Another EV bike :)

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Grumpy-b
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Joined: Sat Dec 01, 2007 12:06 pm

Re: Another EV bike :)

Postby Grumpy-b » Fri Mar 07, 2014 8:39 pm

On EVTV JR was commenting about the Tesla having a small fuse connecting each cell to its common joining plate. That sounds like a brilliant idea, as the Valence Battery I blew up had around 25 cells welded to each of 4 plates (To get to the 12v +)and when one went short circuit it overheated and basically took the battery into meltdown. Not a pretty site. The Tesla solution very simply takes that problem away.

Grumpy-b

ChrisBarron
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Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Fri Mar 07, 2014 10:54 pm

I have some fuses coming for the balancer leads, but not for each of the cells. Had to refresh my knowledge regarding failure current and breaking current. I've selected devices suitable for 300A breaking current and 10A failure.

The cell which 'exploded' due to a screw being misguided when I fixed the side plate, gave a noticeable pop when it let out it's electrolyte and a lot of heat. Then I was glad of the sideplates ! Needed to wash off the electrolyte because it's condictive, I think to the point that it could cause alopng duration leakage problems

The important thing was that it failed open circuit, more or less, and when removed it had a standing voltage of 0.8V. The neighbouring cells did not continue to pump current into it and cause the feared runaway condition, so a massive failure of a single cell is readily contained within the physical structure of these packs and doesn't seem to propogate to any neighbouring cells.

I guess this is the benefit of using such low capacity individual cells, the whole pack inherits an element of safety which a pack made oup of larger cells loses, due to it's the greater ability of a single cell to dump, and absorb current.

I think a fuse on each cell is a good idea if you are making a product for general purchase, to the 'I don't wanna have to do anything but drive it' section of the market, due to their litigious deftness ;) Showing due diligence in every possible way reduces the potential for success of a 'they should've expected and planned for that kind of failure' type of claim.

I did connect an 11A resettable fuse across one pack. It's thick connecting lead vapourised immediately....which reminded me to rate my fuses for a suitable breaking current !

Chris

Grumpy-b
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Re: Another EV bike :)

Postby Grumpy-b » Sat Mar 08, 2014 9:57 am

This is interesing,
https://www.youtube.com/watch?v=O8R0ig0J4wk only the texans would think of such an innovative way to test a battery pack. Not an approach I would have immediately though of!
My own experience of these cells is that whilst this specific chemistry cells dont burn they do get seriously hot and when they get hot the outer battery casing gets melted, and the guts of the cells get extruded out with great force, either through the end or out the side. I found carbondust and silver foil around two metres away.
I will take some shots of the offending battery . I still have it as a reminder of what a low voltage on one of thes packs actually means.
The 12+ v pack initially read just under 9v but responded immediately to a small current and crept over 13v encouragingly. Great, all was not lost I thought. I left a 125a pack on around 4amps for a number of hours, and I guess that was just too much for the cell that was shorted. It was effectively taking down the cells in the parallel layer around it at a low rate but when a higher current was passed through it it just gave up and made a great heat source.
So its not just the separate layer that benefits from the fuse but as Tesla have found if you take out the duff cell you take away the potential problem. Yes the approach and necessity is the difference between a self made project and a commercial enterprise.

Keep up the trials.

Grumpy-b

ChrisBarron
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Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Sat Mar 08, 2014 11:42 am

Grumpy-b wrote: I left a 125a pack on around 4amps for a number of hours, and I guess that was just too much for the cell that was shorted. It was effectively taking down the cells in the parallel layer around it at a low rate but when a higher current was passed through it it just gave up and made a great heat source.
Grumpy-b


When you say you had a 125a pack I presume it to mean that it was a 125Ah pack ? or was it a pack with a maximum discharge current of 125A ? Is it a comparable pack to using 18650 cells, IE, not prismatic 'black' cells with capacities over 10Ah ?

Would yiou say that that pack, whatever it was made up, failed instantly without warning ? I'm interested to know how yoiiu came about having that particular pack, was it in a vehicle ? if it was in a vehicle was there any sort of monitoring ? At the moment I'm just presuming that yoiu didn't stumble across a random pack, but built one up or bought one, and then I wonder what it's life was like immediately before you put 4A on to charge the pack. ?

If the pack was a 125Ah pack, 4A of charge current equates to c/31.25, which i don't think is excessive. You say one cell was shorted, was this a pack of series cells ? I guess not unless yoiu were using large prismatics, T-Sky or similar ?

I'm just trying to draw parallels (!) between the pack yoiu're describing and one which might be quite different. They do have potential risks associated with them but the risks might well be different and therefore demand different precautions in terms of monitoring and management.

Thanks,
Chris

ChrisBarron
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Re: Another EV bike :)

Postby ChrisBarron » Sat Mar 08, 2014 4:14 pm

I have assembled the balancer leads now and connected them all together on an interface board.

This forum isn't allowing me to post any more photos, so here's a link to Flickr
http://www.flickr.com/photos/28496053@N04/13011898075/

Now that all of the 'layers' within each of the 4 packs is connected in parallel they're all balancing themselves up to the same voltage and I have also connected a charger onto the bottom 'layer', to see how long it takes to top off the cells and bring them all to the same voltage.

Closeup of the plugs (also showing clear heatshring on the board as a precaution !) http://www.flickr.com/photos/28496053@N ... otostream/

The coloured twisted pairs going into the side of each pack are connected to the thermistors. They have their own breakout socket on the connection board in the first photo.

Underneath the sockets the socket pins are all connected together with 2.5mm^2 copper, good for 27A ayt 250Vac all day long, photo here http://www.flickr.com/photos/28496053@N ... otostream/

I can get going with the voltage monitor now, all of the ordered microcontrollers have arrived. I'ld like to use a Raspberry Pi as an information centre, but they can be quite power hungry and take a relatively long time to boot up. I suppose I could leave one switched on, with auto shutdown enabled if it detects low pack voltage ?

I also need to start making up enclosures for these battery packs, as well as measure up and make a pattern for a motor mount.

I have the 24 Headway cells (38120s) to include too. They need to be connected in parallel pairs. I've had to buy low voltage microcontrollers to monitor them because of their lower minimum voltage compared to the 18650's

Grumpy-b
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Joined: Sat Dec 01, 2007 12:06 pm

Re: Another EV bike :)

Postby Grumpy-b » Sat Mar 08, 2014 5:35 pm

I will try and take some shots of the Valence battery pack. The valence packs are made into 12v analogue casings with part of an onboard BMS (It needs other parts) its made up of over 90 separate cells like yours, in four layers. But thats more self evident if I take some photsos. I opened up another one that was completely dead and had at some time got water in it, but it shows howe they are constructed.

Grumpy-b

ChrisBarron
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Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Sat Mar 08, 2014 5:59 pm

Grumpy-b wrote:I will try and take some shots of the Valence battery pack. The valence packs are made into 12v analogue casings with part of an onboard BMS (It needs other parts) its made up of over 90 separate cells like yours, in four layers. But thats more self evident if I take some photsos. I opened up another one that was completely dead and had at some time got water in it, but it shows howe they are constructed.

Grumpy-b



What's their hit rate like at constructing good packs ? So far you've only talked about problem Valence batteries... :)

What causes them to fail though ? Are they being overstretched in any way ? Is there some sort of monitoring which could have been added to them in order to detect the onset of a fatal fault condition ?

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timpootle
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Re: Another EV bike :)

Postby timpootle » Sat Mar 08, 2014 8:29 pm

ChrisBarron wrote:This forum isn't allowing me to post any more photo...


Oops, yes. Sorry Chris. That wasn't just you, it was everyone. I have upped the disk quota for photos from 70Mb to 700Mb. That should keep us going for a while.
Tim Crumpton

ChrisBarron
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Re: Another EV bike :)

Postby ChrisBarron » Sat Mar 08, 2014 8:51 pm

timpootle wrote:
ChrisBarron wrote:This forum isn't allowing me to post any more photo...


Oops, yes. Sorry Chris. That wasn't just you, it was everyone. I have upped the disk quota for photos from 70Mb to 700Mb. That should keep us going for a while.



Thanks Tim.

I always try to resize my pictures down to a few hundred kb's so hopefully that s a few years worth of extension !

ChrisBarron
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Re: Another EV bike :)

Postby ChrisBarron » Sat Mar 08, 2014 8:53 pm

My balancer leads !
balance1.JPG
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balance2.JPG
balance2.JPG (117 KiB) Viewed 7110 times
balance3.JPG
balance3.JPG (145.54 KiB) Viewed 7110 times


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