Weber and Coulomb's MX-5

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Post by weber » Tue, 28 Oct 2014, 18:15

Now that MeXy has her full design power, I had great pleasure in giving joy rides/drives to all the new and returning committee members after our Brisbane AEVA AGM last Wednesday (and then to anyone else who wanted one).

As the night wore on, the clutch wore off. It began slipping at lower and lower torques. This was apparently due to temperature, as it was back to "normal" the next day (only slipping after a full-torque gear change).

So it will have to come out. But I won't have time to do that for some weeks. I have an off-grid solar power system to design, test and install. I have decided to use LiFePO4 batteries rather than lead-acid for this for the first time, and with Coulomb's help, to attempt to hack the inverter, PV charger and genset charger to be controllable by our new universal-fitting (for prismatics) cell-top BMaS. That's Battery MAnagement System pronounced "bee em ass" as opposed to Battery MOnitoring System pronounced "bee em oss". Image

You can read about some aspects of this solar power system here:
viewtopic.php?title=pip4048ms-inverter&t=4332

Back to Mexy:

Last Sunday afternoon I asked the wife to come for a drive in Mexy, on the kind of road the MX-5 was made for -- a winding mountain road -- for the first time since attaining full design power. "Oh yeah", she said with a smile, "So where are you going to strand me today?". I said, "How about a drive to Mt Nebo, with a cuppa at a tearoom when we get there? [oh ye of little faith]". A cautious "OK".

My son got his drivers licence last week, so I pointed out that if MeXy did break down, he could come and get us. But to show how confident I was, I didn't pack so much as a Philips screwdriver, let alone a multimeter.

So off we went with the top down. Stopped at The Gap to check the tyre pressures. I put 250 kPa (36 psi) in the front and 270 kPa (39 psi) in the back. This showed that they had been a bit low. They looked and felt good after this.

On the mountain, the one-pedal-driving in second gear was fabulous -- effortless acceleration, even up hills, and just the right amount of regen-braking on accelerator backoff. The wife said she was only a little scared twice. Once on a blind 270° left-hand bend (it just goes on and on -- fabulous) where I was hugging the inside so tightly she though I was going to drop off the bitumen on the inside. And once after some motorbikes passed me at an insane place, with double-white-lines, I slowed down to let the next one past at a sensible place, but they didn't pass and just beeped their horn, so I thought I'd give them a run for their money, and the tyres started to scrub a bit on the corners.

The police were there, looking after public safety -- by positioning themselves at the safest part of the whole road -- the only long straight -- out of sight -- with a radar gun.

Anyway, we got to our tearoom in one piece and enjoyed a delightful cuppa and a bikkie on a verandah, surrounded by king parrots, rosellas and cockatoos.

When we got into MeXy to come home, I turned the key to Start, the power steering came on as usual, but there was no response to the accelerator. Uh oh. Embarrassed look at wife -- smiling resignedly.

To cut a long story short, by getting her to flip the key on and off while I felt each contactor box in turn, I discovered that none of the isolation or segmentation contactors for the "A" half-pack were operating. So I then felt the automotive relays that drive their coils and found both "A" and "B" relays were clicking just fine. But apparently the contacts in the "A" relay were toast. They must have given their last gasp in a fiery arc as I turned MeXy off before we went for our cuppa. Fortunately these relays are of the kind that have two quick-connect pins connected together, so I used these to bypass the relay and leave the "A" pack permanently on, and we set off for home.

But this was not until after I had phoned my son and he had collected the tools I asked for and set off to the rescue in the Nissan Leaf. We phoned him on our way, and he pulled off at McCaffey's lookout and waited for us to go past.

Of course to folks like us on this forum, a breakdown and makeshift repair like that just topped off the perfect afternoon. Image
One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).

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Post by Johny » Tue, 28 Oct 2014, 18:39

Well you know what the flyboys say Dave - "any landing that you walk away from...".
I'm impressed that you located and "fixed" the problem without needing the cavalry - even though they had already saddled up (Mrs weber is probably quietly impressed too!).
It'll be interesting to know if the relay is dealing with some back EMF from something or it's just unlucky failure. I thought they only switched EV200s in MeXy.

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Post by weber » Tue, 28 Oct 2014, 20:23

Johny wrote: Well you know what the flyboys say Dave - "any landing that you walk away from...".
I'm impressed that you located and "fixed" the problem without needing the cavalry - even though they had already saddled up (Mrs weber is probably quietly impressed too!).
Thanks Johnny. Let me put it this way: I wouldn't like my chances of getting her to go bush with me in MeXy again, if I hadn't done that. But perhaps next time I'll quietly pack the toolkit. Image
It'll be interesting to know if the relay is dealing with some back EMF from something or it's just unlucky failure. I thought they only switched EV200s in MeXy.

Yes, only EV200s, with their economiser circuits that are supposed to take care of back-EMF. 5 of them on the "A" relay and 6 on the "B" (although they had 6 and 7 for most of their life, when we had separate contactors for the DC-DC converters). The relays are rated at 30 amps from Supercheap Auto.

I'm going to assume the relays were fine but the economisers are less than perfect in that regard, and so I will add diodes to chassis from each relay contact (switched +12 V), effectively across the EV200 coils. I'll replace both relays.
Last edited by weber on Tue, 28 Oct 2014, 09:31, edited 1 time in total.
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Post by Johny » Tue, 28 Oct 2014, 21:13

Sorry to be a pain but could you put a CRO on the output before you do that and see what is going on?

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Post by weber » Tue, 28 Oct 2014, 21:18

Johny wrote: Sorry to be a pain but could you put a CRO on the output before you do that and see what is going on?

That's a good idea, but I probably won't get to it any time soon.
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Post by Johny » Tue, 28 Oct 2014, 21:28

weber wrote:That's a good idea, but I probably won't get to it any time soon.
That's fine - I might check it out but I have 2 of the EV200s and 2 of SW80s so mine won't show the same data.
Just concerned that the EV200s do something weird as the 12V shuts down - like FET goes effectively shorted so we are looking at the coil almost directly for a few mS - in which case diodes would help. (I have a 3A diode on mine across the SW80s.)

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Post by Damnthematrix » Tue, 28 Oct 2014, 23:26


The police were there, looking after public safety -- by positioning themselves at the safest part of the whole road -- the only long straight -- out of sight -- with a radar gun.

Bastards........ I used to live up there in the 90's, did 190km/hr a couple of times in our Honda Integra on that straight.... They were the days..... I guess with that many bike accidents these days they had to do something...

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Post by weber » Sat, 28 Mar 2015, 06:35

It has been a while since I've posted anything about MeXy the electric the MX-5. But just over a week ago I finally upgraded her tyres as the engineer insisted I must. They were the Mazda standard size of 185/60R14 82H, which have a 475 kg load rating (the "82" code tells us that).

Her present kerb weight of 1350 kg is balanced nearly 50:50 front and rear. But when fully loaded, to the engineer's compliance-plate limit of 1530 kg (two 80 kg occupants and 20 kg of luggage in the boot), each front tyre has 365 kg, and each rear tyre has 400 kg (48:52).

Her original kerb weight was 970 kg and if loaded the same she would have been 1150 kg. So fully laden mass has increased 33%, hence the suspension and brake upgrades, and braking test, as well as the tyre upgrade.

Of all the legal tyre sizes for an NA model MX-5, without changing the stock 14" rim size, the one with the highest load rating is the 195/60R14 86H at 530 kg.

When considering all available rim sizes, the only one better is 205/45R16 87W at 545 kg, only 15 kg more. We had been asking for a luggage allowance of 40 kg, but when I told the engineer (David Blythe) that I didn't want to spend $800 or more on a set of 16" rims*, we agreed to drop the luggage allowance by 20 kg.

*And besides, I like the fact that MeXy looks to be completely stock (unless you look up her exhaust pipe Image)

Although we passed the braking test OK, I always felt the passenger-side front caliper wasn't working properly. I could never make it lock up, while the driver's side did so readily on a panic-brake (no ABS).

So I got my local mechanic (Andrew Burrows of Chandler's Corner Auto) to overhaul the brakes at the same time as replacing the tyres and doing a wheel alignment. When he was done, and after a short run-in period for the brakes, braking felt strong and even, and it was hard to lock up any wheel. I suddenly felt a whole lot safer. Thanks Andrew and Co.

I got the 195/60R14's in a low rolling-resistance type, namely Yokohama BluEarth (AE01). I gave Andrew the wheel loads, and he asked his tyre supplier to recommend the pressures. He said they recommended 260 kPa (38 psi), which I thought was a very high pressure at the time, and he implied that they recommended that pressure all round, which made little sense to me. But he said the main concern was protecting the sidewalls. At the time, I assumed he was worried about the front tyres rolling sideways during hard cornering due to the higher mass.

But last weekend I visited the Coulomb castle, and Coulomb enjoyed a long overdue drive, and on my way home I happened to drive through heavy rain the whole way. On arriving home I emailed the following to Coulomb:
Lots of water on the road on the way home. Some big rooster-tails. Once where I couldn't see a thing for a second or two because the water is sprayed forward by the front wheels and then blows back on the windscreen. But no problems at all. The new tyres felt like they were gluing MeXy to the road. No issues with electronics getting wet either.

That's a good car we built.
I thought to do an insulation test of the hazardous-voltage system about half an hour after I got home, by simply turning the key back to start, and she passed -- no beeps from the IMUs.

The engineer had also pointed out that I needed to change the tyre pressure sticker on the end of the driver's door -- that I was the manufacturer now, and so I must specify what they should be, so there could be no nasty mistakes where someone borrowing the car, or a mechanic, might put the original 180 kPa (26 psi) in them and then drive on a wet or winding road.

So I did some web research on tyre pressures and found one paper, which I can't find now, that claimed that for a given tyre size, scaling of pressure should not be linear with weight, but a 1.25 power, and that it had previously been thought that a 1.7 power was appropriate.

I found that I could only calculate a pressure as high as the recommended 260 kPa (38 psi) by scaling down linearly from the tyre's maximum pressure of 350 kPa (51 psi) at its maximum load of 530 kg, but that was only for the rear tyres. Linear scaling gives 240 kPa (35 psi) in the front and 260 kPa (38 psi) in the rear.

But hey, it had stuck like glue at 90 km/h through some serious water, so who was I to argue! However, before I went ahead and made the new label, I thought I'd better check what was actually in the tyres. Guess what? They were 240 kPa (35 psi) in the front and 260 kPa (38 psi) in the rear! That clinched it.

So here's a photo of the new label. It's printed on plain paper, but sandwiched between two layers of Contact-brand clear self-adhesive polypropylene, and stuck over the old label.

Image

[Edit: Improved grammar and added clarification. Added thanks to Andrew (mechanic).]
Last edited by weber on Sat, 28 Mar 2015, 10:40, edited 1 time in total.
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Weber and Coulomb's MX-5

Post by offgridQLD » Sat, 28 Mar 2015, 14:53

Nice,
     I guess that investment in new rubber marks the end of the smoke signals on exit from now on Image

Years ago when I did a defensive driving / track day the first thing the guy mentioned to most people was to forget what the factory tyre plate said and had me put about 40psi all round. That was 195/50/15 on a stripped down 700kg fwd car.

I have had few what if scenario's going through my head in super heavy rain in the 360v eom imiev. I'm glad you guys know what your doing playing in puddles with double that voltage.


Kurt
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Post by weber » Sat, 28 Mar 2015, 21:24

Thanks Kurt. Yes, I had my fun with the old tyres. I have to be a good boy now. Image

And yes, the only safety effect of over-inflation (provided it is less than the rated maximum) is the slightly increased risk of damage to the tyre on impact (e.g. with a sharp edged pothole). The other bad effects of overinflation are merely economic and comfort-related -- uneven wear (more in the middle) and harsher ride.

On the other hand, under-inflation definitely has major safety concerns as it can lead to complete failure of the tyre due to fatigue, and to aquaplaning in the wet, in addition to the economic consequences of uneven wear (more near the edges) and greater energy consumption / shorter range.

So, best to err on the side of over-inflation initially, and keep an eye on the wear pattern. And sure, for a track day, uneven wear, impact damage and a harsher ride are irrelevant, so go up 10% or 15% (but of course, not more than the max printed on the sidewall).

Almost all EV converters need to know how to determine new tyre pressures to compensate for increased weight. Thank goodness I found that paper again (linked in an email I sent to Jeff Owen a year ago).

There is so much conflicting opinion out there. This paper is the only thing I've found that appears to be based on empirical evidence from credible sources. Thanks Doctor Daws! BTW, all the tables and figures are at the end.
http://www.dawsengineering.com/linked_f ... dloads.pdf

And in case you want to argue with him, check out his CV first.
http://www.dawsengineering.com/linked_f ... ae-web.pdf

My summary of the paper is: To maintain the same deflection of the tyre, the pressure should be scaled to somewhere between

Pressure = MaxPressure * (Load / MaxLoad)^1.25

and

Pressure = MaxPressure * (Load / MaxLoad)

where MaxPressure and MaxLoad are read off the sidewall, and Load is determined by scales under the wheels, with the vehicle loaded with occupants and luggage.

The 1.25 power applies to small tyres, say 13" and smaller, and the 1.0 power applies to large tyres, say 22" and larger, with others in between.

For example, in my case I have the sidewall info from the Yokohama BluEarth (AE01 model) 195/60R14:
MaxPressure = 350 kPa (51 psi)
MaxLoad = 530 kg

And for the front wheels, when fully loaded, I have measured
Load = 365 kg.
Since it's a 14" tyre I use a power of 1.2 and get
Pressure = 350 kPa * (365 kg/530 kg)^1.2
= 224 kPa (32 psi)

For the rear wheels I have
Load = 400 kg, so
Pressure = 350 kPa * (400 kg/530 kg)^1.2
= 250 kPa (36 psi)

So, if I find they are wearing faster in the middle at their present 240 and 260 kPa (35 and 38 psi), then I'll try dropping them down to the above calculated values, and change my door sticker again.

[Edit: Grammar]
Last edited by weber on Sat, 28 Mar 2015, 12:39, edited 1 time in total.
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Post by weber » Sat, 11 Apr 2015, 17:44

I've been having a wonderful time with MeXy this past week, giving joy rides and drives. In particular to two people who helped in the early stages of her development, but weren't aware that she now had her full design power. Namely Mho (Ross Pink) inspirer of the whole project, through his own induction motor van conversion, and Pascal (Warrick Beattie) designer of MeXy's taperlock flywheel, and designer and maker of her adapter plate.

In the process, Ross proved (while taking his wife for a drive) that MeXy is capable of a 160 km range. But first he and his son Anthony proved (while taking turns with me as passenger) that even when driven like a bat-out-of-hell on the Mt Nebo road she will still do 120 km. Image

Prior to that, MeXy had been off the road for a couple of weeks with a dead cell. It had gone down to 1.5 volts through an unusual combination of events and needed to be replaced, and of course it wasn't in either of the two readily accessible battery boxes (of the 10). But that's a story for another post. Suffice it for me to thank my son Hunter for helping me take the battery box out, and Kurt (offgridQLD) for helping me put it back in again. I want to tell a different story here.

On the day in between Mho's and Pascal's drives, I realised that I had never driven MeXy on the Mt Nebo road myself with full power and the confidence that comes with new tyres and a brake overhaul. So off I went, by myself, late in the afternoon. It was a week-day so the traffic was minimal.

This is the kind of road the MX-5 was built for, and the regen setting in second gear is just perfect for one-pedal-driving, although one does have to be careful with strong regen. Coming into one corner I decided regen had not burned off enough speed and I hit the friction brakes and there was some brief rear-wheel drift. But one of the fun things about the MX-5 is its controllabilty in oversteer. 33% more weight seems to have made little difference to that. It was so much fun I did not want to stop at Mt Nebo. I went on to Mt Glorious and decided to go for a walk in the Maiala rainforest.

It was near dusk as I crept to the far end of the carpark. I passed one other car, a station wagon. A little later, as I walked back from the composting toilet ("If you didn't eat it, don't put it down the chute") I caught a glimpse of the car's sole occupant at a picnic table in the distance.

Miala rainforest has been something of a pilgrimage for me, since childhood. By the time I had walked a hundred metres into it I felt enclosed in a safe dark cocoon, surrounded by catbird calls and rustlings in the undergrowth, and I knew that this forest had been here, much the same, for hundreds, maybe thousands of years, and it barely knows that humans exist. Such balm for the soul.

For some wonderful photos, scroll halfway through this page:
https://mildlyextreme.wordpress.com/tag ... ainforest/

But this time, before entering the rainforest, I had been confronted by a sign, with photographs, announcing that it was infected with "Myrtle Rust" a fungal disease "easily spread by human activity". And so an imaginary conversation with the other occupant of the park formed in my mind. I imagined him approaching me along the track as I watched the last rays of sun glint off what looked like chips of mirror on the bark of a tree (they turned out to be the wings of small insects). It went like this:

Me: Bloody humans. They ruin everything.

Him: F**king tree-hugger! And I suppose you drove up here in a solar powered electric car?

Image

After my walk, as I drove slowly out of the carpark again, the real person who had inspired my imagined antagonist, was putting things away in the back of her station wagon. This motherly-looking woman turned and gave me the most beautiful smile. It was then that I realised I had a stupid grin on my own face, which may well have prompted hers. Still smiling I said "Lovely isn't it". Still smiling she replied "Yes. It is".

[Edit: Spelling]
Last edited by weber on Sat, 11 Apr 2015, 19:48, edited 1 time in total.
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Post by weber » Sun, 12 Apr 2015, 18:21

The nice thing about the Mt Nebo road is that you can have fun without breaking the speed limit. In dry weather you just double the numbers on those yellow advisory signs before the corners. Image

But that wasn't good enough for my evil twin Maxwell. Last night as he pulled onto a 3 lane 100 km/h divided freeway he couldn't resist moving to the clear right lane and winding MeXy out in 3rd gear before immediately backing off and regenning back to 100 km/h and staying there. It was probably all over in about 6 seconds (3 seconds up and 3 seconds down). But bad luck for him, a police officer witnessed the whole thing, because a little while later he heard a siren and saw the red and blue lights in the rear view mirror.

He pulled over, thinking "Yikes this is going to be expensive. Will I lose my license?". Of course he cooperated fully, stayed seated, handed over his license, blew in the breathalyser. But when the officer tried to get him to estimate his speed he declined, saying he wouldn't like to guess. He denied having looked at his speedo at the time, although he admitted he'd given it a quick burst over the speed limit, "just for fun". The officer said he'd been forced to exceed the speed limit significantly to catch up to Maxwell.

He looked up Maxwell's history of speeding tickets and gave him a spiel about road safety and the unnecessary deaths on the roads over Easter, to which Maxwell agreed solemnly, but in the end the officer said he wasn't going to give Maxwell an infringement notice because he hadn't been able to measure his speed at the time he was actually speeding. "But next time, do it on a race track". Maxwell replied, "I really appreciate that, and now that we've got that out of the way, you might be interested to know it's an electric vehicle."

The officer said, "You mean this is electric?". Maxwell said "Yes". And the officer couldn't help smiling as he said, "Jeez, It goes alright for an electric car!" Image

Mexy the electric MX-5 -- breaking EV stereotypes -- one speeding ticket at a time. Image

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Post by weber » Mon, 20 Apr 2015, 00:35

Since my last post, MeXy has given joy rides/drives to three more of her contributors, Newton (Jeff Owen) the brains behind most of the mechanical engineering that went into her, and Tritium engineers James Kennedy and Alex Rudzki who tested and supplied the brilliant 750 volt WaveSculptor motor controller and the two open-source EV Driver Controls units (EVCUs), and helped us whenever we managed to screw things up with our software or our motor tuning parameters.

Two posts back I mentioned a destroyed cell. It's time to tell that story. Like all great engineering disasters, three or four things had to go wrong at once to bring it about. Not that this was a great engineering disaster.

One Sunday night, many weeks ago, I convinced my wife to let me drive us in MeXy, to a birthday party for her mum at Pinjara hills. Regular readers of this column may remember what has happened on other occasions when my wife has been a passenger. The word "Jinx" comes to mind. Image

MeXy was not fully charged, but the "fuel" gauge showed about 70% and it was only a 35 km round trip (MeXy has easily a 130 km range). We got there without incident, but on the way home I noticed that the cell stress gauge (re-purposed oil gauge) was showing increasing stress spikes on acceleration and hills, and soon our EVCU software began to dial back the WaveSculptor's power limit. As soon as I could do so safely, I pulled over to consider our options. The fuel gauge showed about 60% and yet at least one cell was clearly going undervoltage. How could we be running flat with the gauge showing 60%?

It was around 9 pm. Janelle resignedly suggested we just phone the RACQ and get a tow-truck, but I said "Where's the fun in that?". We were near a major shopping centre, and I remembered Jeff Owen's stories, so we crawled into the shopping centre and crawled around the outside looking for power points. Way around the back of the shops, where normal people don't go, we found one and plugged in. I went to find a loo, leaving Janelle with the car.

A little while later a cleaner came out, looked at the lead coming out of MeXy's closed boot, followed it with his eyes down to the ground and up to the power point, and said, "What's going on here?". Janelle replied, "It's an electric vehicle. We just need a little power to get home." I came back and said with a smile, "Yeah, it's an electric vehicle, want to see under the bonnet?". He replied with the immortal words of Sargeant Schultz, "I don't want to see anything. I didn't see anything.", and went back to work. Image

We noticed there was a security camera, with its ring of infrared LEDs, looking pretty much straight at us. I figured we needed to charge for an hour to get home, and thought it was best if we weren't there if someone else discovered the lead, so we went for a walk to circumnavigate the shopping centre looking for other power points in case we got moved on. And indeed, when we got back, the cleaner was waiting, and said he was leaving now and asked if we would please do the same. So we moved to where we'd found a DVD vending machine plugged into a double power point, and plugged in beside it.

At one stage a security vehicle came past and stopped for a long hard look. The charging lead was not visible from their point of view, Janelle was reclined in the passenger seat with her eyes closed and I was about 20 metres away studiously pretending to use my phone. They drove on. After the hour's charging at 10 amps (240 Vac) we drove gently home without further incident, and with only the tiniest indication of cell stress as we drove up the final slope. Hoorah!

I did not plug it in to charge when I got home, because I suspected a badly out-of-balance cell and a complete charge would have balanced it. Although that was kind of silly because every CMU (Cell Management Unit) records its cell's lowest and highest voltages and temperatures until manually reset via a laptop.

Because of various other commitments it was almost a week before I got to look at the problem. I hooked up the laptop and was told by the CMU on cell A62 that it had not heard from cell A61 for some time. And when I finally got physical access to A61 I found it at 1.5 volts (too low for its microprocessor to operate).

Any lithium ion type cell that has been below about 2.0 V (continuously in its rested state as opposed to briefly under load) loses capacity and becomes an unpredictable danger, and so it had to be replaced. Why? Search on "copper" in this document:
http://americansolarchallenge.org/ASC/w ... ontrol.pdf

The cell had also bulged with pressure due to whatever non-condensing non-dissolving gasses are formed under these conditions, presumably by breakdown of the electrolyte or its solvents. It beats me what the supposed vents on the tops of these (Sky Energy) cells are for, since they never seem to relieve this pressure. So I needed to completely remove the battery box to undo the cell-row clamping-rods sufficiently, to get the cell out. Cell A61 is in the forward fuel-tank box, and this requires first craning out the rollbar and its battery box. Thanks again to Hunter (my son) and Kurt (offgridQLD) for helping me with this process.

Image

Image

The question of course is how did it get down to 1.5 V? It clearly wasn't anywhere near that when we got home. We'd have to have been crawling the last several kilometres in turtle mode.

I will eventually describe all the compounding causes, but the final stage of its demise was "Death by CMU". As I jokingly said to Coulomb, "I hate it when Jack Rickard is right". Image We clearly have a problem with the MOSFETs that we use to switch the bypass resistors on and off, because they occasionally and randomly go leaky and start drawing about 10 mA from the cell (averaging about one failure out of the 218 every 6 months). Normal CMU current is only about 1.6 mA. So as well as the cell, I had to replace the MOSFET on its CMU (an Si2312BDS).

Any ideas about the cause of this in-service onset of leakage, and what to do about it, would be welcome. The MOSFET simply switches 6.6 ohms of resistors across the cell, via a fuse and a diode. It has a 47k gate pulldown and its gate is driven directly from the MSP430 microcontroller powered from a regulated 2.5 V supply.

A clue to the second partial-cause came when I asked my recent off-grid solar power system customer (the one with Black Monolith #1) how his system had fared during recent consecutive days of overcast weather. He said the State-of-Charge meter had never been below 80% and "Could there be something wrong with the meter?". Combine this with my own question, "How could MeXy be running flat with the fuel gauge showing 60%?", and a light-bulb came on (a highly efficient LED light bulb Image).

The Black Monolith uses the same BMS as MeXy, including the IMU (Current Monitoring Unit), but because of the much longer time-constant of its PI-control-loop (which, in the end, we couldn't use), it samples the current-shunt and sends status-bytes at a leisurely 1.9 Hz compared to MeXy's frenetic 15 Hz. Because of this, I modified the coulomb-counting code to handle different update rates differing by powers of two. The coulomb-counter is reset at full-charge (when all cells are bypassing) and so it really measures Depth-of-Discharge (DoD), which is converted to State-of-Charge (SoC) for display by subtracting it from 100%. During testing of the Monolith's coulomb-counting code, I decided it was overestimating the Depth-of-Discharge by a factor of two, and thought I must have been out by one shift-instruction, and so I "fixed" it. This code was later propagated back into MeXy.

So what was happening with both MeXy and the Monolith was that they were underestimating the DoD by a factor of two. 50% SoC on the gauge was really 0% SoC. And 60% on the gauge was really only 20% SoC.

The third contributing factor was my failure to fully recharge, and hence balance, the "A" half-pack for weeks prior to the run-flat, which allowed cell A61's leaky (10 mA) MOSFET to pull it 20% below the other cells. But why didn't I fully recharge the A-pack?

Contributing factor number four is the fact that even when MeXy is not being driven she is programmed to turn on the traction battery contactors as needed, so the DC-DC converters can recharge the 12 V auxiliary battery. Except that I still haven't got around to replacing the microcontroller in the EVCU for the "B" half-pack with one whose analog section isn't blown up, and so it fails to participate in these events, and only the "A" half-pack expends itself in maintaining the auxiliary battery. This leads to the A-pack taking longer to charge than the B-pack, particularly since MeXy was not being driven very often back then, and when I did decide to drive her I was impatient.

So there you have it. Behavioural lesson: Fully recharge your EV at least once a week so balancing can compensate for any differences in discharge between CMUs. Technical lesson: Find out why these MOSFETs are failing, or failing-that, just replace them with devices having better values of all the specs that could matter. And otherwise try to ensure that all CMUs always draw the same small current.
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Post by offgridQLD » Mon, 20 Apr 2015, 01:37

Battery SOC meters interesting.

On my offgrid lithium bank I unintentionally ended up with three SOC meters ( all working off the same shunts) on the one system. I have one critical meter I trust and use every day. Then then a 2nd on the inverter and 3rd on the BCU display. They never totally agree but they are always close to one another (within 5% SOC usually less) It's nice to cross reference them now and then.

If I'm understanding it correct. You infected you MX5's battery managment unit with PIP4048 DOD/SOC programing giving false high SOC Reading's?


If so it reminds me of a Air crash investigation Documentary I watched. Where a mantanance mechanic installed the wrong fuel gauge in the passenger jet. Turns out it was for other model jet that used a different sender unit. The result was a false high fuel level reading. Combined with human error while doing manual calculation cross cech of the fuel tanker docket numbers it ended in disaster.

They made changes so the guages could not phisicaly fit in the wrong aircraft. So even with double redundancy things can go wrong. Perhaps that 3rd SOC meter will save me one day .

Perhaps you need to do the same so each battery management unit has some kind of ID that only accept the correct programming.

Kurt



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Post by weber » Mon, 20 Apr 2015, 02:40

I love Air Crash Investigation, although I have mostly only watched the ones where everyone survives, usually due to the cleverness of the crew. Image But I tracked down the one you mention, and will watch it.
https://www.youtube.com/watch?v=Z_yW1d7Ij-w
offgridQLD wrote:If I'm understanding it correct. You infected you MX5's battery managment unit with PIP4048 DOD/SOC programing giving false high SOC Reading's?
No. It's not really like that. If I had accidentally loaded the Solar Power System BMS software into the EV, or vice versa, it would have been immediately obvious. For one thing, there's the factor of eight difference in the flash rates of the heartbeat LEDs.

Many source code files are shared between the two projects so they are easier to maintain. A bug in the common code only has to be fixed in one place. In this case a bug was introduced into the common code at the time when it was first made to be common. When I learned that both the Monolith and Mexy were giving false high SoC readings, I knew immediately the problem was in the shared code and easily fixed it. So that's not an ongoing issue.

What is an ongoing issue, is that small MOSFETs that test OK at CMU assembly time may decide to go leaky after 6 months or a year or two years in service, or never. And why do they only ever leak about 10 mA. One theory I have is that due to cumulative damage from very fast voltage spikes induced by RFI, and the fact that they have "only" a 20 V rating, they are leaking drain to gate, and the 10 mA is the maximum that the micro can sink via the pin that's pulling the gate down. Needs further investigation.
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Post by T1 Terry » Mon, 20 Apr 2015, 03:22

The way to get a puffed up cell out of a pack is to peel the label off the valve cap and remove it. It will pop just as the thread runs out on the valve cap, well that is he case with Winston cells anyway. Now take the cone spring out and refit the cap. With a wide flat plate work it down the sides of the cell to compress it back into shape, that the valve cap off again and refit the spring. Pull the plates out and the cell should come out with them, a strap made from a doubled length of tie down ratchet webbing, melt a bolt hold through the 2 layers with a heated Phillips screw driver and use a piece of flat aluminium to act as a big washer each end and refit the terminal bolts. The whole thing, cell and flat plates will slide up and out, the new cell drops back in as there is plenty of room for it made by the swelled up cell.
It is incredible how much pressure the cells can apply when they start to swell, in one of my house cell packs 3mm thick 50mm strap was stretch across the width of a 100Ah cell(143mm) between each cell group separator bar until it snapped the welds or tore a piece of the strap out if the weld held fast. In the end 2 cells actually exploded the upper section of the plastic case completely out.
Why do the cells bulge rather than the gas escaping through the valve? The plastic becomes very soft when heated, it's the heat that boils the electrolyte that forms the gas. A few of the cells actually flattened out the ridges in the case as they couldn't expand any more because of the other cells holding them flat and straight, the top ballooned and so did the base.
Solar was connected virtually directly to the battery for 6 weeks, 22 of the 32 cells still work, they are tough buggers, but they will not survive even a small reverse charge.

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Post by weber » Tue, 21 Apr 2015, 01:04

That's great information T1 Terry. Any photos of the damage?

We certainly could have used the idea of the webbing-strap handle bolted to the cell terminals, to lift the cell out more easily. And the idea of releasing the pressure in the cell before doing so.

I agree that if the plastic case has been softened by heat, it is not surprising that it may balloon out before the valve vents the pressure. But it is not merely boiling of the electrolyte that causes the pressure. If that was the case, after cooling, it would re-condense and there would be no residual pressure. It is clear that, as I wrote earlier, a non-condensing non-dissolving gas or gasses are produced by breakdown of the electrolyte. These may include Oxygen, Carbon monoxide, Carbon dioxide, Methane.

In my case there was no heat involved. The gasses were caused by undervoltage, not overvoltage or overtemperature. The destructive overdischarge of the cell took place over several days at a slow rate. In this case I suspect the breakdown of the electrolyte solvent into non-condensing gasses is catalysed by the copper ions from the copper-foil current-collectors dissolving in the electrolyte.
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Post by T1 Terry » Tue, 21 Apr 2015, 02:22

The destructive heat is generated at both ends of the voltage curve, just far more destructive at the lower end. A cell can handle a few trips over the 4v mark and live, but 1 trip below the 0v mark and it's dead. I guess the difference is when over voltage charged there is a shortage of some where for the lithium ions in the electrolyte to go as the graphite is saturated that cause the heat generation, in the low voltage side all the lithium ion are stripped out of the graphite and then stripped out of the electrolyte itself as the natural voltage of an LiFeP04 cell is some where around 1.5v and it will destroy itself trying to maintain that voltage. Once the lithium ions are stripped from the electrolyte and the electrolyte heating separates the higher volatility hydrocarbons, the material these hydrocarbons hold is suspension are coated on the electrode that the lithium ions are move into. When charging the ions are settling into the leaf structure of the graphite, when discharging the ions enter the porous structure of the iron phosphate material. Once they are all out of the electrolyte and the residue from the separated electrolyte coats the now rich lithium iron phosphate electrode, those ions are trapped, there are no ions to do the interchange through the electrolyte that is part of the heat stabilization process, so the heat goes into a run away and eventually leads to internal shorting between the plates are sections build up and penetrate the separator. With an over charge there is high current available to burn off the first tips of these bridging pieces of dendrite for want of a better word, but in a low voltage build up that energy is no longer there, the dendrite simply links the 2 plates and the cell now becomes an electrolyte heater no matter which way the energy is flowing. even 1 plate with a few bridging pieces will lead to very rapid cell destruction.
Some photos of toasted cells, not a pretty sight,   ImageImage

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Post by offgridQLD » Tue, 21 Apr 2015, 02:40

I'm dizzy trying to run through that paragraph. I would need a diagram to go with that one Image.

Though I disagree with the cell heating due to low voltage particularly as Webber mentioned it was taken down very slowly.

I have played around with lifepo4 cells for a while now (small cells that I could kill and not worry about the cost) I have not witnessed any heat when they go low voltage even draining the cell to 0 volts.

They might get hot if you reverse the cell and try and charge it up again.

Overcharging they sure get toasty!

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Post by T1 Terry » Tue, 21 Apr 2015, 03:05

Trust me, the cells get just as hot at the bottom end as they do at the top end, watch a few of the serious discharge tests and see how long it takes for the cell to heat up and the electrolyte start to vent, no reverse charging there. The slow discharge may not build as much heat rapidly so the cell can dissipate that heat reasonably well, but the heat is still generated between the plates and that is where the damage is occurring. I have a collection of murdered cells now, I think 12 are from zero voltage discharge, the others toasted to death, these things happen when you test so many things for such a long period, always pushing the envelope to determine just where the boundaries lie and building cheap off the shelf and simple to assemble control systems to keep the cells in the safe zone. One rather expensive error was using a system that required the load cut to be energised, the logic behind it was to minimise phantom loads so the cut off device was normally closed.... hind sight is wonderful thing, when the voltage drops too low to hold the load cut device open, it closes again Image
Lesson learnt, it is better to loose a little to the phantom load rather than the whole battery pack.

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Post by Adverse Effects » Tue, 21 Apr 2015, 03:49

weber wrote:I will eventually describe all the compounding causes, but the final stage of its demise was "Death by CMU". As I jokingly said to Coulomb, "I hate it when Jack Rickard is right". Image


Image Image Image Image

and for thos that dont know what he is talking about here is jack talking about it this week in his EVTV show

EVTV Friday Show - April 17, 2015 EVTV Friday Show - April 17, 2015 (starting at 1 hour and 7 min in)

i just think that he was talking about it again at this time is funny

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Post by offgridQLD » Tue, 21 Apr 2015, 04:16

It was funny. Well not funny no one likes a dead cell. But I did think of Weber's post when Jack mentioned it.

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Post by weber » Tue, 21 Apr 2015, 05:38

Thanks for the link, Mr Effects. Unfortunately Jack's "solution", to the problem of cell monitoring units (CMUs) occasionally failing and discharging a cell to death, is only practical when the number of cells is small, and all cell terminals are accessible.

The idea of working my way through MeXy's 218 cells, most of which are inaccessible, with a resistor and a meter on a pair of alligator clips is nothing short of ludicrous.

And can you point me to where he describes how to terminate charging of a bottom-balanced BMS-free 218-series-cell pack without any cell going overvoltage? Preferably in writing, not video. Life's too short. Image

With the help of others on this forum, and elsewhere, I'm sure we will solve this problem, as we have every other one, and produce a robust and reliable CMU.
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Post by Adverse Effects » Tue, 21 Apr 2015, 07:38

his method is simple

if a cell has a max V of 4.2 (liop's that i use) and min V of 3.0

you would bottom balance them all to 3.0

then if the max V is 4.2 you never charge them to that you would only charge them to 4.1 (the extra 0.1V is almost nothing in Wh of storage)

so you get a charger that will stop charging at (4.1 X 218 cells) 893C to 894V

and you would stop driving when your pack voltage at rest got down to around 676VDC (about 3.1V X 218 cells) not that you would ever realy push a EV that far (or shouldn't)

No balancing No floating no nothing you solely run off a full pack voltage (there is a PS:- in there of "NOTHING A CAN DRAW FROM ANY CELL OR CELLS UNLESS ITS THE TOTAL PACK NOT JUST SOME")

this way no cell will drift and you will never need to re balance the pack again

thos numbers are just off the top of my head but are close for the type of battery i use they will be different for other types of chemistry

i do this (just over a year now) with my 6S 5800mah and have yet to lose a cell , have them go out of balance , explode not to mention i am getting almost 3 times the amount of charge cycles the other guys i fly with are getting

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Post by weber » Tue, 21 Apr 2015, 08:13

Right. So if one cell in the bottom-balanced pack has slightly lower capacity than the others, so that it is completely full when the others are very nearly full at 4.09 V, then with your 6S pack, the lower capacity cell will be at
6 * 4.1 - 5 * 4.09
= 4.1 + 5 * 0.01
= 4.15 V
No problem.

With my 218S pack the slightly-lower-capacity cell will be at
218 * 4.1 - 217 * 4.09
= 4.1 + 217 * 0.01
= 6.27 V
See the problem yet? Image
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