Common pack design mistakes, how to avoid?

[Punx0r]

But what evidence is there than these arrangements are "perfect"? Or even "better"?

If they result in each cell seeing the same resistance to the discharge leads then they are "perfect". If they fail to achieve this despite a labyrinth of nickel strip then the layout is a folly. A complex design needs to justify itself with increased functionality else it's just making hard work for yourself.

Your latest analysis beat me to this point - I assumed the layout in question was supposed to achieve equal resistance to all cells. It seems it doesn't and yes, you have to ask "how much better is it than a simple layout? What is the real advantage?". It becomes subjective and all we can say is current should be as good as reasonably practicable. Much like how balanced should your pack be? 0.1V? 0.01V?

 

[Buk___]

It becomes subjective and all we can say is current should be as good as reasonably practicable.

Hm. I don't think there is anything "subjective" about it.

This is both an empirically measurable property, and a something that can be characterized according to well known and well-defined physical laws; and thus reasonably accurately simulated.

But graphical representations of what 'looks right' are fiendishly deceptive when it comes to high order parallel resistances.

 

[Punx0r]

Subjective in that you must decide what is acceptable for a given application. Whereas with equal resistances all you can simply say "job done".

But graphical representations of what 'looks right' are fiendishly deceptive when it comes to high order parallel resistances.

You have no argument from me there.

 

[tsourorf]

Hello and sorry I haven't replied for a few days. Really busy at work!
I have created a demo for the various connections seen on the SmartGauge page:
View attachment 15P connections demo.txt
This shows two things:
1) The SmartGauge website is right and the first two types of connection are just a pain for the cells.
2) I have made a huge mistake with my schema. I should never go with 15P. It cannot really work with that balancing configuration.
After working a bit with the falstad circuit simulator, I have now come to the conclusion that I have to go with a 13S16P pack. The 'perfect' connection is the following:


However, I still have some changes in mind regarding the connecting strips. There is some significant voltage drop because of the total resistance on each cell. This can be helped by using nickel and copper strips or nickel strips and copper wires. I am currently making the calculations and I'll be back with my results as soon as possible. :wink:

 

[tsourorf]

So, the configuration I am going forward with is the 13S16P with the cells connected as shown in the following images:





The connections between cells next to each other will be done with spot welding 7mm wide 0.15mm thick pure nickel strip. Thus, all cells will be connected with the nickel strips in pairs in the first place. Then, the 3.6cm length connections, connecting second with fourth or first with third will be done with spot welding a 10mm wide and 0.1mm thick copper strip that will be split right in the middle for the spot welder to do its job. Finally, the quadrants (rows in the images above) will be connected using a 12AWG wire and old fashion flux welding.

The final result should be a 48V 13S16P battery pack with each cell facing 2.073mOhms of resistance because of the above connections. You can see the circuit in practice using http://www.falstad.com/circuit/circuitjs.html with the text here: View attachment Perfect 13S16P pack.txt

I am looking for both feedback on my attempt and your own numbers to compare if possible.

Feeling excited now! Looking forward to actually finishing the pack and see the practical results.

 

[tsourorf]

So, I have finished my first 16P pack. It went better than I thought. I only used spot welding @40A for nickel strips and @60A for the split copper strips. I only need to solder a wire for the positive and a wire for the negative pole of the pack to finish it. And... do it 12 more times of course! :?

You can see the result here:



So, the steps I followed were:
1) Spot weld 16 cells in pairs using 25x7x0.15 mm pure nickel strips
2) Connected pairs of cell pairs in a row by spot welding 1st to 3rd cell on positive end and 2nd to 4th cell on negative (Positive of 2nd and negative of 3rd are covered by electrical tape). Used 40x10x0.1 mm pure copper strips split right in the middle to form two 40x5x0.1 mm parallel strips.
3) Used hot glue to stick the four fours of cells and have one solid pack.
4) Spot weld one small 1cm piece of nickel strip on the positive of the 2nd cell of each row and on the negative of the 3rd cell of each row.
5) On the positive end, I spot weld 2nd cell of row 1 to 2nd cell of row 2 as well as 2nd cell of row 3 to 2nd cell of row 4, using 25x10x0.1 mm copper strips split in half to form two 25x5x0.1 mm strips. Also used the same strips to connect the negatives of the 3rd cell of row 1 to the 3rd of row 2 as well as the 3rd of row 3 to the 3rd of row 4.
6) On the positive end I covered with electrical tape the 3rd row and on the negative end I covered the 2nd row.
7) Spot weld a small 1cm nickel strip piece on the positive end of 2nd cell of row 2 as well as the 2nd cell of row 4. Then, I spot weld a 40x10x0.1 copper strip (split in half again) on top of those cells.
8) Like step 7, I have spot weld a 1cm piece of nickel strip on the negative end of 3rd cell of row 1 as well as the 3rd cell of row 3. Then, I spot weld 40x10x0.1 copper strip (split in half again) on top of those cells.

That's it! It might have taken me some time to do (And it will take a long time to do it again for 12 more packs), but I am very pleased with the result and myself! I am proud I figured out a way of spot-welding copper using my cheap £100 spot welder!

 

[Headrc]

Has anyone on here now tried any of the liquid/spray on based protective coatings for your battery packs?

 

[Philaphlous]

Has anyone on here now tried any of the liquid/spray on based protective coatings for your battery packs?

Like the stuff they have on TV that keeps a boat afloat with a screen door as the hull? Lol... Not that I've seen, nor do I think it's the best idea. If you're planning on spraying it directly on the batteries I'd think it'd make it nearly impossible to do any maintenance on the pack if needed...plus it might mess with the CID on the cells...not sure. I'd stick to tape or shrink wrap...

 

[spinningmagnets]

The spray insulation "might" be OK on the negative ends of the cells (fast, cheap, convenient), but...I'd keep the vents open on the positive end.

 

[fechter]

I've seen some packs that are completely potted, even over the vents (Grin LiGo, for example). I guess if the cell gets to the point where it wants to vent, you have bigger problems anyway. The foam stuff won't hold back a venting cell much anyway. If you could find some foam that doesn't expand much and is pretty hard when cured, it might add some good protection. Forget about ever repairing the pack though. Another approach is to cover the pack with a thin plastic bag, then foam around that so the foam doesn't actually stick to the cells. It would be like a custom foam cushion that could be cut away if needed.

 

[Joachim]

This pack good for 80A max discharge 1mm pure copper wire for the series 6x
The pack:3s 28P 10.8V 81Ah 8AWG wiring no direct heat to the cells on soldering the copper serie/parallels

 

[Headrc]

No, I was not thinking of great stuff! LOL. But I read through every thread of this discussion last night and one of the posters mentioned what I am talking about. For example:

https://www.amazon.com/gp/product/B00RZUOCOS/ref=ox_sc_act_title_1?smid=ATVPDKIKX0DER&psc=1

or

https://www.amazon.com/3M-1602-R-Electrical-Insulating-Sealer/dp/B001CGVHN0/ref=sr_1_1?ie=UTF8&qid=1518538978&sr=8-1&keywords=3M+1602-R+Electrical+Insulating+Sealer+12-oz+Can+Red

On another note, the folks in Powerwall land ...as well as those who use 18650 packs for EV cars seem quite concerned about fusing individual cells. So on those forums/Youtube etc. there seems to be a lot of use of Bus Bar wire for connecting series cells. An example of it is here, skip to about 12:15 if you do want to watch the whole procedure:

https://www.youtube.com/watch?v=xOVEnbbhbRY&t=118s

So what about this folks? The 20awg busbar wire he is using would only be good for 5 amps to my knowledge, so that doesn't add up for high amp draw situations. But are there any concerns and approaches here for fusing individual cells?

Thanks again

RH

 

[Headrc]

Here is another one, can someone pinpoint any problems of the design of this pack ...and it is specifically designed for an Electric bike. From what I am understanding here is this seems to follow the proper high current pack design ...except for the resistor leads to connect cells?

https://www.youtube.com/watch?v=adUTP92VFqg

If you don't want to watch the whole thing you can see the finished pack around 25:12

 

[spinningmagnets]

I'm not a fan of soldering onto cell ends, especially the negative electrode, but I have to say I would definitely not use the technique shown. He says to get a good soldering iron, and then he shows a 40W unit with a fairly small tip. Then he holds the hot tip onto the negative electrode as an example for about 20 seconds to get a solid solder connection.

I use a 100W soldering iron with a fat tip (for thermal mass, so the tip doesn't cool down as soon as it touches the subject), and I can tin the end of a cell with solder in just a couple seconds, so the heat doesn't reach down deep into the cell.

 

[Headrc]

Nods ...I agree on his soldering technique. But what about the pack design itself? It looks like it is following all the rules here for a design ...except for the possible resistor wires for the fuses/connections. Again, it seems like that is not a concern on this forum as well. On the soldering, from what I have researched ...it depends on how well it is done. The top two (that I can see) followed powerwall/EV pack builders on Youtube both use soldering instead of spot welding. It started from what I can see as a result of the cost of a good spot welder but they seem to have stuck with this technique and are happy with it. Of course, a powerwall does not get the physical abuse that an EV bike probably gets but an EV vehicle could with the condition of roads nowadays! Thanks for taking the time to respond. RH

 

[spinningmagnets]

The main reason the Powerwall guys use individual fuses on the cells (even though it's a fixed non-mobile installation) is because it's popular to build with used cells that are typically $1 each. Sometimes they overheat under normal use...

 

[Headrc]

Right, but Tesla also does it.

 

[tsourorf]

As pointed out in a previous post, connecting the input/output on the first of all these parallel cells is a big mistake. The first 7 cells of the pack will take most and give most of the power, with each one of the rest of the 10 parallel rows contributing less and less as we move farther from the top.

I know the copper bar will minimize the losses and the difference, but there still going to be a difference and the pack will under-perform.

 

[Headrc]

So exactly how would this design be approved upon? I guess I better go back and read through all of this discussion ...I thought the busbar on those parallel in's and outs were solving this problem.

 

[RootedSuperuser]

So, anything wrong that is previously unknown?

Here is a identification chart of batteries and the wrap that they have....
https://drive.google.com/open?id=1p8yGDk8hVn7LbdOhq8-uOypXN8ZomzzN

 

[brumbrum]

Holy moley batman, my 5 year old daughter could make a pack better than that! :lol:

 

[RootedSuperuser]

Holy moley batman, my 5 year old daughter could make a pack better than that! :lol:

This type of build will probably cause The Insurance Bureau of Canada to have a exclusionary rider for any type of lithium EV battery stored in/on your premises, attached or unattached garage or out building including a garden shed, this might even extend to eCars and Hybrids.

I'm sure that this will be followed through in many other countries that have experienced residential fires due to Lithium batteries as this has the heaviest cost to loss of life.
Laptops may even be legislated to be powered by a converter with no battery when stored/used in a residence. Laptops and now VAPING fires are a huge contributor to the Li issue, phones, not so much now.
I have been checking on insurance rates for my ebike.

It's coming, the stats are there for the insurance company's to crunch, and as the crooks they are, anything to low ball or prevent a payout they will take advantage of.

Notice the bunk beds?

 

[Buk___]

This pack good for 80A max discharge 1mm pure copper wire for the series 6x
The pack:3s 28P 10.8V 81Ah 8AWG wiring no direct heat to the cells on soldering the copper serie/parallels

As best as I can model, that is a near perfect pack; with all the individual cells delivering 2.86A or 2.85A when the pack is delivering 80A.



Its too complex for a url link, so you'd need to paste the following text into File->Import from text, to recreate it:

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[tsourorf]

This is nice Buk! Basically, you are using copper to minimize resistance and trying to keep equal distances from root to each parallel cell. I say it's perfect enough for pragmatic applications.

After the last complicated 16p pack I've made, I though I should try a much easier and possibly better way of connecting my cells. I have ordered a 0.9mm thick C101 copper sheet to cut to pieces that cover all the parallel ends. Then spot weld 2cm long 7x0.15mm nickel strips on the cell ends. Cut through the copper right where the nickel strip is, to let the nickel strip go through the cut. Then fold the remaining nickel strip (Approximately 1cm long) on top of the copper that lies on top of the parallel cells and spot weld again! This should practically give me no resistance (ok, it's just minimal) from the wire to each cell because the current will go through the whole copper until it reaches a very small portion of nickel and then go though the cell.

Once I receive the copper sheet I've ordered and try it, I'll post pictures.

 

[Buk___]

This is nice Buk! Basically, you are using copper to minimize resistance and trying to keep equal distances from root to each parallel cell. I say it's perfect enough for pragmatic applications.

All the props go to Joachim, he built it. All I did was try to model it