Terminating the 15Ah and 20Ah Cell-Man cells

[olaf-lampe]

Just tossing out a wild idea...
For the hemispherical mounting clamp, how about using copper/brass wool between the top part of the clamp and the sandwiched tabs? That is, the orange stuff in Olaf's diagram is the brass wool. It would compensate for not-perfectly-matched tube halfs and ensure connections over many points and surfaces of the two tabs. And it conducts electricity and is very resistant to heat.

Otherwise, use a dense neoprene foam strip, perhaps 1/6" -1/8" thick, as the orange material. This stuff would ensure perfect compression along every bit of both tabs as long as the neoprene never gets fully compressed. There are neoprenes foams that can take any temperature those tabs might see but even the standard foam goes to 158F and those tabs will never be at that temp for long. And you can get the foam in rolls with a self-adhesive back!

Good idea to put something conductive between the clamps, but don't forget how narrow the whole story is.
I also thought about copper paste ( seem's to be nonconductive ) or graphite powder ( is conductive, but how can it be spread evenly )
The latest and greatest idea came overnight, but I have to sketch a picture to describe it. It only takes one (copper) tube and the pressure would be spread evenly over the whole surface and I only need 1 bolt from sideways.
Stay tuned...
Olaf

[olaf-lampe]

Here are the sketches of my new simple tab clamp.
It's made of a copper/aluminium tube with a slot on both side. The slot starts and ends with a drilled hole to take care of stress caused by compression. The compression part is a silicone/rubber tube with nut/bold and washers on the ends.

Installation is simple : you guide the battery tabs through both slots and insert the silicone assembly. Then tighten the bolt until you have a nice pressure on the tabs. It works the same way as on some Dremel tools.

Of course the position of the slot is the critical part. Also smooth edges are mandatory.

http://img651.imageshack.us/img651/2540/fullpipec.jpg
http://img36.imageshack.us/img36/7748/fullpipe2.jpg

-Olaf

[cell_man]

Hi Olaf,

imageshack is blocked in China Any chance you could attach or forward to my email or PM

I've got my latest and greatest effort so far pretty much ready. Will post few pics tomorrow when it's finished. I tried the solder but wasn't happy with it if I'm honest. By the time you have taken care to sink the tabs, avoid any shorts, prepare the tabs etc etc I think a mechanical solution would have been time better spent. However I may not have the best soldering iron, maybe too hot and the solder I was supplied is maybe too corrosive. I used standard solder in the end. As long as the tabs are adequately sinked by clamping them down onto metallic bars, the heat doesn't reach the cells. So until I can find a good alternative solution mechanical clamping is gonna be the way I go.

I think you'll be presently surprised by my latest effort. It's quite a departure from previous efforts and seems to tick all the required boxes, apart from maybe it's still quite a bit of work... but definitely less than previous efforts and functionally superior too. My idea is not a half cylinder like yours but does have some similar ideas.

[heathyoung]

Blocked - pah. Download firefox for your browser, and install TOR - bypass the blocks.

[liveforphysics]

Hi Olaf,

imageshack is blocked in China Any chance you could attach or forward to my email or PM

I've got my latest and greatest effort so far pretty much ready. Will post few pics tomorrow when it's finished. I tried the solder but wasn't happy with it if I'm honest. By the time you have taken care to sink the tabs, avoid any shorts, prepare the tabs etc etc I think a mechanical solution would have been time better spent. However I may not have the best soldering iron, maybe too hot and the solder I was supplied is maybe too corrosive. I used standard solder in the end. As long as the tabs are adequately sinked by clamping them down onto metallic bars, the heat doesn't reach the cells. So until I can find a good alternative solution mechanical clamping is gonna be the way I go.

I think you'll be presently surprised by my latest effort. It's quite a departure from previous efforts and seems to tick all the required boxes, apart from maybe it's still quite a bit of work... but definitely less than previous efforts and functionally superior too. My idea is not a half cylinder like yours but does have some similar ideas.

I'm working on refining a soldering solution. I also had trouble getting heat quickly spread out over the wide and thick cell tab. I'm making a custom iron the uses a pair of heated copper blocks to clamp upon the cell tabs. When I get a setup refined that works quickly and ensures a perfect connection every time, I will mail it off to you.

[voicecoils]

imageshack is blocked in China Any chance you could attach or forward to my email or PM

See attached.

Have lower temperature silver solders already been discussed?

We have a product that some have had good luck with in Oz for soldering tabs to cells which contains:
95.5% Tin, 4% Silver and 0.5% Copper
(I have not used it myself however)

[cell_man]

Hi Olaf,

imageshack is blocked in China Any chance you could attach or forward to my email or PM

I've got my latest and greatest effort so far pretty much ready. Will post few pics tomorrow when it's finished. I tried the solder but wasn't happy with it if I'm honest. By the time you have taken care to sink the tabs, avoid any shorts, prepare the tabs etc etc I think a mechanical solution would have been time better spent. However I may not have the best soldering iron, maybe too hot and the solder I was supplied is maybe too corrosive. I used standard solder in the end. As long as the tabs are adequately sinked by clamping them down onto metallic bars, the heat doesn't reach the cells. So until I can find a good alternative solution mechanical clamping is gonna be the way I go.

I think you'll be presently surprised by my latest effort. It's quite a departure from previous efforts and seems to tick all the required boxes, apart from maybe it's still quite a bit of work... but definitely less than previous efforts and functionally superior too. My idea is not a half cylinder like yours but does have some similar ideas.

I'm working on refining a soldering solution. I also had trouble getting heat quickly spread out over the wide and thick cell tab. I'm making a custom iron the uses a pair of heated copper blocks to clamp upon the cell tabs. When I get a setup refined that works quickly and ensures a perfect connection every time, I will mail it off to you.

Thanks Luke

I was thinking of something similar. The way I see it you need to get the heat evenly distributed across the tab so was thinking about heating a block of a suitable material to a suitable tempertaure and then clamp that to the cell tabs whilst they are pressed up against maybe a ceramic block. I'm not so happy about the solder I was supplied. I hope I haven't trashed any cell tabs with it. The flux was kind of nasty stuff and the soldering iron was maybe too hot. There is a lot of crap in the solder once you've got it to take and lots of aluminium like stuff is deposited on the iron. I'm hoping it's just the oxidised alumium surface surface and it's not eaten into the alumium tab too much. Will just have to keep those cells for myself.


You would also need a suitable heat sinking arrangement for any tab soldering and likely need to tin the tabs beforehand. However whilst I do think it's very do able I'm tending towards a good mechanical solution instead. The parts required on my latest version are simpler than previous versions and would offer many advantage over a soldered version. My latest idea would not require complete disassembly of the pack to change a cell either, just partial disassembly of a 4 cell block (to be decided). Will get some pics up later.

[cell_man]

imageshack is blocked in China Any chance you could attach or forward to my email or PM

See attached.

Have lower temperature silver solders already been discussed?

We have a product that some have had good luck with in Oz for soldering tabs to cells which contains:
95.5% Tin, 4% Silver and 0.5% Copper
(I have not used it myself however)

I haven't tried yet but will try if I can get hold of some. Thanks

[CamLight]

You would also need a suitable heat sinking arrangement for any tab soldering and likely need to tin the tabs beforehand.

How about using heat putty? I've seen it used to stop heat flow along pipes and metal when brazing or welding metal and you could form a rod-shaped hunk of it firmly around the base of the tab(s). Mcmaster-Carr has it here in the US for 13USD for a one-pint jar, but it should be available from any well-stocked welding supply house.

http://www.mcmaster.com, use "page 3364" in the search box, bottom right of the page.

[liveforphysics]

The chunks you see are AlO3, aluminum oxide. The job of that flux is to lift it off the surface of the cell because it otherwise prevents any solder surface wetting. Minutes before I TIG weld aluminum, I run a couple passes over the surface with a corse stainless steel wire brush. This does a pretty damn good job of removing the surface layer of AlO3, and let's the welds look very clean, as well as easy to dip the filler rod.

This is an application that must use paste only IMO to ensure a fast and full soldered connection. Heat must also come roughly equally from the top and bottom of the tabs being soldered. My CNC machine just arived, and I've got lots of copper blocks laying around. I'm going to make you a little spring loaded pliars-like iron that is spring loaded and has an insulated grip. This way, you put a smear of solder paste on the surface of a tab, use a little spring loaded heat-sink clamp, then spring open your custom soldering iron, let it clamp down on the tabs, count to 3 (or whatever I determine it takes), then remove it, wait another 10 seconds or so, remove the heat sink, and move on to the next set of tabs to connect. Probably take 30-45seconds per connection once you get a process smoothed out.

[cell_man]

Thanks John and Luke

This place really is a hive of really useful info and great people.

With all this talk of quick soldering techniques I feel a bit bad to show my latest termination method. I need to refine a couple of small points but nothing major. I really think this is THE ONE

Ok we start with a strip of alumium sheet with some holes drilled into it and some wedge shaped clamps. There are 10deg cuts on the clamps and the clamps shown are 6mm but will go up to 8 or 10mm once I've had a chance to get the materials. Inserts fitted into rear and the holes have been drilled 4mm through the tab and then 6mm leaving a couple of mm so the insert sits inside there nice and tight.... just the way I like it..

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The alumium sheet is folded over and then the 2 tabs are folded over the sheet. The aluminium sheet serves a few purposes. Firstly it prevents the tabs being folded over too tightly, the 0.5mm sheet allows a small radius on the bend, it retains the tabs in position when it is bolted to the rear clamp and finally it allows the balance wires to be connected to the tab. They are isolated otherwise.

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Then finally the clamp is bolted to the front bracket and the spacing of the tabs and alumium strip keeps everything at a safe distance. The bolts are all wrapped in insulating tape (well these aren't but I've checked this method before and it works, the spacing is wrong on these brackets to allow room for the tape) so there is no potential on the bolts. Countersunk bolts may be replaced with pan head to try and get the clamping foprce more over the edge of the tabs rather than past the edge of the tab which will likely result in more flex on clamps and less clamping IMO. Even with the less than ideal clamps and bolts there doesn't seem to be much bowing of the inner clamp and considering that pretty much the entire tab is clamped there must be more than enough tightly clamped surface area to minimise any attenuation. Worse case an M3 or M4 bolt through the centre could be easily added.

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Keeping the front clamp as a single piece will increase stiffness massively. The rear clamps will be increased in size to maybe 8-10mm thick. Keeping just 4 tabs clamped together allows for easier breaking down of the pack and also allows for blocks of 4 cells to be prepared and then just tied together as required.

It looks like I will likely be building a parallel pack, 32S, 3P soon if all works out So I just have to get this 1P method sorted 100% or as near as damn it soon, then get on with a parallel setup. The parallel setup will require copper and lots of it IMO. In some ways I feel it may be a little easier but I really shouldn't say that and jinx myself up I may need some advice on just how big the copper needs to be

[jonescg]

"It looks like I will likely be building a parallel pack, 32S, 3P soon if all works out"

I can't wait Hopefully the use of copper blocks won't add too much weight to the whole show. Also, how much extra length does the termination add to the cell? The dimensions are roughly 230 mm long + about 15 mm for the blocks and tabs?

Cheers,
CHRIS

[cell_man]

Hi Chris,

hopefully you don't have to wait too long

The length is 227 but that is actually including the edge of the foil pack at the back edge. If you fold that up you could bring it down closer to 220mm length. If you took a similar approach to what I've done with the latest design above the inner clamp could be within that 220mm and then just have maybe a 5-10mm thick copper bus bar on the front which would link 6 cells together, 3 positive and 3 negative tabs. If space is really at a premium you could maybe get the total ength down to 230mm or a little less. We need to confirm how much copper cross sectional area is required to keep the losses to the minimum but I would have thought 5mm thick, 50mm wide copper, 250mm^2 is not gonna have much loss even at circa 1000A but will need to do the calcs to confirm. The way I see it working, some of that current will not even be flowing through the bar as 2 of the 6 cells could be directly clamped together, positive to negative so the current from only 4 of the 6 cells would flow through the bus bar.

With serious current and the possibility of very hot temperatures where you'll be running the bike I think some cooling panels would be better. They could be brought out to either side of the cases and maybe the back side as well if that is possible (but i think not). Aluminium sheet would be ok but some of the corrogated sheet shown previously would be better IMO but it would add more volume to the cases and would probably need some fans to be properly effective. First things first is to knock up some examples of the possible terminals and go from there but the more I think about it, what I've shown above in copper might fit the bill just fine with a few tweeks here and there.

[bigmoose]

It is great to watch this design evolve. Each iteration gets better and better

[cell_man]

Thanks Bigmoose,

it's taken some time but I think it's now pretty close to being right and it seems to tick all the boxes. If a similar idea can be implemeted for the parallel configuaration I think that will work out really well. Chris's requirements for his dual Agni powered motorbike is very weight and size conscious so I'm gonna have to keep materials down to what is actually required, no more and make best use of the available space. I've got a machine shop that I'm looking to start using to knock up some parts rather than trying to do everything myself as well. There's lots of things I need to be getting on with other than just cutting and machining parts all day long. It's draining my creative juices you know...

I'll be calling on the ES gurus over the next few weeks for some advice on the parallel design. Will put a drawing my idea up soon and see what people think but basically I'm thinking 6 tabs joined together to each bus bar (3p pack). First 2 negative tabs linked together and then clamped to the bus bar, then 1 negative and a postive linked and clamped to the bus bar and then finally 2 positive tabs linked and clamped to the bus bar. Basically just the same as I've done above but in copper with a few changes obviously. With this arrangement you would only have current flowing through the bus bar from 2 of the 3 parallel cells, the third is directly tab to tab, reducing current flow and reducing the required size of the bus bar or reduces losses. The rear clamps that are facing the body of the cell maybe do not even need to be conductive but probably better just to stick with copper

[liveforphysics]

I'm going to throw this post I made last night for the cell testing thread into this thread as well, because I think it's very relevant to this pack design thread and the cooling needs. As my tests showed here (and surprised me!) these cells don't need heatsinking, but rather just need heat-spreader plates sandwiched between the cells.



Here goes the post:


So, this is the reason why my FET setups kept exploding until I went to the 100ohm gate drive resistors with the 7.1v zenner clamp diodes. The inductive kickback is brutal! This is the reason why caps are so very important on things with FETs, and it's a damn shame I can't use them on a cell testing setup (because it would influence the cell performance readings).



Ok, I snuck my test-rig into work to have access to the $14,000usd fluke thermal imager, and took some very slick pics.

I layed out some engineering blueprints to protect the table, then setup the whole mess right on the 20ft long confrence table. lol



These are thermal tests from the minty fresh undamaged cells. I tested the 15Ah cell until I ran it to LVC, then swapped to the 20Ah cell, and ran it almost to LVC, but I had an unexpected problem with my test setup that you will see later in the pics. lol

The colors auto re-calibrate to fit the temp range, so blue might be 500deg if there are parts in the picture at 1000deg, or white/red could be -20deg if there are parts in the picture at -100deg, so you must use the temp tags to read the temps, or look at how the temp scale is setup for each indivdual picture.

This is 2 minutes at 150amps on the 15Ah cell. (5Ah, or 33% of cell capacity used)
97deg peak temp. Ambient temp was 77deg F, so 150amps for 2minutes just brings the cell up to human body temp, and all localized at the cathode. Pretty damn cool running cell.




I loaded the loadbank up to 300-315amps, and let it sit another 1 minute (Another 5Ah, 66% of cell capacity used).
107.7deg F temp peaks. Feels about Luke-warm to the touch in that cathode area, about body temp for the rest of the cell. Still very cool running for a cell getting hit with 20C discharge continously...



I was going to step up the discharge to 450amps for the last 5Ah left in the cell, but I was dicking around tweaking the manual focus and trying to range the camera that it all ready had another 30seconds at 300amps, so I just let it finish at 300amps to LVC. (Another 5Ah, ~90-100% of the cell used.)




At this point, I was pretty proud of how my setup was working, and I wanted to take a pic of the FETs.

Shoot! Barely warm! Little bastards were doing a hell of a job, and I owe that to BigMoose's advise about the higher resistance gate drive resistors and the zeners to protect gate bounce. Thanks Moose!


So, feeling pretty confident with my FET setup, and wanting to make the most of my time with the IR imager, I decided to hit the 20Ah cell with a worst case scenerio sort of situation. I did 600amps, 80% duty cycle (480amps PWM average), 10khz switching frequency.

So, it starts out humming right along, this was roughly 40seconds in (5.3Ah, 26% of the cell used), I didn't get a clean time marker when I started exactly. Oops! I decided to just run the cell into LVC and snap pics along the way. lol This is pretty much an absolute worst case scenerio jump in your EV and just stay wide open on max current limit from a full charge all the way to empty. lol If a cell can handle this, it can handle pretty much anything an EV is going to throw at it.






The FET bank was really making a strong humming sound, so I decided to snap another pic.







I could see the loadbank start to relax, and I could see waves of heat riseing off of it. lol I wanted to check on the FETs again.
WHOA! 371deg F FETs!!! Not good! lol I decided to just let it run it's course and see what happens though.




This was about 95% discharged, keeping up the brutal amp rate the whole time. Why didn't I run to 100% discharge? You will see why below.



The solder on my FET bank MELTED!!!! WHOA!!! Amazingly, I tested the FETs afterwards, and they still seem to work perfect! Notice that crazy delta-T between the copper and the FETs! Can you say thermal run-away? lol

[cell_man]

Thanks again Luke,

so you feel that a simple sheet of Alumium between each plate is sufficient? I've tried some 0.5mm sheet and it seems to help. I feel that if you were gonna go to the trouble of putting a sheet between each cell you may as well bring them out to some form of heat sink, even if that was just a thicker alumium sheet. Could always wrap it in something when the temperatures drop.

So what would you guestimate is the point where some form of heat sink or heat spreader is necessary? I'm thinking somehwhere over 10C CONSTANT (need to consider the environment they are intended to be used in too) to be on the safe side as with all those cells stacked up the cells in the middle will get a bit hotter. I believe it's more a case of the average power over time that will cause heating rather than bursts of high discharge mixed with lower discharge levels.

[j3tch1u]

I'm going to throw this post I made last night for the cell testing thread into this thread as well, because I think it's very relevant to this pack design thread and the cooling needs.

thanks for that excellent test luke--it is indeed very relevant to the termination method for these cells. i don't expect to even approach the worst case scenario so i've decided to try and make my package as slim as possible without any sinks or spreaders.

this is a first cut of my monoblock termination cap. it is for a 12s pack. the material looks like kraft chedder (not sure what it is called) but it is cheap and my machine cuts thru it like cheese. after i nail down the feed rates and optimize tool paths, i'll cut one out of delrin for better durability.

the cells fit thru slots in the back and stack on top of each other without any spreaders. there are thru-slots for the tabs which are folded over each other and clamped with a 3mm copper plate. i may add 1.2mm flat springs (with slight curvature) to even out the contact pressure of the tabs and eliminate any possible voltage drop. in the middle of the block are channels for the balance wires. there are grooves in the back along the outer edge to secure 2mm carbon fibre panels for the housing. a matching end cap will lock the assembly together.

assembling each cell is very fast, safe and solder-free.

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[liveforphysics]

Hey! That looks nice too j3ch 1u

Don't forget to electrically isolate the foil pouches from each other though. For some reason, lipo, Kokam pouches, and about every pouch cell I've ever worked with do get a little continuity between tab and the foil of the pouch in time. Just a 1mil sheet of any plastic film, or double side tape (this is what RC lipo packs use to isolate the cells) would be adquate.

[swbluto]

Really awesome thermal pictures. It seems silver-based solder does have its advantages. Of course, the lead solder you used apparently works well as a cheap resettable thermal fuse.

Btw, is that oscillograph one of V_gs (Voltage between gate and source?) or is it V_ds? Or something else?

[liveforphysics]

Really awesome thermal pictures. It seems silver-based solder does have its advantages. Of course, the lead solder you used apparently works well as a cheap resettable thermal fuse.

Btw, is that oscillograph one of V_gs (Voltage between gate and source?) or is it V_ds? Or something else?

That scope pic is across the cell terminals! No kidding! Inductive feedback is a crazy powerful thing! My cell tester obviously needs to be fixed right now, but I was able to measure ~32v voltage difference from 1 side of my copper FET buss-bar to the other side! This is what was smoking my FET drivers and dicking up my FET gate control. The poor FET driver was getting so hot or explodeing because it was trying to clamp the voltage polarity swap across it, and it didn't have a chance. lol I changed all the ground locations to just pick 1 point as a reference, and ran a seperate lead to everything from that single point, and it all works fine now. I thought Moose was crazy when he suggested that the voltage difference across the bus bar could be more than VCC to the fet driver chips... lol

[cell_man]

Hey! That looks nice too j3ch 1u

Don't forget to electrically isolate the foil pouches from each other though. For some reason, lipo, Kokam pouches, and about every pouch cell I've ever worked with do get a little continuity between tab and the foil of the pouch in time. Just a 1mil sheet of any plastic film, or double side tape (this is what RC lipo packs use to isolate the cells) would be adquate.

I second that. Make sure all edges are isolated. I've just recenly found that the foil edge have some continuity to the negative tab. I will send a bulletin out to anyone that has bought the cells and ensure that any exposed foil edge has some some suitable tape applied. It's mainly the opposite end of the cell to the tabs you need to watch and even if there is tape it could be easily abraded through. The sides are folded over and do not have the foil edge exposed. Best to check to ensure there is no potential voltage between the tabs (any tab should suffice in the pack) and any conductive material used in the case or in close proximity to the cells. This also leads me to think that if the protective coating on the cell body is abraded through it could also cause some current leakage.

I've just done some checks on an 8S pack and was able to get about 2mA by shorting my Ammeter between the foil edge of cell 8 with the negative terminal of cell 1. So we are not gonna see sparks flying but it could slowly cause a cell to discharge or imbalance the pack.

Great work Ben,

the buyers are showing me up again If anyone hasn't checked out Bens, j3tch1u build they really should and his workshop is pretty nice too. Hopefully I can see them in the flesh on my planned Taiwan visit in March

Are you cutting the tabs down for the assembly?

[MitchJi]

Hi,

the cells fit thru slots in the back and stack on top of each other without any spreaders. there are thru-slots for the tabs which are folded over each other and clamped with a 3mm copper plate. i may add 1.2mm flat springs (with slight curvature) to even out the contact pressure of the tabs and eliminate any possible voltage drop.

Excellent !

Even with a spring in the middle will the 3mm copper plate bow too much?

BEGIN EDIT Addition:

Liveforphysics - Thanks for the thermal imaging. This forum never cease to amaze me. Makes me shy of not being able to contribute much to the community.

Oops, I completely forgot to mention Luke's excellent work/documentation/post on Cell Testing.

Outstanding!
END EDIT Addition:

[Velocipede]

j3tch1u - This is beautiful. I love how the recessed connections keeps the tabs from any short. This is in my opinion the best solution on this thread yet.

Liveforphysics - Thanks for the thermal imaging. This forum never cease to amaze me. Makes me shy of not being able to contribute much to the community.

[CroDriver]

Hey! That looks nice too j3ch 1u

Don't forget to electrically isolate the foil pouches from each other though. For some reason, lipo, Kokam pouches, and about every pouch cell I've ever worked with do get a little continuity between tab and the foil of the pouch in time. Just a 1mil sheet of any plastic film, or double side tape (this is what RC lipo packs use to isolate the cells) would be adquate.

I second that. Make sure all edges are isolated. I've just recenly found that the foil edge have some continuity to the negative tab. I will send a bulletin out to anyone that has bought the cells and ensure that any exposed foil edge has some some suitable tape applied. It's mainly the opposite end of the cell to the tabs you need to watch and even if there is tape it could be easily abraded through. The sides are folded over and do not have the foil edge exposed. Best to check to ensure there is no potential voltage between the tabs (any tab should suffice in the pack) and any conductive material used in the case or in close proximity to the cells. This also leads me to think that if the protective coating on the cell body is abraded through it could also cause some current leakage.

I've just done some checks on an 8S pack and was able to get about 2mA by shorting my Ammeter between the foil edge of cell 8 with the negative terminal of cell 1. So we are not gonna see sparks flying but it could slowly cause a cell to discharge or imbalance the pack.

So you would recommend to isolate the whole cell? Every cell?