Another EV bike :)

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

Re: Another EV bike :)

Postby ChrisBarron » Tue Oct 01, 2013 1:58 pm

No response from the welder seller yet, but I think it's a Chinese holiday at the moment.

Well anyway, the 12-cell pack seemed to take an age to charge, but this morning I got it onto the discharger and set for 3A continuous (the maximum) discharge.

So far the pack has delivered 19880mAh (about 20Ah) and the voltage is still at 3.3V, with a discharge termination at 2.9V.

I'm guessing conservatively that I'll end up seeing about 28Ah from these 12 cells. About half of these are the highest capacity cells from my pulls so not representative of the whole lot of cells in general, I would guess that a pack made from the full mix of cells that I have would yield about 15% - 25% less capacity.

Scaling up to 96 cells (x8) would seem to produce about 220Ah (very highest estimate) when discharged at 3A x 8 = 24A

Due to cell differences maybe 160Ah is more realistic, and for a continuous current of 100A then who knows how much the capacity drops off by....


I guess the thing to do now is to get a data logger organised and build a load which draws 10 - 12A of current, and then test it again. R= V/I = 3.7/12 . A 0.33 Ohm 50W resistor will get me in the ballpark.....I'll see what I can find lying about in the garage

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

Re: Another EV bike :)

Postby ChrisBarron » Thu Oct 03, 2013 11:17 pm

I dug out the datalogger and loaded up the 12 cell pack for a 9Amp draw. Cutoff voltage set to 2.9V. Run time for calculation are from 20:51hrs to 23:19hrs(at 2.9V when discharge ended)

A graph of the discharge is here http://www.flickr.com/photos/28496053@N04/10076452724/

That's roughly 2.5 hours of running, at between 9A-8A (lower current as the voltage dropped), which seems to give a little over 20Ah.

I'm going to try another discharge, but at a current of at least 15A, preferably 20A

Chris

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

Re: Another EV bike :)

Postby mattcarr » Fri Oct 04, 2013 2:21 pm

Thats not too bad, drawing 9 amps for 2.5 hours, is what, about 22 ah. Will you be able to test the a larger pack when you have built one at higher currents? I wouldn't like to try and drain 100+ amps unless I had the correct equipment to do it.

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

Re: Another EV bike :)

Postby ChrisBarron » Fri Oct 04, 2013 4:05 pm

I've made another resitor bank up so next test will be at 14.5A at 3.9V. It's going to be more meaningful to me because if the capacity doesn't drop much below 18Ah then once the pack scales up in size to 8 times it's present size then all other things should scale with it.

As for testing at higher currents once I've assembled the complete pack, yes, I'm a 14 stone load bank and will be stradling a steel motorcycle. I'm sure I'll be demanding the controller's maximum 250A here and there :)

Seriously though, If I can expect 18Ah from 14.5A load, scaling up by a factor of eight gives 144Ah at 116A. 116A is probably reasonable for a constant 40-50mph, averaged travelling up and down inclines. Say at 40mph that's 1.25 hours, the distance will be 50 miles.

Still, not all the cells I have can perform as well as half of those in this small test pack, so again, 30 miles range or even 15 mile range with some extravagance of throttle doesn't feel too unrealistic at this point.

However, the all important 14.5A load test hasn't happened yet either :) Once the pack is charged (later this evening or tomorrow, I'll run this 14.5A discharge cycle and then I'll finally know whether to cry or not ! :)

Chris

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

Re: Another EV bike :)

Postby ChrisBarron » Sat Oct 05, 2013 7:09 am

I managed to set up the outomatic alarm system on the logger to kill the discharge via a relay when the voltage dropped to 2.9V, allowing me to let it run after I went to bed (making sure the batteries were on a metal surface etc first !)

I remade connections in the circuit to produce a start current of between 14-15A when the voltage was 3.45V at the start of the run (quite a sag under load !)

The discharge ran from 11.51 until 01.24, so just over 1.5hrs runtime at a minimum of 14A, or approximately 21Ah.

I've left the whole graph together because it shopws the interesting way that the battery voltage rises again after discharge, to 3.7V, but with absolutely no capacity left over (maybe a little at much lower currents ?).

Overall, considering that some of these cells were the best of my lot, then I'm happy to carry on building these packs up and assuming at least 18Ah at 14A load is in the ballpark. This higher current run suggests that when scaled up by a factor of eight to represent the final pack, then I could potentially have a pack with around 140Ah capacity at 112A.

Even derating the pack further to 120Ah to allow for some miscalculation here I think I'll still see my target of at least 30 miles range :)

The 14A discharge graph follows. Interesting to see the voltage creep back up during the night to 3.75V. If you put a meter on that and were a bit unwary then you might think the cells were in good shape !
http://www.flickr.com/photos/28496053@N04/10095608675/

Chris

EDIT - not sure why (boys with their toys...) but I just wanted to graph the recharge cycle of the cells too, so deciding to make sure they were discharged down to 2.9V before I put the 14A load back on........ and it took a further 14 minutes for the voltage to reach 2.9V again. It's things like that which I find encouraging :)

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

Re: Another EV bike :)

Postby ChrisBarron » Sat Oct 12, 2013 3:41 pm

This week has been all about the welder. apart form some comical misunderstanding, they're sending me a new welder control board.

In the process of looking at this fault, identifying the blown triac as the reason of failure, it looks like the triac snubber circuit (designed to prevent spurious activation of the triac) is wired on the wrong side of the load (pulsing transformer), and is of the wrong values for the inductive.

Using a 'scope to check that the pulse control side of the circuit still works it looks like the 'dual pulse' feature of this welder doesn't exist. A single 40us pulse is used to control the triac.

The seller agreed to take the welder back, and pay for the £102 return shipping, but I wasn't sure they would pay in the end (some negative feedbacks on ebay which they don't care about) annd offered to replace the triac and rewire the unit correctly if they gave me a partial refund.

It's not an ideal ending, but it's enough to keep the ball rolling and put this behind me


My battery tab material turned up.....it is 7mm x 0.3mm and it is just too thick for the welder to cope with (when it's working), so I've ordered a reel of 7mm x 0.15mm tabbing strip, which is still on the wrong side of the welder's capabiltity, but only just, and I think my rewiring of the welder will make it more reliable and powerful.


The pack of 12 cells already assembled has surprised me. After half a dozen charge-discharge cycles it's capacity has stayed constant.

I'm waiting until payday before ordering the plastic spacers, £80 worth. I have found a supplier who can sell them at $0.20 each, so if anybody else would like some to play with I don't mind bulking up the order (there is a single postage cost anyway) and repacking them once they arrive here.

Chris

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

Re: Another EV bike :)

Postby ChrisBarron » Sat Oct 19, 2013 11:36 am

The welder vendor has just emailed to say that my new control board is on it's way.
No mention of the reason for failure, or why the snubber circuit is connected incorrectly, or why the fuse is on the neutral (and I think the reason why the triac failed)

But anyway, i gave up waiting and ordered two identical triacs from a UK supplier which arrived yesterday. Lastnight I fitted them to the welder board, two in parallel to replace the one which had failed. I also relocated the snubber circuit and rewired the mains input wires so that the fuse is on the live wire and the triac now switches the high side of the transformer.

Glad to say that it works, and works well now. Setting the power all the way down to 1% produces the smallest amount of current and perhaps a tiny spark. Turning the power setting up to 40% makes almost decent welds on 10 thou thick plate - the original spec is for 5 thou maximum. Welding will be happening here again soon !

I'm waiting until pay day next week before ordering the plastic plates to make the endplates from. I've started to think that the best thing to do is instead of making 3.7V packs from 90 parallel cells, make 3.7V packs from 2 x 45 cell groups in parallel, because this would leave the door open for reconfiguring the pack as 20 x 3.7V packs in series to run at 72V at a later date, if all is still well. and then as funds allow I could buy a higher spec motor and controller

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

Re: Another EV bike :)

Postby mattcarr » Mon Oct 21, 2013 4:52 pm

Nice to see you have your welder back in action. Shame the cheap Chinese things never seem to work out.

I think if you went for the small packs, then you would have more options for creating a higher voltage pack as well. You may well end up wanting a higher voltage pack for those higher speeds on the bike :-)

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

Re: Another EV bike :)

Postby ChrisBarron » Mon Oct 21, 2013 5:41 pm

Yes I know Matt, but I'm not doing it for the higher speed because I'm older and more sensible than that....I'm not doing it for the.....for the......

Did someone say higher speed ? ;)

It might make the pack interconnections easier to make too. Less current at higher voltage gives more options for interconnects. The present low voltage high current plan (200 Amp minimum connections) has some major design implications - it's no time to skimp on well made (read as expensive) parts and materials.

I've built up a good selection of parts in the garage over the past couple of years, have some nice copper bars and brass nuts and bolts, 72V contactors and so on.

I should probably get on with making a modular solution. Maybe 15 cell packs would be good, 5 cell X 3 cell blocks. I've previously toyed with the idea of using a lot of small relays to wswitch blocks of cells in and out of parallel, because for that pack I had wanted to have the ability to take out of circuit any cell pack which overheated.

Small packs are not only easily isolated (I have a few hundred 24V coil DPDT relays with 16A (dc rated)contacts) but they're also easy to replace. If I build a siongle large pack then as cells start to fail and capacity is reduced, the only way to determine where the faulty cells are is to break the whole pack down....


Hmmmm, now I'm thinking :)

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

Re: Another EV bike :)

Postby ChrisBarron » Tue Oct 29, 2013 4:26 pm

I've bought the interlocking cell holder plates now. Hopefully they'll arrive from China in a week or two, and the replacement welder control board has arrived so I have that as a spare now....and the welding strap arrived , the 7mm x 0.15mm stuff, which starts to get slightly warm when carrying 12A, but that's fine because I don't intend subjecting it to more than 9A. (The beefed up welder might manage to weld the 0.3mm thick strip now...)

I've settled on making 12-cell blocks, the cells wired in parallel.....which will then be assembled into 10-block packs, stacked as two 5-block packs, one over the other. Difficult to explain, but it means I'll have 7 packs at 37V, each one capable of 50Amps, and that means I can use good quality 90A connectors which the RC hobbysits use.

This means that the packs can be reconfigured for different voltages, plus it also means that because each pack has just 120 cells, making another pack isn't going to be too difficult if I come across more cells at bargain prices.

Starting to look forward to building this battery now !
Chris


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