Weber and Coulomb's MX-5

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Post by weber » Tue, 15 Jul 2014, 23:03

Weber & Coulomb's Fibre Optic BMS now does masterless control of a TC charger

Mexy is running well. It was great to be able to drive her out to Queensland Raceway at Willowbank every day when the Electric Superbikes were there. On the Saturday, I took my EV mentor and former employer Ross Pink, who is a motorbike racer from way back, and insisted he drive Mexy both ways. We both really enjoyed the day at the races (mostly talking to all the guys in the pits), and the drive.

I once asked Ross what SI unit name he would like to be called by. He requested "mho" because he has a moustache. I pointed out that the mho (a unit of conductance) isn't actually an SI unit, but belonged to an earlier metric system. But I said I'd let him have it anyway, because I really didn't want to be calling him "siemens". Image

Coulomb and I have often joked that Mexy was merely the test bed for our BMS development, which was where much of the 5 years went. We couldn't be happy with just any BMS. We had to have the safest and most versatile BMS on the planet, within the bounds of reasonable cost, mainly because we were doing one of the highest-voltage conversions on the planet, and many cells would be inaccessible.

So I'm happy to report that the BMS is still functioning flawlessly and, along with our software in our modified Tritium EV Driver-Controls unit, is controlling charging and balancing, and is preventing idiot drivers from damaging the cells, like me the first time I came home from the raceway and took a wrong turn and ended up at Springfield. I just managed to crawl home, the last km or so in turtle mode.

We need to get back to work on, and finish, the new "universal" version of our CMUs (cell management units, which we previously called BMUs) that make up our BMS. They will be "universal" in the sense that they will fit all common sizes of prismatic lithium cells, from Winston, Sinopoly, CALB etc., not to mention the old Thundersky and Sky Energy cells. We hope to offer them for sale in future, if we can get enough orders to make it worthwhile paying the pick-and-place setup fee.

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Last weekend I wrote some new functionality into the CMU code. I made it so that you no longer need a master unit for our BMS to control the popular TC (Tiecheng/Elcon/Chennic) chargers. All you need is the following adapter cable to go from the BMS output optic fibre to the 7-pin control connector on the charger. The circuitry in the adapter cable consists of 3 wires, and two resistors which are inside the heatshrink surrounding the fibre receive connector (photo-transistor) at the lower left.

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All CMUs have identical software, since they can all be reprogrammed in one go, in situ, but now any CMU can be told to run a PI controller which sends messages to the charger to adjust its current, based on feedback from the fullest cell. And when all cells are bypassing, it tells the charger to terminate the charge. Of course it only makes sense to tell the last CMU to do that. This currently requires the "CAN bus" version of the charger, although it does not require the CAN bus adapter. However the CMU software could easily be modified to control the non-CAN version via its enable input, requiring a different, but equally simple, adapter cable.

This masterless PI control was tested by using it to charge half the cells in the MX-5. For the "A" half, the $275 Tritium EV Driver-Controls unit was replaced with a dumb fibre-to-fibre coupler. This simply pointed the BMS output fibre at the charger input fibre. For the "B" half, charge was controlled by our existing software in a Tritium Driver-Controls unit as usual. Charging of both half-packs was indistinguishable. Hoorah!

I note that we still need the two Driver Controls units in the MX-5 as they do much more than control charging. They also listen to the accelerator pedal, switches and sensors, and talk to the WaveSculptor, gauges and contactors.
Last edited by weber on Tue, 15 Jul 2014, 13:26, edited 1 time in total.
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Post by Johny » Tue, 15 Jul 2014, 23:17

I'm certainly envious of your CMUs. Well done guys.

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Post by GRMarks » Mon, 18 Aug 2014, 23:21

Weber

I have a question (maybe dumb so forgive me) but I have a 132V controller running a 118V (3.3V per CALB cell) thats 36 cells. so at 3.65V (when my simple BMS turns on the resistor to bypass gives me 131.4V (the maximum for the controller).
Now if I want to add 2 extra cells so I have 125.4V (at 3.3V per cell) and there for I only want 3.47V as a bypass voltage so as to keep pack voltage to 132V (for the controller (and charger) sake).
My question is why does no one ever do this? Why does everyone charge to the maximum when those 0.35 volts is used in no time adding very little rage to the pack. Wouldn't it be better for the life of the cells to cut off at 3.4 volts? The other problem is to find a BMS that lets you set the bypass voltage (and max voltage) for a non-can bus charger.
How would this affect the TC charge with its fixed charging curve?    
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Post by weber » Tue, 19 Aug 2014, 00:51

You are quite right that it would be better for the (LiFePO4) cells, to terminate the charge after balancing at 3.4 volts.

Why don't most people do it? The main reason is because you need a BMS and charger where the BMS can control the charger current smoothly right down to zero (actually it only needs to go down to the minimum bypass current). The BMS would do this based on keeping the cell with the highest voltage at 3.40 volts for however long it takes for all cells to reach that voltage.

If your BMS or charger is such that the BMS can only turn the charger on or off, then if it turned off when the first cell reached 3.4 V, not only would no balancing take place, but even that first cell would not be full, because part of its 3.4 volts would be due to the voltage rise caused by the full charging current times the internal resistance of the cell.

Normally an on/off type LiFePO4 BMS will turn on bypass on any cell that goes over some threshold typically between 3.5 and 3.65 V, but will not shut off the charger until that cell reaches a higher threshold, typically between 3.65 and 4.0 V. There must be a significant difference between these two thresholds in the on/off scenario because the time it takes the first full cell to go from the bypass threshold to the alarm threshold is the only time available for any balancing to occur, i.e. for any of the other cells to catch up.

But in the smoothly controlled scenario the current can be tapered off as required to hold that first cell just barely above its bypass threshold for as long as required for all the other cells to catch up.

Some non-CAN TC chargers have an input which lets you smoothly control their current with a 2 V to 5 V analog signal. Our BMS can work with either this type or the CAN bus type, to balance at any voltage you want.

However, there is another proviso if you want to balance at anything below about 3.45 V, and that is that your individual cell voltage measurements must be very accurate because you are no longer on the steep part of the charge voltage curve.
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Post by weber » Tue, 19 Aug 2014, 01:10

To answer your second question: "How would this affect the TC charger with its fixed charging curve?"

If you can control the charging current of the TC charger, it doesn't matter about its voltage setting, provided this is higher than you require.
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Post by GRMarks » Tue, 19 Aug 2014, 02:28

OK makes sense. Now that you have explained that, is that why the small RC lipo charger I have turns off to check the cell voltage then turns on again, to get an accurate voltage reading? And if so - there's a new mod for your BMS :)

Some ideas I have had, but have no idea if it is possible are as follows:-

1) is it possible to build a single cell isolated charger to charge at 12A for a low cost. Then have one per cell (36 cells = 36 chargers). No need for BMS to worry about anything but low cell voltage. No balancing required as each charger turns off when its cell reaches the set voltage.

2) Can you have the cell thats at 4.65+ volts connect (via a control unit) to the lowest voltage cell to help discharge its self and help charge the low voltage cell faster? After charging the same system could also keep cells that discharge quicker stay at the same voltage as the rest of the pack giving greater rage?
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Post by Adverse Effects » Tue, 19 Aug 2014, 03:46

there are hobby chargers (admittedly very high end ones) that will balance charge all the way like the PowerLab 8 (v2) Workstation

this just needs to be up-scaled and it would be the best care for cells

you can set min V max V it graph's everything and so on
Last edited by Adverse Effects on Mon, 18 Aug 2014, 17:47, edited 1 time in total.

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Post by weber » Tue, 19 Aug 2014, 04:24

Such "active balancing" schemes are possible, but not usually at low enough cost, because in practice very little balancing is required after the initial balance, and if one cell is significantly limiting your range it may be cheaper to just replace it.
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Post by weber » Tue, 19 Aug 2014, 19:38

The high voltage WaveSculptor has finally arrived!

I picked it up this morning, and James kindly gave me the tour of Tritium's new office and workshop at Coorparoo.

Thanks James and Alex, for getting the testing done despite not being fully settled in your new premises.

And thanks Jeff, for negotiating this deal four and a half years ago.

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I'm smiling in the photo, but I have to admit to a certain bittersweetness, as the arrival of this final component forces me to face the fact that a very enjoyable project will soon come to an end.

I really need to find some paying work now, and I'd like it to be EV-related (but Solar Power / Energy Storage System related would be good too). So if you know of anything in Brisbane, please let me know by forum private message, or phone me on 07 33662660.

Regards,
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Post by coulomb » Tue, 19 Aug 2014, 20:21

Weber,

sorry, it needs to go back. It says battery voltage 750 VDC, yet the warning is only for 500 VDC. Just not safe. Off you go now...

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Post by Johny » Tue, 19 Aug 2014, 20:28

Well you know what Sheik Spear said:
"A potential difference by any other value would hurt as much."

Weber I wouldn't be thinking it's over yet. You're about to double the pack voltage. What foibles might emerge now....


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Post by Adverse Effects » Tue, 19 Aug 2014, 20:38

Johny wrote:What foibles might emerge now....


*covers his ears and go's* LA LA LA LA LA i carnt hear you LA LA LA

SHHHH you will jinx them lol

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Post by TooQik » Tue, 19 Aug 2014, 20:39

Very nice!!!

Now the fun begins, or is that continues. Image

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Post by weber » Tue, 19 Aug 2014, 21:26

I wondered how long it would take someone to comment on the inadequacy of the "DANGER ... 500 V DC". Yes, I can just see someone thinking, "Only 500 V DC. In that case I won't bother waiting for the caps to discharge or towelling myself dry after my swim before I start poking around in there." Image

I think we can cut them some slack since it's the only one in existence. Essentially a prototype. The white panel on the right with the new ratings is very neatly stuck over the old WaveSculptor label.

But hey! James just sent me some new firmware which I uploaded to the label and it's all fixed. Image

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Post by TooQik » Tue, 19 Aug 2014, 21:36

I like the fact the kid in the picture is wearing a bright orange jumper....very appropriate. Image

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Post by weber » Tue, 19 Aug 2014, 21:46

And just so's nobody gets any funny ideas, I'm lettin' yers know, Minxy the guard cat's on duty.

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Post by jonescg » Tue, 19 Aug 2014, 22:52

Looking forward to seeing how this new controller works out. I gather the field weakening is working better on these inverters now? If I recall the earlier incarnations never had it.

I see high voltage is a common trend amongst EVs now. Voltron Evo is very happy at 700 V, as is Mexy, Tesla's getting close to the 400 V mark, and the new Renovo coupe has a 740 V pack.

Despite all the protestations from many sources that it's not necessary, there is a bit of a trend towards higher pack voltages. If carefully managed, it's just a number. I think after all is said and done, it's hard to argue with the physics of more "V" resulting in less "I" for the same "P".
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Post by Adverse Effects » Wed, 20 Aug 2014, 00:00

higher V = less losses and more efficient if i am not mistaken

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Post by jonescg » Wed, 20 Aug 2014, 00:20

Yep, but more than anything else, the 350 V+ inverters and motors seem to offer better power to weight, and much higher efficiencies. So it's a matter of going where the progress is heading, and that means up towards 720 V at top of charge. Makes DC Fast charging a bit harder to manage since all the commercial units rarely put out more than 500 V DC.
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Post by weber » Wed, 20 Aug 2014, 04:16

jonescg wrote: Looking forward to seeing how this new controller works out. I gather the field weakening is working better on these inverters now? If I recall the earlier incarnations never had it.
Tritium have always had flawless field weakening as far as I know. You may be remembering that we had a problem with field weakening. But it was no fault of the WaveSculptor. However Tritium found the problem for us anyway. It turned out to be because some of our motor parameters were out by a factor of 5, plus we had intermittent ground loop noise on our accelerator pedal signal.
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Post by jonescg » Wed, 20 Aug 2014, 04:37

Ah OK. Just that the Varley guys were saying they can't get their Carbon motors to spin fast enough and I assumed it's due to field weakening. It might be that the ironless motors don't lend themselves to it?
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Post by weber » Wed, 20 Aug 2014, 14:55

jonescg wrote: Ah OK. Just that the Varley guys were saying they can't get their Carbon motors to spin fast enough and I assumed it's due to field weakening. It might be that the ironless motors don't lend themselves to it?

That's it alright. I just googled "field weakening ironless motors" without the quotes, and immediately found this excellent exposition by a well known author.

The first WaveSculptor was made for an ironless motor. You want more speed with an ironless motor, you need more voltage. But I suspect Varley just need someone to design their system properly. I have some ideas. And hey, I'm available guys. I can help you be in there with a chance against Voltron Evo. A very slim chance. Image
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Post by weber » Wed, 20 Aug 2014, 22:38

Varley describe the Dual Carbon motor as having a feature called "Dynamic Overdrive". That might be some tricky means of field weakening these ironless motors that I've never heard of, but more likely it's marketing-speak for series-parallel switching.
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Post by weber » Sat, 30 Aug 2014, 22:26

I guess everyone's wondering how the 750 V WaveSculptor is going. Sorry to tell, we've spent the last two Fridays working on the MX-5 but haven't even started the switchover from 360 V to 720 V (parallel to series half-packs). Why?

Well, I've been having intermittent loss of one half pack for some time now. Since they are in parallel I don't usually find out about it until the remaining half-pack unexpectedly runs flat (power starts to be limited by cell stress). But typically I can turn the key off and on and get both half packs again and carry on. It might be a bad connection in the coil drive to one of the many pack-breakup contactors.

While this is not too serious with half-packs in parallel, it would be a lot worse with them in series, not only because you'd lose power completely, but because the opening contactor would have the full pack voltage across it and might not last long with the arcing that would occur if it opened at high current.

And to top it off, on the way home from the Brisbane AEVA meeting at which I proudly displayed the new WaveSculptor, both half-packs briefly dropped out half a dozen times!

So we really need to get to the bottom of this, before doing the big series/parallel switchover.

We can get an indication of whether we're running on one half-pack by looking at the voltage sag under load, taking into account the state of charge. But you have to drive it hard to see this. You can't just do it in the carport. Intermittent faults are such a pain.

So we decided to finally install our IMUs (Insulation and current(I) Monitoring Units) as this would allow us to see the current in the two half-packs separately. If you need reminding of what these do, or how they work, see
viewtopic.php?title=weber-and-coulombs- ... 111#p48111.

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It took me a while to get the bugs out of the code I wrote for the Tritium Driver Controls units to interrogate their respective IMUs and display both currents alternately on the tacho, but that's working nicely now.

We're only using the IMUs for current measurement so far. We haven't yet implemented the insulation test on startup, but that worked great in bench testing of the IMUs. I'm particularly pleased with the self-test mode that we wouldn't have, except for the modification suggested by honorary SI unit 7TauRadians a.k.a. 7Circle. No, really, Pi is wrong.

But while testing the IMUs in the vehicle, we noticed a problem with BMS comms on the 'A' half-pack. About one in every 2000 characters is corrupted. This low level of errors doesn't affect the operation of the vehicle, since that's why we have checksums for command and response packets, and checkbits in the status bytes. But we would like to get to the bottom of it. So now we're nested two levels deep in things we want to get to the bottom of, before going to full voltage and installing the new WaveSculptor. Sigh.
Last edited by weber on Sat, 30 Aug 2014, 14:14, edited 1 time in total.
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Post by PlanB » Sat, 30 Aug 2014, 23:43

Oh sure just replace Pi with Tau no problem

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