Another EV bike :)

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ChrisBarron
Posts: 244
Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Fri Sep 20, 2013 7:48 am

The postman arrived with my first lot of cell spacers. I bought just a few to play with first before taking the plunge.

http://www.flickr.com/photos/28496053@N04/9832511044/

The weight for 12 cells is 580g, working out at about 48.5gm each. I've yet to sort out the cells into capacity groups for mixing together in equal proportions, as well as figure out how to pack the flat cells but I think amethod will become apparent once i start building the larger packs of cylindrical cells.

I like the fact that there is a generous gap between cells... http://www.flickr.com/photos/28496053@N ... otostream/

I also have to grind the tops of the cells to remove the weld pips that were left from the previous pack dismantling, but I don't want to do that just yet and remove any finish to the metal tops before welding, otherwise they could start to oxidise and then when it comes to welding the tabs on things might not go as well as they could.

It looks like 8mm wide nickel tabbing will just fit in the lots across the top, so I'll order a kilo of 8mm x 0.2mm nickel strip and a load more cell spacers. http://www.flickr.com/photos/28496053@N ... otostream/


Chris

ChrisBarron
Posts: 244
Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Mon Sep 23, 2013 11:01 am

I've desided to err on the side of caution, although the gap/trough between cells measures exactly 8mm, I've purchased 7mm wide tabbing strip, 0.3mm thick. Haven't worked out the current capacity but several strips from different cells in parallel should be enough to spread the load and make sure the tabs don't even get warm.

Instead of building a welder I've put an offer of $82.99 (plus $65 shipping) on one on ebay, a double pulse variable current unit. I'm sure I can sell it on after I'm done with it too, more easily than a homemade unit, so I don't mind such an expenditure.

Still to do, sorting of the 1,000 cells into capacity groups.

Chris

mattcarr
Posts: 389
Joined: Sat May 12, 2007 2:27 pm
Location: Hampshire

Re: Another EV bike :)

Postby mattcarr » Mon Sep 23, 2013 12:53 pm

Hi Chris, Looks like you will have some smart packs there. Be very interested to see what sort of performance you get from them.

ChrisBarron
Posts: 244
Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Mon Sep 23, 2013 1:50 pm

Hi Matt, I'm very interested too. If I could, at this present time, weld some tabs onto a small batch of cells and make, say, a 20 cell pack for experimental abuse reasons, I would be on it by now :)

There's obviously a risk in using used cells but I hope my screening process lets me make packs which are robust enough to get some enjoyment from, even for a short while. In terms of investment so far, moneywise it isn't very much, time wise it is all labour intensive.

I'm watching the price of new cells coming down all the time, unfortunately some cells rated for very high capacities are being shown to be a nonsense, especially when yoiu want high current, so although they're cheap their value is pretty low too.

The biggest problem with reliability that I face is going to be the large number connections required for each pack. That's probably the main reason why I'm splashing out on a custom built tab welder - although I could builod my own adjustable output capacitor discharge welder the question will be 'how good is good enough ?' and I'm willing to spend a bit of my money on not having to do all that work finding out too !

Slowly slowly catchee monkey :)

I've seen a lot of used laptop packs selling for silly money recently. I think I worked out that I would get 25% useful cells from every 'faulty' pack, which turned out to be a very conservative estimate for me, but then again that value varies from batch to batch very much.

So for every 4 cell pack you might get one useful one. Considering that you can buy a new cell for £1, (in large volumes) then I put a value of 25p per cell on my bids on eBay and luckily I got everything I wanted for a price of about 10p per cell. Just last week I saw a lot of 30 packs , containing 4 or 6 cells each, (total 120-180 cells) sell for £60, which means they were sold for 50p - 33p per cell. I think if the price is over 25p per cell when in 'faulty packs' then you may as well just buy new and save yourself the trouble of finding out which are good, which are bad, and then have to fettle and measure the capacity of every cell. Unless of course you don't have a big budget, like me !

Chris

ChrisBarron
Posts: 244
Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Mon Sep 23, 2013 2:11 pm

My ebay bid for the tab welder has been accepted, so it comes to me at £95.20 including shipping, possibly with some customs duty when it gets to the UK (on the £51.87 cost) so it's still pretty reasonable I think.

Shipping is going to take a week or so. I'll have to modify the welder to make a handheld welding head but that ought to be trivial. Here's the type I've bought...
http://www.ebay.co.uk/itm/400502719187? ... _918wt_932

They accepted my offer of $82.99.

I have some lengths of tabbing strip left from the disassembled packs if my reel of 7mm x 0.3mm strip hasn't arrived by the time the welder arrives, so I hope to finally get a feel for how this pack will behave within a couple of week's time.

I'm starting to look at pack management, but I still feel that the massively parallel cell construction will make individual pack balancing pointless. Some individual pack voltage monitoring will be useful though, long term, and I have an old datalogger which I can use for monitoring upto 16 series connected packs with while testing, and a battery monitoring project here http://groups.yahoo.com/neo/groups/Cell ... s/messages

Chris

ChrisBarron
Posts: 244
Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Mon Sep 23, 2013 10:48 pm

I got my act together and spent an hour counting the cells out into capacity denominated groups.
http://www.flickr.com/photos/28496053@N04/9905174614/

I set the acceptance level at cells rated at 5000 seconds and above, keeping those with lower ratings to make up the numbers when the packs are finally made.

Of the flat cells, there are 110 in the 6000-7000 range and 31 in the 7000-8000 range

Of the 18650 sized cells I have...

5000-6000 x 120
6000-7000 x 185
7000-8000 x 263
8000-9000 x 194
9000-10000 x 85

To make ten packs I'll use 1/10th of each count from above, which for 18650's is 12+18+26+19+8 = 83 cells and for the flat cells, 12 of each. Seems to be a total of 95 cells per pack and ten packs in all. If each cell only had a capacity of 1Ah, that would be 95Ah packs, at a total battery voltage of 37V it should work out at about 3.5kWh.

In reality, all cells have a capacity above 1Ah each, and some are almost 2Ah, so I think 3.5kWh in total is easily achievable.

The motor controller tops out at 250Amps, so if all cells were identical then the maximum demand from each cell will be about 2.6A at full load. Some cells will have given their best before others so I will have to wait and see how the pack behaves under load. With 95 cells a 100Amp continuous draw seems to not be too much of a problem and in reality I think I could cruise at a much lower constant current. But time will tell.

As for the pack weight, 95 cells in ten packs = 950 cells. each cell is about 48gm so A total cell weight of about 45kg is about what I guessed it would be. I'll need to add a few kilos for the casing and protection though, but it's still light when compared to an equivalent lead pack. I actually don't think I'll bother adding any lead for deep discharge and heavy current protection, just some large capacitors to help smooth the thyristor controller pulse demands... !

Now I just need to ask for a good price for those plastic cell holders. I've reckoned on 56 x 3-cell holders per pack, 560 in total for ten packs.

Chris

mattcarr
Posts: 389
Joined: Sat May 12, 2007 2:27 pm
Location: Hampshire

Re: Another EV bike :)

Postby mattcarr » Tue Sep 24, 2013 3:35 pm

Lots of great information and it sounds like it wont be too long before you have a production line going turning out the battery packs there.

ChrisBarron
Posts: 244
Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Mon Sep 30, 2013 9:17 am

The good news is that the welder arrived this morning.

Naturally, I took the cover off to see what I got for my money (and being a member of the IET I wanted to make sure it was safe)

The bad news is that the internal fuse is fitted on the neutral side of the supply. Whiler that in itself doesn't make the unit unsafe I would recommend to anyone who buys a similar device that they only operate it throug a protected supply (RCB and ELCB protected ). That being said, it means that the HV side of the pulse transformer is kept live and the output feeds the weld probes so there is a lot depending on the construction of that transformer being up to scratch. I will probably rewire it at some point but for now, because I've got a fully protected supply in my garage (thanks to B&Q doing special offers on distribution panels last year !) I've done a few test welds.

Very quickly, using the 2mm x 0.1mm tabbing strip supplied with the welder, I've been able to weld up the pack of 12 cells shown in one of the photos above and right now it's on the charger getting a 3.5A charge to 4.2V. I'm not sure what the maximum current my battery analyser can load the pack up with, but this will be the first test of any parallel connected cells I've done so I'm intrigued to see what I can get from a 12 cell pack, which has the smallest possible cell interconnections (2mm x 0.1mm is a tiny strip !)

Chris

ChrisBarron
Posts: 244
Joined: Tue Dec 11, 2007 11:26 pm

Re: Another EV bike :)

Postby ChrisBarron » Mon Sep 30, 2013 12:57 pm

No sooner had I spoke when.... The welder died.

I tried turning the power down from 25% to 1% to make some thin foil welds, The first one sparked as if it was still at 25% power and the second one killed the unit, blowing the fuse. I've replaced the fuse but it blows instantly.

I'e opened up the unit and it looks like the main triac, an ST made BTA41600 (40A at 600V) has let the smoke out because I'm measuring 0.9 Ohms across the switch connectors.

I've emailed the seller, this is going to be expensive to return so I'm hoping I can do some wheeling and dealing, but I haven't mentioned that I've looked inside ;)

A new triac is £3.

Perhaps I should've built my own welder after all...or bought a British one !

The battery pack I assembled is still happily sucking in juice. I put my ammeter and usual load across two of the 2mm x 0.7mm tabs, which stick out of the end of a pack, and they glowed bright orange, then vapourised in under half a second before I could get a reading.....ummmmm......this could be fun !

mattcarr
Posts: 389
Joined: Sat May 12, 2007 2:27 pm
Location: Hampshire

Re: Another EV bike :)

Postby mattcarr » Mon Sep 30, 2013 5:05 pm

well, at least you got one pack assembled before the welder bust. I hope you get it sorted out okay.


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