Energy cells split into in two 114 cell half-packs. Max charge voltage will be 417 V per half-pack, too much for any cheap SMP supplies. Not only input voltage but also HF-transformer insulation voltage rating is an issue.

We can get a suitable supply for about $1000. But we'd need two of them, with outputs paralleled, to keep the half-packs balanced when we eventually go to the full 228 cells in series. $2000 can buy a lot of PITA. And we're keeping the clutch and gearbox, and the gearbox has a neutral switch, so there's no problem with having an idle mode for the motor. And we are going to have a pulley driving the aircon and power steering anyway.
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Post by a4x4kiwi » Thu, 03 Jun 2010, 22:16

How about you put a suitable zener (or diode if 3.3v device)on the low side of the voltage divider so it always has what it wants? This would solve the over voltage problem.

If there is a similar under voltage problem, you could remove the low side resistor altogether and replace with an appropriate zener or diode.

TJs solution does give an over voltage protection over 15v however.
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