Weber and Coulomb's MX-5

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Post by weber » Sun, 06 Feb 2011, 20:18

PlanB wrote: Retro MX How are these guys getting these 300+mile ranges?

I don't believe it -- not with normal driving anyway -- unless there's no boot or passenger seat.

But why do so many kit-car designers etc. choose an MX-5 chassis? Because it is engineering perfection.
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Post by Electrocycle » Sun, 06 Feb 2011, 23:47

a convertible is never going to be engineering perfection :P

Getting an MX5 anywhere near the rigidity of a similar sized car with a roof ends up being substantially heavier.
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Post by coulomb » Mon, 07 Feb 2011, 01:42

weber wrote:
PlanB wrote: Retro MX How are these guys getting these 300+mile ranges?

I don't believe it

More here:

TGMY converts Mitsuoka Himiko to electric, gets 550 km range (DIYelectriccar.com)

That seems to be a really long bonnet, they are using LiPo (as in polymer) batteries, which are more energy dense than LiFe, and I believe the pack is really heavy. The cells used aren't common; TGMY seems to be the company making the cells and "just" showing off their cells with this car. So it may be real. I find it hard to believe that the thing is based on an MX-5 chassis. Maybe stretched a fair bit...

[ Edit: note the distance from the front of the door to the blinker, a bit further than on an MX-5, then from the blinker to the front tyre... they seem to have added about 1/2 of a metre! Though the front tyre is closer to the front of the vehicle, making more useful battery space available.]

Image     Image

Image

Last edited by coulomb on Sun, 06 Feb 2011, 14:56, edited 1 time in total.
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Post by woody » Mon, 07 Feb 2011, 02:17

The doors and flanks and windscreen all look very similar - but the wheelbase is at least a foot longer.
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Post by weber » Mon, 07 Feb 2011, 03:00

The video makes it clear that they only _hope_ to get 550 km under the same conditions that a Tesla Raodster got 501 km.

The Himiko has 168 cells in 21 x 8-cell (25.9 V) packs. Each cell is 100 Ah. That's 54.4 kWh. A Tesla Roadster pack is 53 kWh and the US EPA rates the Tesla at a 393 km range.

So under what conditions did a Tesla Roadster get 501 km?

From Wikipedia: "The world distance record of 501 km for a production electric car on a single charge was set by a Tesla Roadster on October 27, 2009, during the Global Green Challenge in outback Australia, in which it averaged a speed of 40 km/h."

In other words, under "hypermiling" conditions, not normal driving conditions.

[Edit: Corrected Ah to kWh. Added "25.9 V" as shown along with "100 Ah" on a card in the video.]
Last edited by weber on Sun, 06 Feb 2011, 16:04, edited 1 time in total.
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Post by weber » Mon, 07 Feb 2011, 03:13

Perhaps it's 8 x 21-cell packs, not 21 x 8-cell packs as I wrote above. If they are LiPo then that 25.9 V on the card must have been 7 cells at 3.7 V each. That bumps the capacity to 62.2 kWh. That's just over twice the capacity of our battery. We can see 3 packs in the video. It's hard to imagine where they could be hiding the other 5.
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Post by EV2Go » Mon, 07 Feb 2011, 03:29

Perhaps they repositioned the front suspension???

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Post by coulomb » Thu, 10 Feb 2011, 16:50

Weber did some welding of stainless steel rods last night:

Image

The bricks keep the rods straight after being aligned by eye. The cable clip on the left is a standard car jumper lead; it seems to handle the welding current just fine. He did need to use the 2.5 voltage setting with 5 wire speed on the Transmig 165, which he suspects would have blown away mild steel rod of the same diameter (M5).

Image

In the photo, the welded rods have not been ground back yet. That's a set sufficient for one box (our largest, 60 cells). We don't know yet if such long rods will sag too much, or stretch too much; we'll find out soon. Sag can be fixed with intermediate bushes; stretch will require more work. These are the longest strings of cells (23) that we have anywhere in the MX-5.

The idea is to get this box put together, to test how things will fit, how to handle a 90+ kg box, and to finally get some coulombs out of the charger. We figure that 60 cells should squeak past the charger's minimum pack voltage limit (below which it refuses to charge).
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Post by coulomb » Thu, 10 Feb 2011, 17:44

More on Charger Control

Yesterday we completed testing of the charging software. We decided to keep it simple, and have the Driver Controls Unit (DCU - it connects to the pedal and motor controller, and does mundane things like brake lights) talk to the charger and BMU string with its two spare serial ports. (We considered a second DCU just to convert CAN to 2400 bps serial for the Elcon charger(s), so we won't need a second DCU in later chargers if they ever fix the CAN bus bottleneck, or if we use a different charger, but decided against it). We'll still need a second DCU if and when we get a second charger; it's analogue interface, CAN bus, etc won't be used. At least this way, we can have the same software in both DCUs.

Here are the mods we made to the DCU:

Image

[ Edit: The green mods haven't been made yet; we're considering whether to put the mods inside the DCU, outside, or in the cable shroud. For now, we're connecting one side of the charger output's differential drive to ground. ]

So we've sacrificed the gear selection LED/lamps (12 V) for 3.3 V differential drivers and receivers for what we're calling our IDCL (TM) (Isolated Differential Current Loop) interface. That's what we use for BMUs; we'll be using the same thing to the charger as well. Near the charger, we'll have a box with more or less the same circuit as the CAN interface that came with the charger. In fact, we've used a prototype BMU board with end-of-segment circuitry, which converts the charger's single ended signal to differential drive. Hopefully, that will minimise noise sensitivity, and we're used to using it, and have our Novus RS485 boxes to test it with.

The charger algorithm is quite simple. It initially ramps the charger up to full current (see below), with a voltage limit that corresponds to an average cell voltage of 3.65 VPC. (Really the voltage limit is irrelevant; it's just a very small safeguard in case all the other safeguards fail). Every time a badness byte is received from the BMU string (two times per second at present), the charger software sends a voltage request to the next BMU, in a round robin fashion. This is mainly to find out when all BMUs are in bypass (at 3.60 V). All-in-bypass is currently detected by all reported voltages >= 3.59 (when a BMU is bypassing, the bypass current comes on and off as needed, and if it happens to be off, it could be a little under the 3.60 V bypass threshold). The software remembers the start of the most recent string of BMUs that are all in bypass. If that becomes the whole pack, then charge is complete, and we start a "soak" phase. This lets the cells stay at 3.60 V for five minutes, at 0.5 A current (50% duty cycle), so the last cell to come up to voltage gets a chance to stay there for a while. At the end of this soak period, the software sends the command to turn the charger off.

We have a better scheme whereby the BMUs can determine when they are all in bypass on their own, but as of yesterday, we had zero (0) bytes free, and could not implement the scheme. Fortunately, TI have come out recently with the G series chips; some of these have 8K bytes of flash (compared to 2K now) and 256 bytes of RAM (compared to 128 bytes now). All for less money; technology is wonderful. We've ordered two samples of these. We intend to use those on the diffamp boards that we have ordered two prototypes of.

The charger will also cut back to 0.9 A at any badness (over-voltage, over-temperature, and possibly problematically, under-voltage). It will gradually raise the current 0.1A every second after that. Hopefully, this will cause current to sawtooth from 0.9 A to whatever current the string can currently handle, so the charge is not needlessly slowed down, but the cells remain safe. It also starts the charge off at 0.9 A, ramping up to maximum over some 46 seconds.

The shunts arrived recently, as well as a bag of SMD resistors for about $60. We always seems to be wanting to try out something and have to wait for the next order to buy a 2.5c component; now we have E12 values in stock. Hundreds of them.
Last edited by coulomb on Thu, 10 Feb 2011, 06:50, edited 1 time in total.
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Post by Tritium_James » Thu, 10 Feb 2011, 18:00

Re: badness, cell volts, charge algorithms, etc, why not just run a PID control loop that controls charger current based on highest cell voltage. Then current then backs off automatically, there's no real concept of charging vs balancing, etc. It all just happens nicely by itself. If you change the bypass current in the future, the charge algorithm just compensates by itself too.

Re: 2.5c components, you know if you order online Farnell has free shipping at the moment, right? :)

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Post by Johny » Thu, 10 Feb 2011, 18:29

Tritium_James wrote:Re: 2.5c components, you know if you order online Farnell has free shipping at the moment, right? :)
Minimum order $10 before GST - but I use it a lot, right on the min. order threshhold. Next day free shipping helps stop the now-I-want-a-new-component slow-down. You just end up with lots of spare stuff.

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Post by coulomb » Mon, 28 Feb 2011, 23:00

We're preparing for our first battery charge. Now that we have the hardware interface to the charger sorted and the charger software written, we need about 60 cells in series so that the charger doesn't reject the battery as too low voltage.

This was the first battery rack that we made up, and we've left things a bit tight:

Image

The arrow indicates one of the bolts that is rather hard to access behind the vertical angle iron. But that's OK; we could push the pack towards one end, and the other end has enough clearance to get fingers in. When the cells were manually compressed, that allowed the nuts on the threaded rods to be tightened at least finger tight. The ones in the middle we could use tools on (pushing the nuts up or down to clear the nearby nut).

We had a humorous moment when we tilted the pack to get better access to the hidden nuts:

Image

We found that sometimes the cells needed a little persuasion to get them into alignment:

Image     Image

[ Edit: the cells will eventually get BMUs on them; we just don't have 60 BMUs made yet. ]
Last edited by coulomb on Mon, 28 Feb 2011, 12:06, edited 1 time in total.
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Post by T1 Terry » Mon, 28 Feb 2011, 23:07

Image
Now there's someone who does use a hammer very often Image love the white overalls too.

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Post by coulomb » Tue, 01 Mar 2011, 00:19

Weber and Coulomb have been pumping coulombs!



Here is the mess of hardware to talk to the charger:

Image

It's great to see the charger working after all the turmoil getting the interface working.
Last edited by coulomb on Mon, 28 Feb 2011, 13:22, edited 1 time in total.
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Post by antiscab » Tue, 01 Mar 2011, 00:28

you can now buy electric torque limited drills,
I use them for torquing down bolts now.

much faster than doing it manually, particularly if you are planning on doing up the ~220 cells of yours more than once :)

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Post by Tritium_James » Tue, 01 Mar 2011, 14:40

Nice, guys! Don't forget when you reassemble the pack with the BMUs in there that you should wire-brush the cell terminals and busbars and apply a smear of anti-oxidation compound in there before it goes back together.

Matt, what drills do the electric torque limit? I'd really like one, but all the types I've seen have a mechanical (clutch?) torque limit.

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Post by antiscab » Tue, 01 Mar 2011, 17:53

I saw one at bunnings ($100), a cheap chinese one.
it was calibrated to a specific Nm per se, but there were 21 different torque limits (all same speed).

probably a brushless motor with 21 different current limits.

This one is a cheapie:


Dewalt also make a torque limited drill
Cheap torque limited drill

even cheaper torque limited drill

the $200 one I was looking at did indeed havea clutch, but I don't think they cheap ones do...

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Post by coulomb » Tue, 01 Mar 2011, 20:00

Tritium_James wrote: Don't forget when you reassemble the pack with the BMUs in there that you should wire-brush the cell terminals and busbars and apply a smear of anti-oxidation compound in there before it goes back together.

We attempted various cleaning methods. Most look good until a day or two has gone by. Metho (as a final rinse) was surprisingly bad, leaving green salts(?) on the straps.

Wire brushing, perhaps on a grinder or similar, might still be the way to go. We were hoping to clean between the laminations with chemical baths, and of course brushing won't do that, unless we completely disassemble them and redo the heat shrink.

These are the sorts of things that really add to the labour of a conversion. And of course with high voltage AC, we've quintupled the amount of work on our many cells (228 cells verses typically 45).
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Post by seligtype3 » Wed, 02 Mar 2011, 22:55

The stainless rods are a good idea!
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Post by coulomb » Thu, 03 Mar 2011, 01:14

seligtype3 wrote: The stainless rods are a good idea!

Thanks. But they come with the Sky Energy / CALB cells (or at least did when we ordered them in a bulk order). They're mostly of the correct length for 8 cells; we figured that was a nice size, and if we wanted more, we could weld them together or buy our own and cut to size.

[ Edit: you get to choose the number of cells in a string when you order; at that time, we didn't know how many cells would be in each string. We guessed a few, and ordered the rest as strings of 8. ]

Image

These are part of the much-lamented Sky Energy clamping system; Thunder Sky cells came (at one point anyway) with a much more sophisticated clamping system. These seem to be adequate, even if the clamping bars are frustratingly too long, and when we drill a new hole, it overlaps with the old hole slightly. The Thunder Sky system seems to be fixed length, not screw adjustable as the Sky Energy system is. We find that a string of 23 cells (as we have in some battery boxes) compresses about 10 mm (about 1%) with a little pressure. I doubt that we'd be able to use the Thunder Sky system with anything like 23 cells in a single row.
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Post by seligtype3 » Thu, 03 Mar 2011, 01:37

coulomb wrote: ...compresses about 10 mm (about 1%) with a little pressure.


Good to know! Thanks. I'll be looking to devise something along these lines I think.
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Post by coulomb » Tue, 08 Mar 2011, 18:04

Some EV days you don't seem to have much to show for it. But the planning is important too, and hopefully will result in a better final result that is easier to replicate.

Our most recent EV day produced this diagram:

Image

It may not seem like much, but it's a huge relief to finally have a plan of what cells go where, what isomer the BMUs will have (i.e. negative terminal on the left or right), even what colour Anderson connector will use on each box. We tried to find a sensible standard to follow for the Anderson colours, but in the end, we decided that the good old resistor colour code (at least, the ordering thereof) is what we'll use. So the boxes with the highest number of cells (28 cells) will have white Andersons (white coding for 9 in the resistor colour code). At the other end, boxes with just 13 cells will use black (0 in the resistor colour code).

As noted, the ordering we chose also corresponds approximately (and arguably) to luminance order. So the higest voltage boxes' Anderson connectors will be easiest to see in dim light.

Anderson do have a suggested colour coding, but using that standard, our connectors would be only about 3 colours, and we happen to have two sets of six colours. Also, their standard seems rather arbitrary.

The nominal pack voltage is using 3.33 VPC. That's a typical no-load cell voltage. At 3.2 VPC, they would be 365 V. I wish that there was a definite voltage standard for LiFePO4 cells too.

The next job is to make a 28-cell box (because the metalwork is done and we have enough BMUs) and actually charge it. We'll have to put the 60-cell pack in series with it. We won't be able to charge it fully, since none of the 60-cell box cells will have BMUs on them. I suppose we could run around with multimeters (we've found the cell with the highest voltage at present) so that we can get a decent sort of charge into the next 28 cells.

We might build up packs of approximately 16 cells and give them minimal charges, since most of the cells have been lying idle for about 20 months.

We've been waiting for a couple of weeks now for our prototype boards with the differential amplifier circuit needed for the current shunts. The boards have apparently been made, but they haven't progressed to "shipped" status.

Edit: changed diagram to use British Standard colour abbreviations; acknowledged donation of Anderson connectors from Soanar Australia
Last edited by coulomb on Tue, 08 Mar 2011, 11:41, edited 1 time in total.
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Post by coulomb » Tue, 08 Mar 2011, 18:26

Re the BMU isomers: we decided that odd-length strings of BMUs were just a bad idea, at least manufacturing them. It gets tricky, so I won't go into details, bit it turns out that even-length BMU strings will always work with each other (provided that they start with the same isomer), but odd-length strings may or may not work.

We finally figured out that horseshoe and other configurations of cells can be conceptualised as folded linear strings of cells. So we can consider each half-pack to be a single string of 114 cells, as far as isomers goes. We can just take strings of (say) 8 BMUs, and cut them as needed, and they are guaranteed to be the right isomer to fit the end of the existing string of cells.

We want the communications data to stream moderately sensibly from one end of the half-pack to the other. We decided to have the comms start at the front of the car, and work towards the back. That means that the end-of-row isolation circuits, which operate at 3.3 V, will on average be further from the BMS master near the start of the chain under the hood somewhere. We have the communications moving left to right (from above with the hood "up") and top to bottom, i.e. western reading direction. The communications direction determines the position of the links between cells of the half-packs (there are two ways of linking a linear string of cells).

So that determines where every cell and link goes, and which direction each cell faces (e.g. positive on the left or right).

Now all we have to do is actually implement this plan   Image

Edit: string of cells -> string of BMUs
Last edited by coulomb on Tue, 08 Mar 2011, 07:48, edited 1 time in total.
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Post by weber » Tue, 29 Mar 2011, 20:30

Just thought we'd better post these photos for Newton (Jeff Owen). Hope your arm is better.

When we posted some photos a few weeks back, of clamped batteries showing interfering clamp bars and rods, Jeff kindly phoned to remind us that he had solved that problem for us months ago. The plan was to cut and weld the clamp bars so they are continuous across multiple rows of cells and then use only one threaded rod between cells (per clamp bar).

The photo shows that we have implemented his brilliant (and much neater) solution. We even did it where one row is shorter than the others, in the manner he suggested.

Image

In the following photo you can see our temporary use of webbing tie-downs and pieces of wood, to get started. And the use of a cordless driver on its lowest torque setting to achieve a constant tension on the clamps.

We are assembling the cells in the lid of the box to avoid the box corner angles which make it difficult to get at the clamping nuts. When done we lifted the whole block into the box.

Image
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Post by antiscab » Wed, 30 Mar 2011, 19:22

coulomb wrote: Image


Hi Coulomb,

How long are those bolts supplied with the SE cells?

cheers,
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