Another EV bike :)

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ChrisBarron
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Re: Another EV bike :)

Postby ChrisBarron » Mon Jan 27, 2014 2:33 pm

The welder took shape (of sorts) this weekend.

In the photo, the capacitor is the large aluminium piece over on the right. On the breadboard are 3 IGBT's (2 connected) on the right. The large resistor is a 200W 2R5 resistor for charging/discharging the capacitor through, to achieve the correct weld voltage in the range 0-24V.

The board with the lcd is the control board, which controls the pulse output to the IGBT drivers, and a power supply at the back left, supplying, 5V,15V and 24V.

I got the thing working and started doing some test welds. The capacitor earth was connected via a 100Amp car jump lead as opposed to bolting it in place because it gives me a quick safety disconnect point. A few test welds with this setup and everything seemed to be working fine, the scope shoed the IGBTs were switching cleanly and quickly, so i replaced the jump lead with some very heavy gauge cable.

A few test welds and I hadn't quite got as far into setting the variables as needed, when first one and then the other IGBT blew.

A 165F capacitor at 24V such as this can dump so much current , that I guess my 10mS pulse duration allowed the current to exceed the maximum of the IGBT's.

I'm a bit stuck now. I don't want to blow a small fortune on 4 or 5 IGBT's, but I want to weld these cells up, so maybe my best bet now is to look on eBay for a suitable welder which can be modified, used, and then returned to standard trim for resale afterwards.

The small 70Amp MIG welder I have here justr isn't powerful enough and acts like a resistance welder, heating the tabbing strip quickly to a cherry red colour, but, I suspect it would be just too hot for the lithium cells.

It's possible that I could go down the route of a much smaller capacitor, of 2F at 15V, but then that is more expense which will return nothing back to me afterwards.

It's time to start Googling again ! :x
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Kevin Sharpe
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Re: Another EV bike :)

Postby Kevin Sharpe » Mon Jan 27, 2014 5:27 pm

I don't know if you've seen but Jehu Garcia is now soldering his cells...

http://youtu.be/pVWgVHzyibI
Kevin Sharpe - Founder and Patron for UK registered charity Zero Carbon World. Founder and Chairman Mainpine Group. http://about.me/kevinsharpe

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

Re: Another EV bike :)

Postby ChrisBarron » Mon Jan 27, 2014 7:11 pm

Hmmm, he's taking a big risk.

I had turned my back on soldering based on some tests i have done with different wattage irons and different techniques. The problem has been in most cases that the heat at the +ve terminal often damages any protection circuit or the plastic vent mechanism which is prone to melting quickly.

Maybe I should review the idea ? I can imagine punching a hole in each strip where it lines up with the end of a cell and then spot soldering inside the ring to the end of the cell, so that heat is extracted in all directions as quickly as possible from the soldered area by the nickel strip. Maybe I don't even need to do that ?

Just having a look at solders, I've previously used lead free gear, melting at 215 Celsius, but it is still possible to buy lead based solder with a melting point at 179 Celsius (every little helps). In fact with a good enough iron or small blowtorch it ought to be possible to preheat the strip (and tin it ?)and put a small solder pellet on the cell so that when the strip is applied it just melts the solder before cooling rapidly.


I was thinking of buying another temperature controlled soldering station anyway after the last one died.

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

Re: Another EV bike :)

Postby ChrisBarron » Sun Feb 02, 2014 1:42 pm

After trying a variety of solders and fluxes it seems that my new 50W iron is powerful enough to let me use regular lead free solder and appropriate flux.

The 60/40 lead/tin with a melt point of 183C works very well, but is expensive and doesn't seem to allow me to work any more quickly than with lead-free solder.

I've packed 40 modules, each of 21 cells of mixed capacities. I've done my best evenly distribute cells of equal capacity within each module.

The plan now is to solder a small 0.15mm tab of nickel plated steel to each cell and then connect them together using the 0.3mm strip I originally bought and I think this will give me very low resistance across the pack. Using an individual tab has several benefits, although it is time consiuming to do.

I can use thin strip to connect to each cell because it only has to carry the peak current of one cell, and when soldering to the cell I can use the minimum possible amount of heat. (actually I use high heat for a minimum period of time). Ultimately this also makes the pack easily serviceable. When tinning both the cathode and anode I can get fast and full wetting of the cell contacts. The big fear I have of overheating these cells (based on previous experience with an underpowered iron) is probably not going to be an issue using this method.

The choice to make 21-cell modules came after seeing Jehu Garcia's packs made of 18650 cells. I was going to make similar sized packs of hundreds of cells soldered in parallel too. But then what happens when cells (inevitably) fail, with respect to servicing the battery ?

For that reason I've gone with 21-cell modules because I can easily diasassemble a battery down to module level, and then by desoldering it's tab I can replace a single failed cell.

This is all very boring work, I have assembled 2 modules in 1 hour. I hope to get up to 3 modules per hour but will be going along at a steady pace for a while, but hope to get some quiet days at work which will help !

Chris

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ChrisBarron
Posts: 244
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Re: Another EV bike :)

Postby ChrisBarron » Sun Feb 02, 2014 11:48 pm

Very little of interest on the television tonight so I practiced my module assembly technique and have succeeded in making my first half pack, at 18.5V. Each battery will be comprised of two of these packs to get the 37V I need.

I need to make another 7 of these and don't feel they're going to take too long to build. I'm pleased with the method I've chosen, although soldering is generally bad for these cells the only damaging heat they're subjected to is the quick soldering of the tabs to the cells themselves. When the long strips are soldered to the tabs the heat is dissipated in the tabs and the strips to a great extent, protecting the cells from excessive heat.

I just connected this half pack to the motor in the bike frame and it kicked into life without hesitation, once turning the current settled at 15Amp and the voltage sagged from 18.3V to 18.15V at 15A.

The half pack is on charge at the moment and given that I'm using only 2Amps of charge current it will be on charge until tomorrow. The 21 cell modules which make up the half pack are all sitting at different voltages and need a balance, which I'll sort out tomorrow before I try loading the pack hard.
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Kevin Sharpe
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Re: Another EV bike :)

Postby Kevin Sharpe » Mon Feb 03, 2014 1:22 pm

very cool :D

Here's the latest on the eSamba, Ep 23 - Recycled Laptop 18650 Battery test

http://youtu.be/aaTBuWXA4-4
Kevin Sharpe - Founder and Patron for UK registered charity Zero Carbon World. Founder and Chairman Mainpine Group. http://about.me/kevinsharpe

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

Re: Another EV bike :)

Postby ChrisBarron » Mon Feb 03, 2014 1:31 pm

Thanks Kevin, I saw that episode, but the weather gets the better of him and he can't complete his load testing

Can't wait for Ep 24, hopefully he'll have some data.

I put one 21 cell module on to charge lastnight, it was standing at 3.6V and I set a target V of 4.0V and max current 2A. At 12 lunchtime today it is still sucking just over 1 Amp and is almost at 4V, which ties in with my guesstimate capacity of over 30Ah. The weakest cells in each pack are 4 cells with a capcity of around 1500mAh, all the rest are above that with the majority at 1700-2000mAh.

I'm hoping that by the end of the week I'll have all 5 series connected modules at the same 4.0V level to do a bit of testing with (particularly interested in voltage sag at various loads)

I've ordered some more lead/tin solder today, as well as some Anderson connectors for hooking the packs up with. I realised lastnight that I am getting better wetting with the lower temperature solder, although I think it's probably just a better flux. The flux cored lead/tin solder I've ordered claims to be particularly good for bonding nickel plated and steel pieces together. (the tabbing strip is nickel plated steel)

Chris

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Kevin Sharpe
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Re: Another EV bike :)

Postby Kevin Sharpe » Mon Feb 03, 2014 2:29 pm

keep up the good work Chris... very interesting :D
Kevin Sharpe - Founder and Patron for UK registered charity Zero Carbon World. Founder and Chairman Mainpine Group. http://about.me/kevinsharpe

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

Re: Another EV bike :)

Postby ChrisBarron » Fri Feb 07, 2014 11:28 pm

Jehu Garcia released the awaited Ep 24 , but unfortunately the weather once again got the better of him.
http://www.youtube.com/watch?v=R7ufhAgzI6w

I've been charging the individual modules this week but because I've been busy with my day job it has been very ad-hoc and I wasn't taking many measurements.

For the last module I've been making a note of the actual capacity being charged by my small charger, which cuts out between 9900mAh and 10890mAh. This last module wasn't completely discharged, and has so far taken 27Ah of energy, so these submodules might well have my hoped for 30Ah capacity.

I've been working on my workflow too, managing to tin the ends of every 18650 cell, as well as tin many small pieces of tabbing strip to solder to the ends of the cells. Using lead/tin solder and a hot iron means I can tin an anode in under 2 seconds, the lead based sodler forming a complete shiny button of solder for the tab to adhere to. The cathode needs the 50w iron turning up to full power, but within 2 seconds of forming the tinned button it is almost cold to the touch, such is the heatsink effect of all the surrounding metal.

Tonight saw the assembly of another half pack which I'll probably mate up with the first half pack at some point over the weekend. This was just a 2 hour job, thanks the benefit of having tinned everything before starting. 3 more full packs to go, so 12-15 hours work there.

I made an offer on ebay for two adjustable 24-48V Astec supplies at £22 and have won them, so they'll be here soon and should make it easy to charge whole 37V packs, or if I connect them in series, 74V packs :)

I've started to look at how to make metal enclosures for these packs, I can go for either aluminium right angle strrip corners with sheet cladding, or, buy a cheap metal brake and fold up my own enclosures from scratch. It's going to be a while before I'm ready for that though, so there's plenty of time to investigate.

Chris

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

Re: Another EV bike :)

Postby ChrisBarron » Sat Feb 08, 2014 8:40 am

The submodule which continued to be charged overnight is still taking current this morning, at 370mA (4.2V), and the total capacity charged so far is a little over 35Ah. The cells were not completely discharged before I started this charge cycle (they had a standing voltage of 3.74V as a pack) , the cells had come off the discharger because they had dropped to a voltage of 2.9V while 850mA was being drawn from them. In practical terms there isn't very much more left for them to give which would be useful in meeting the demands of an electric vehicle, but it just goes to show that by selecting the best of the bunch cells it may be possible to build a useful pack.

Following are a few more pics, showing how well the cells can be tinned, particularly with lead based solder and active rosin fluxes....just look at that nice shiny solder blob. It's been a few years since I've seen this because lead-free solders usually go dull as they cool.

If anyone were to try to replicate this methodology, I strongly recommend working in batches, methodically. EG when tinning the cells set the iron to the temperature which gives best results for the anode end, and work through the whole lot of cells tinning each anode. Then set the iron to the temperature required for the cathodes and work through the batch again. Then cut the tabs, and tin those all in one go. The reason I say this is because where it took me over 4 hours to build my first half pack, cutting and tinning tabs as I went along, tinning opposite ends of each cell at a time, and so on, the second half pack took half the time to assemble because all the fiddly stiff was behind me and I could focus on getting the interconnections correct.

'Mis en place', chef !

Before the end of this weekend I hope to have the two half packs fixed together, and will continue to charge submodules until the whole pack is full. Until the adjustable power supplies come this is probably the best way for me to do it. The better way will be to discharge each submodule to a nominal voltage, say 3.5V, and then charge them in series with the adjustable supplies once they arrive.

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