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4Springs' Brumby

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jonescg
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Re: 4Springs' Brumby

Post by jonescg »

The road is a brutal environment for cells. They really need to be well protected, snug, and unable to rub of fret against anything. Or reside in tension - eventually something gives way, and the opportunities for cascading problems start to pile up. Likely there was a leakage current to chassis? Did you do a check for stray voltages between (+) and chassis? If you do spot something, it's worth checking the current with a decent ammeter (don't forget to swap the cables back once you're done!). Should be well under 30 mA.
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Re: 4Springs' Brumby

Post by 4Springs »

jonescg wrote:
Mon, 06 Apr 2020, 19:01
The road is a brutal environment for cells. They really need to be well protected, snug, and unable to rub of fret against anything. Or reside in tension - eventually something gives way, and the opportunities for cascading problems start to pile up. Likely there was a leakage current to chassis? Did you do a check for stray voltages between (+) and chassis? If you do spot something, it's worth checking the current with a decent ammeter (don't forget to swap the cables back once you're done!). Should be well under 30 mA.
Yes, did a current leakage test. From memory it was a couple of mA. I use the method described in E/ECE/324, regulation N. 100. This test puts a resistor across to the chassis from the traction pack (both + and -). You measure the voltage and calculate the leakage current.
The aluminium cases of the modules are not earthed, or at least not intentionally earthed. The two in the front are in wooden boxes, with the rear ones sitting on Astroturf on top of the metal tray.

It is suspicious that most of the cells I've found so far are from the front two battery modules. So far I've thoroughly checked the 35 cells in the front, and 30 of the cells in the back. 6 faulty in the front (including the two leaking ones), and 2 in the back.
I'm not sure what would be different about being in the front though.

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Re: 4Springs' Brumby

Post by 4Springs »

jonescg wrote:
Mon, 06 Apr 2020, 19:01
Did you do a check for stray voltages between (+) and chassis? If you do spot something, it's worth checking the current with a decent ammeter (don't forget to swap the cables back once you're done!). Should be well under 30 mA.
I've just thought of something.
The cells butt up against an aluminium guide. I push then all up against the lip so that they line up nicely. This is where the cells are sandwiched together. Is that an aluminium cover underneath the Mylar covering the cell? Is it connected electrically to the cell? Could that possibly poke through at the ends and provide electrical conduction through to the guide?

The guide is shown in these photos:
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IMG_0370.JPG
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The 'guide' is the side of the module, and is folded at right-angles to run down the end. In the photo you're looking at the end of the module, and you can see the guides running down the left and right hand sides. The one on the right has a few notches cut out where the BMS wires go through. All the cells butt their corners up against it.

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Re: 4Springs' Brumby

Post by jonescg »

Yes I certainly wouldn't be packaging pouch cells against bare metal, especially not where it might rub. The pouch itself is a composite of aluminium and polymer, but the polymer is vanishingly thin - and effectively peppered with microscopic pinholes. Any moisture or humidity that works its way in will start to oxidise the microns-thick foil and eventually stuff leaks out.
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Re: 4Springs' Brumby

Post by francisco.shi »

I run the cells for at least 10 cycles at 1C from 4.2v to 3.0v and at least 4 cycles at 2C. This was when I was checking the capacity. After that I have used for energy storage for a battery tester but at very low C rates. The battery tester cycles NiMH prius cells. It is mainly used for recovering the energy during the discharge cycle.

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Re: 4Springs' Brumby

Post by 4Springs »

francisco.shi wrote:
Mon, 06 Apr 2020, 19:54
I run the cells for at least 10 cycles at 1C from 4.2v to 3.0v and at least 4 cycles at 2C. This was when I was checking the capacity. After that I have used for energy storage for a battery tester but at very low C rates. The battery tester cycles NiMH prius cells. It is mainly used for recovering the energy during the discharge cycle.
How many cells have you tested? My discharge rates would not have approached 2C, more like 1.5C peak.

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Re: 4Springs' Brumby

Post by francisco.shi »

It is a 60 cell pack.
I have sent the pictures to the cell supplier and I am waiting for a reply. They are on holidays until tomorrow. I will keep you updated.

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Re: 4Springs' Brumby

Post by 4Springs »

jonescg wrote:
Mon, 06 Apr 2020, 19:48
Yes I certainly wouldn't be packaging pouch cells against bare metal, especially not where it might rub. The pouch itself is a composite of aluminium and polymer, but the polymer is vanishingly thin - and effectively peppered with microscopic pinholes. Any moisture or humidity that works its way in will start to oxidise the microns-thick foil and eventually stuff leaks out.
Hmm... Ok,
Any suggestions on how to coat the aluminium to make it no longer electrically conductive? It needs to be thin, but also tough enough to stand up to rubbing.
I guess paint, or a self-adhesive book covering?

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Re: 4Springs' Brumby

Post by jonescg »

I'd coat it in Kapton tape, or at least some kind of contact adhesive, and make sure that you put a layer of something with a bit of give, like thin EPA or double-sided tape. Ideally you would glue the cells in position, but if you want the option of pulling them apart again, these would be the least-worst options.
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Re: 4Springs' Brumby

Post by 4Springs »

jonescg wrote:
Tue, 07 Apr 2020, 08:26
I'd coat it in Kapton tape, or at least some kind of contact adhesive, and make sure that you put a layer of something with a bit of give, like thin EPA or double-sided tape. Ideally you would glue the cells in position, but if you want the option of pulling them apart again, these would be the least-worst options.
The aluminium guide is just a guide for assembly, and then acts as a cover afterwards. It doesn't hold the cells in place once compressed, the cells are held by the compression and by the terminals. The cover is free to move next to the cells - I can slide it around to some extent once compressed. It can move until it touches a cell, then it can't move any more in that direction. So there is not much pressure on this.
I will need to insulate a few square metres of this aluminium, and it needs to be less than a couple of mm thick, if I'm not to go in for a complete re-design.
Folded Perspex would have been fine for this, but there isn't room for Perspex now - it would be too thick.

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Re: 4Springs' Brumby

Post by blwnhr »

Is it possible the lack of mechanical pressure on the cells is causing problems? Or perhaps the rigidity of the pack itself.

The dynamic loads on any component in a vehicle are significant. With no diagonal bracing of the pack it is highly likely there is movement. If a cell moves differently to the adjacent cell it would place significant pressure on the tabs.

It is well documented in research papers that pressure on the cell is important for the cell health.
eg.
Regarding pouch cells "However, robust packaging is required for performance and safety criteria due to their low mechanical stability, which results in them being compressed in the module/pack." A study of the effects of external pressure on the electrical performance of a lithium-ion pouch cell

"The application of external pressure has a significant beneficial impact, more specifically a 19% increase in capacity combined with a significant decrease of 50% of the discharge ohmic resistance." Analysis of the effect of applying external mechanical pressure on next generation silicon alloy lithium-ion cells

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Re: 4Springs' Brumby

Post by T1 Terry »

I seem to remember a few years back that there was a similar issue on the DIY forum regarding pouch cells, but I never followed the thread so no idea if there was a fix or reason established. I don't think they were NMC pouches either, I think they were LiFeP04 chemistry, but I could be wrong.

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EDIT: Found a recent thread that talks about these pouch cells and failures https://www.diyelectriccar.com/forums/s ... ouch+cells but not much response 2 mths later, so .......
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Re: 4Springs' Brumby

Post by francisco.shi »

Can you tell me how many cells are leaking?

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Re: 4Springs' Brumby

Post by 4Springs »

francisco.shi wrote:
Tue, 07 Apr 2020, 10:44
Can you tell me how many cells are leaking?
So far there are 8 out of the 65 that I've checked that have some damage.
2 are leaking outside of the pouch and have burn marks.
4 have burn marks but no external leakage
2 have signs of leaking within the pouch but no burn marks. The aluminium next to them does look like it got hot though. These two look exactly like the original ones I found before assembly, and they were in the first module I assembled, so they may have been like it from the start, but I didn't notice.

The other 130 have no obvious signs of damage, but I haven't pulled those modules apart yet to inspect thoroughly.
Feel free to email me if you'd like any extra photos etc.

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Re: 4Springs' Brumby

Post by 4Springs »

Ok, here we go.
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I reckon this quite possibly my problem!
I tested a few cells, and some display this feature more easily than others. On this one, which is one of the ones that got hot, it was very easy to find a place that would give me a voltage. The part of the cell where I'm getting the voltage is exactly the part that I use to line them all up against the aluminium guide. The aluminium is not not pushed up hard against the cells, so there would not normally be very many contacting, but the aluminium might move under the forces in a moving vehicle. Particularly in the front boxes, where the modules are end-on to the direction of ecceleration and braking. The modules in the rear are rotated 90°.

I reckon I can cover these ends with Kaptan tape, and also line the aluminium with self adhesive vinyl.

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Re: 4Springs' Brumby

Post by jonescg »

*twitches*

polarity... all... wrong... :x
:lol:
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Re: 4Springs' Brumby

Post by brendon_m »

jonescg wrote:
Tue, 07 Apr 2020, 14:37
*twitches*

polarity... all... wrong... :x
:lol:
Nah, that's just how you measure positrons.

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Re: 4Springs' Brumby

Post by jonescg »

Christopher - I would suggest you also check the continuity there. Set the meter to amps and see what you get. If you get sparks, well there's your answer...
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Re: 4Springs' Brumby

Post by francisco.shi »

I had a thought that perhaps the problem is that the aluminium from the pouch is touching something. Then it will conduct some current which will damage the pouch. So I had a check of the cells in the pack from a year ago and had a look and the way the aluminium plates from the cooling system hold the cells has no chance of touching the edge of the pouch.
Then I put the multimeter between the tab and the edge of the pouch where the edge is exposed and I got about 1.5v.
Then I started wondering if the problem is the edge has made contact with the aluminium plates and shorted the inside of the pouches for different cells. This could potentially put a high voltage between the aluminium inside of the pouch to the electrolyte.

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Re: 4Springs' Brumby

Post by 4Springs »

Back to work again for the weekend and I found out some more info.
I checked the current flowing from between random points on the ends of the cells. In most cases it was negligible - so little that just measuring the voltage with the multimeter made that voltage drop over a few seconds. I tried one of the bad cells though (already had a bubble), and it was more solid. I put a 3 k resistor across the voltage and it dropped to 0.3 V. So still a very small current, but more than the others.

I got back to my half-finished pile of cells, where I had been re-packing them into a module, using the un-modified aluminium sides. And I found this:
IMG_0466.JPG
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This is a cell that had no signs of a problem when I assembled it last week. But today there is a very obvious bubble on the top. So this has formed over the past week. In this module there are 4 cells connected together on one end only. I've removed them by the time the photo was taken, but the aluminium sides were in place.

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Re: 4Springs' Brumby

Post by 4Springs »

Here's what I'm doing now, following the advice gained here (mostly from Chris).
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Steel springs now have Kaptan tape on the side that touches the cell tabs (which are either aluminium or plated copper).
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More Kaptan tape, covering the corners of the cells to stop them making electrical contact with the aluminium sides. I've also placed the green stuff (which I've been calling Astroturf, but it probably isn't) in strategic places. Some at the bottom of the pile to both raise it slightly to make absolutely sure it is clear of the fold at the end of the aluminium tray, and also to insulate the broad surface of it from that aluminium tray. Some more between cells, to provide a bit extra height to help have enough room with the connections on the tabs.
IMG_0472.JPG
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Can't really see it, but I've laminated the inside of the aluminium sides with a clear vynyl self-adhesive wrap.
IMG_0473.JPG
IMG_0473.JPG (36.18 KiB) Viewed 425 times

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Re: 4Springs' Brumby

Post by jonescg »

That Brillo scourer material is quite abrasive (if that's what it is).
But don't be shy with the Kapton tape - that stuff can make the difference.
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Re: 4Springs' Brumby

Post by 4Springs »

jonescg wrote:
Sat, 11 Apr 2020, 09:07
That Brillo scourer material is quite abrasive (if that's what it is).
No, not scourer. This stuff is more dense, thinner and softer.

I've spent the past couple of days dismantling and re-assembling battery modules. Re-polishing all the connections and doing up the roughly 300 fiddly little bolts again. Before I finish I'll have put on about 1000 pieces of Kaptan tape, too!
I noticed today when putting cells into one of the smaller modules that the aluminium guide is quite tight. On most of them it's loose, but on this one the cells fit in very snugly. So cells in this one touch both ends onto the aluminium. This also happens to be one of the two modules that had lots of problems. So I took pliers to it and bent the bend to something greater than right-angle.
I'll take this as more evidence to say that my problem was cells shorting out on the aluminium....

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