Common pack design mistakes, how to avoid?

LiFeCycle said:

I've never liked hot glue - far too weak. Planning to build a battery pack glued together with liquid nails. Should be a lot stronger than hot glue. Any concerns?

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I challenge you to get a hot glued pack apart without destroying things. I've tried. Properly built, it's stronger than the plastic wrappers (they tear when you try to disassemble things).

BionX has been shipping hot glued packs for most of a decade, and I've not seen any of them come apart or even be flimsy. They do shrink wrap everything together afterwards, though.

Syonyk said:

I challenge you to get a hot glued pack apart without destroying things.

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I always found it to be easy enough. https://www.youtube.com/watch?v=dNNqSXgj6DE

In the 10 last years i have built dozens of battery packs ofvarious engineering level.

For all those i have kept the same common rules:

1: Good current sharing across every cells,
2: good thermal management
3: good mechanical resistance against vibration and moisture


For the point 1, this is one very important that most people pay attention to, however some very stupid errors can ruin the pack very easy.
For exemple, for my mongoose 2.3kWh buld in 2008, i have used parallel connections that connect in serie to each other by connecting them to both end of each row.. but did not used serie connections in the middle and thre is a perticular reason for that: the current is function of the cell internal resistance, and internal resistance is function of cell temperature. The cell temp at the end of each paralle row is lower than the temp in the middle. Think about it, this is very simple! if the Ri of the cell in the middle is lower due to higher temp, it will conduct MORE current and cell that are at both end will be colder and conduct less current... The serie connections made closer to these colder cell actually compensate and make the total curent share over the entire paralleled cells to be uniform. This pack ran over 16500km and only required few rebalancing and it does not have any BMS.

So YES current share and thermal management ARE IMPORTANT :wink: and if well made the pack last longer.

The second point: the thermal management is also very important. as well for mechanical tolerance and cell health... Hot glue is the cheap chineese PROBLEM.. I have dissassembled hunderd of OEM ebike battery that all had problem with bad cell connection due to vibrations due to hot glue that just unstick from the cell... Hot glue is bad for lower temp it become harder and break. Then the cell only rely on the cell connection to mechanically stay in place witch is bad.

Point 3: Good Mechanical resistance. i have dissassembled thousand of Makita packs.. yes thousand! and i also say the improovement made over the various version and years.
I can conclude that the packs that have the most problem are due to: Bad cell (bad apple), and bad cell connections due to hard environment where they are used at. and cells had completly broken spoweld... over teh years Makita have reduced the thickness of the nickel sheet and extanded the surface to allow flexion to take vibrations.. but these cells are now also very nicely hold together by plastic structure witch take most of the mecanical vibrations.

DIY Pack fail due to short circuit between cell case because people think the thinny green skin is strong enough to isolate between cells. ERROR!.. cell wirring is also something to think about... the place where the wires path is is also VERY critical.. that wiring must be secured correctly for any temperature and not rub against other sharp electrical connections... the weight of the pack must not be supported by balance connections wires too... all fact that seem an evidence for some is often completly forgot by others...

This IMPORTANT thread will get alot pages over the month i feel! 8)

Doc

Doctorbass said:

So YES current share and thermal management ARE IMPORTANT :wink: and if well made the pack last longer.

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I went completly artisanal on this one. It's my first build and I'm more an amateur neophyte than an expert like you.

Asides from the springs in my build (I plan to modify that), Are my series connections adequate ??? (photos here) :
https://endless-sphere.com/forums/viewtopic.php?f=14&t=57810&start=150#p1213147

I'm also nickel-plating every single copper piece to limit future oxidation in order to limit copper oxide or "vert-de-gris" formation (photos here):
https://endless-sphere.com/forums/viewtopic.php?f=14&t=60364&start=25#p1235640

What do you think Doc ?

Matador

Matador said:

Doctorbass said:

So YES current share and thermal management ARE IMPORTANT :wink: and if well made the pack last longer.

Click to expand...

I went completly artisanal on this one. It's my first build and I'm more an amateur neophyte than an expert like you.

Asides from the springs in my build (I plan to modify that), Are my series connections adequate ??? (photos here) :
https://endless-sphere.com/forums/viewtopic.php?f=14&t=57810&start=150#p1213147

I'm also nickel-plating every single copper piece to limit future oxidation in order to limit copper oxide or "vert-de-gris" formation (photos here):
https://endless-sphere.com/forums/viewtopic.php?f=14&t=60364&start=25#p1235640

What do you think Doc ?

Matador

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Matador, Congrat for your built, it is well made for a spring type conection. the serie link are correct because the copper bus bar that you have to link all cells of the parallel groups are very conductive compare to the sum of all the spring per row that you have.. i mean you probably have alot of resistance on each spring and even all them in parallel are way less conductive than tghe weakest link you have on the parallel bus made of copper... so all cells should share current equally... just like all parallel transistors does in high power audio amplifier... all them have a resistor in serie to help better current share over all them in paralel.. but in your case these resistors are the springs.

Just pay attention to their resistance relative to the current your controller will draw from your battery.... you dont want to ahve them to overheat! if this happen they will loose their spring feature and will fall into plastic deformation so the force they apply for the electrical contact might become lower and making chain reaction that make them to have more resistance and then overheat more etc.. and ruin the cell affected due to overheat of one of the tab...


Doc

For new readers, here is a thread on getting high-current cells from Makita cordless-tool battery packs from back in 2011, 5 years ago...and this isn't even doctor bass's earliest posts. When he gives advice about batteries, take his advice with careful consideration.

https://endless-sphere.com/forums/viewtopic.php?f=14&t=28285

Doctorbass said:

Matador, Congrat for your built, it is well made for a spring type conection. the serie link are correct because the copper bus bar that you have to link all cells of the parallel groups are very conductive compare to the sum of all the spring per row that you have.. i mean you probably have alot of resistance on each spring and even all them in parallel are way less conductive than tghe weakest link you have on the parallel bus made of copper... so all cells should share current equally... just like all parallel transistors does in high power audio amplifier... all them have a resistor in serie to help better current share over all them in paralel.. but in your case these resistors are the springs.

Just pay attention to their resistance relative to the current your controller will draw from your battery.... you dont want to ahve them to overheat! if this happen they will loose their spring feature and will fall into plastic deformation so the force they apply for the electrical contact might become lower and making chain reaction that make them to have more resistance and then overheat more etc.. and ruin the cell affected due to overheat of one of the tab...
Doc

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Thanks for the cues Doc !
Yes, the high resistance of these springs will make them red and lightning hot at high currents and they will loose there elastic recoil (they'll fall into plastic deformation), therefore amplifying the resistance problem...

I've learned that the hard way since I have already toasted a few of these springs! In fact I had to replace 16 springs of the 126 springs that were in function in my build (my 14S13P battery box was only loaded with 126 cells in a 14S9P config).

I can safely say that I have explored the whole spectum of "Cooked springs" as you can judge by yourself by the following photos. So people, please don't do the same mistakes that I did ! I think I've already spent around 300 canadian dollars on this battery box as of yet (plexiglass is expensive, so are brass nuts and bolts !). But I dont really regret it because i've learned a lot from building it.

As you can see on the pics, once cooked, the springs won't apply much pressure on the 18650, therfore increasing contact resistance.

Hence, my build has one MAJOR flaw... but I was fantasising on making a beautifull SOLDERLESS battery... And using crazy ass big copper busbar for future high current uses (I dont want these cheappy ultrathin nickel strips)... I just could not find more beaffy springs....

Anybody know if very high current springs exists ?? If they do, I wonder if I'll have to do extensive mods to my battery casing.

I tried experimenting a bit with copper to make more conductive springs....
Not easy ! Here's some pics

Feel free to make suggestions if you know of any good high amp springs I coud use.... (I need at least 182 of them for my 14S13P)

PS : for some reason, some pics don't appear... you have to click on the jpg link to see them

I think a solderless pack is a viable design strategy, but I would not run current through the coil springs. I believe the spring and the current-carrying elements should be separate.

spinningmagnets said:

I think a solderless pack is a viable design strategy, but I would not run current through the coil springs. I believe the spring and the current-carrying elements should be separate.

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Right, but the current carrying element should be quite high current rated fo my taste, the equivalent of 14 AWG copper wire (to pass 30 amp/cell)... that's not easy to design for a neophyte like me !

BTW Kurt, what's that reggae song's title in your Youtube BBSHD acid test video ? I enjoyed it...

litespeed said:

I copied maddin's design when I built my 24s15p Konion pack early last year. I think this design of connection is the best home made style.

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Thanks for bringing me up YES it is one of the best DIY design because it provides large cross section and you only need to cut one nickel strip which you can use for series and parallel connection at once (instead of cutting many small ones). Thats why you may like it that much.

Arjan from nkon.nl did accommodate my request and he now has that size in stock
25x0,15mm and 10x0,15mm in PURE Nickel

Allex said:

Great design litespeed, the best actually, but very time consuming and hard to weld

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I did not had trouble when welding those large strips.
The only thing you should be careful is when welding on the positive side of the cells. it could be that you burn a hole through the nickel sheet if the tips of the welder not placed EXACTLY above the tab of the cell (one tip over the tab in the middle on the other outside only on the nickel)
adding those insulation washers will help to not damage the stock insulation if this happens.

when welding many smaller strips, it is IMO much more time consuming because it takes more time for cutting the sheets and additionally you need more welds (sheets must be welded onto others).

Next thing to talk about is how to terminate the ends in the best way, what is the optimal way of adding negative and positive wires to a pack to keep the an even current flow all the way to the wire?

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depends on the intended use.
for batteries for the Adaptto Max-e (or for 100-150A), i beef up the 10mm nickel with 2,5² wire. as main leads i solder two 4² wires beside that one. later connecting it to a single 8² wire outside the pack.
As example the pack has 12p, than i solder one 4² in the middle of each 6p. this will give quite optimal current flow through all cells.

Oh we need pictures madin ))

Here is one with a long row of cells on both positive and negative side, could be a bit tricky to get a good current flow into the main lead wires:
https://endless-sphere.com/forums/viewtopic.php?f=6&t=79612

What about a triangular shaped pack? The space I'm trying to fit a pack into is a triangle. 11Sx36P.

Is it ok to make sub-packs of 1Sx36P and then series them together? It would be easy to shape the sub-pack into a triangle as they are all parallel connections. And then I would need to nest the 11 sub-packs to make a triangle.

I like the idea of separate sub-packs as they will be easy to replace and monitor. Also, less likely for mechanical loads to be transmitted through it. Large packs seem problematic.

I need to draw up a schematic to show the sub-pack triangular shape and connections. Hexagonal shape is ok too. I'm envisioning leads coming off three corners to balance the discharge. Each cell is capable of 10A but I'm limiting it to 7A. I will need a fuse for each sub-pack. There will be no BMS but each pack will contain new panasonic/sanyo batteries and charging will be carefully monitored. Each sub-pack will be connected to a monitor when in use.

I haven't built anything yet so your thoughts are greatly appreciated. I need to avoid design mistakes!!

mkp007 said:

What about a triangular shaped pack? The space I'm trying to fit a pack into is a triangle. 11Sx36P.

Is it ok to make sub-packs of 1Sx36P and then series them together? It would be easy to shape the sub-pack into a triangle as they are all parallel connections. And then I would need to nest the 11 sub-packs to make a triangle.

I like the idea of separate sub-packs as they will be easy to replace and monitor. Also, less likely for mechanical loads to be transmitted through it. Large packs seem problematic.

I need to draw up a schematic to show the sub-pack triangular shape and connections. Hexagonal shape is ok too. I'm envisioning leads coming off three corners to balance the discharge. Each cell is capable of 10A but I'm limiting it to 7A. I will need a fuse for each sub-pack. There will be no BMS but each pack will contain new panasonic/sanyo batteries and charging will be carefully monitored. Each sub-pack will be connected to a monitor when in use.

I haven't built anything yet so your thoughts are greatly appreciated. I need to avoid design mistakes!!

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Split packs can work well if done right. And it is a lot easier when all the packs will be in the same compartment. The main thing you need to think about is the series connections. You are talking about a lot of current with 36p x7A. Will need very beefy connections. Infact so beefy I personally would avoid making it split into 11 packs.

Also give thermal management a thought.

Syonyk said:

I challenge you to get a hot glued pack apart without destroying things. I've tried. Properly built, it's stronger than the plastic wrappers (they tear when you try to disassemble things).

BionX has been shipping hot glued packs for most of a decade, and I've not seen any of them come apart or even be flimsy. They do shrink wrap everything together afterwards, though.

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Challenge accepted.
but before you bet money... better watch/read this:
http://www.instructables.com/id/Easily-Remove-Dried-Hot-Glue-aka-Hot-Melt/

cliffs:
You can easily remove hot glue from cells, with isopropyl alcohol. put it on with a q-tip and the cells will pull right off with zero damage.


BTW like "the best ebike" question.. I think pack "mistakes" is rather dependent upon application.
For example the current sharing comments in the first post wont really apply if you're only pulling a couple amps through it.
Meanwhile for me a huge design mistake would be using round cells in the first place, since I cant pack them as tightly as pouch cells for a given square space. :lol:

That IP Alcohol /hot glue tip should be "stickied" somewhere as a key tool in the pack builders bag of know hows. :wink:
...otherwise it will be lost in the depth of this thread as it has been in others previously.
Model builders have been using it for years, but i guess it just didnt cross over to other applications .

Two tips added to the first post, perhaps copied later to one of the battery building stickies?

Do anyone think that this strategy for a solderless pack could work ?
Photos here : https://endless-sphere.com/forums/viewtopic.php?f=14&t=60364&start=25#p1237600

spinningmagnets said:

Two tips added to the first post, perhaps copied later to one of the battery building stickies?

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Another tip is to put the glued cells in the freezer, once they are very cold you can easily crack your pack/cells into several pieces without damaging the shrink wrap.

Allex said:

spinningmagnets said:

Two tips added to the first post, perhaps copied later to one of the battery building stickies?

Click to expand...

Another tip is to put the glued cells in the freezer, once they are very cold you can easily crack your pack/cells into several pieces without damaging the shrink wrap.

Click to expand...

but putting them in the freezer isnt exactly great for the cells themselves.

Allex said:

Hillhater said:

..just as an example of a good pack design. ..( a stripped commercial pack..Bosch i believe)
note the continuous "buss bar" type strip between cell groups.
also the use of plastic cell holders to retain the cells , rather than glue or relying on the welds.
the only welds are those attaching the strip to the cells

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Lets stick on how to improve packs in this thread and how do design the pack for best current flow, talks about glue/holders and other stuff please keep it in other threads like DIY welding: https://endless-sphere.com/forums/viewtopic.php?f=14&t=68005

Here is the same Bosch pack and how it can be improved. We put 4 cells in a row instead of 4 cells in a square group. This way we have a shorter bus that goes from Neg to Pos and we make 4 connections in total - one per cell instead of one for all 4 cells. Shorter bus and and more series connections gives you less resistance=less voltage sag=less heat that goes to the air instead of your controller/motor. Note that I only have one strip that connect all the cells in same group. This is because you don't need more than one, there is no current that flows between cells in the same P-group so you dont need to add a second one. Now for this particular Bosch it may be totally ok with such design, because the whole power system is only 250W and you probably only see 10-15A tops on the original Nickel bus bar. But for those who runs systems that Are capable of more than 15A the layout of the cells/nickel start to get crucial.

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Very nice post Alex!! Is that holder avaible to buy anywere?

MrDude_1 said:

Allex said:

spinningmagnets said:

Two tips added to the first post, perhaps copied later to one of the battery building stickies?

Click to expand...

Another tip is to put the glued cells in the freezer, once they are very cold you can easily crack your pack/cells into several pieces without damaging the shrink wrap.

Click to expand...

but putting them in the freezer isnt exactly great for the cells themselves.

Click to expand...

To store them like that is ok
http://batteryuniversity.com/learn/article/how_to_store_batteries

I think that during transport, especially boat shipping, the containers see a lot of cold temps.

chucho, the holders like that you will only find for Bosch packs.

mkp007 said:

What about a triangular shaped pack? The space I'm trying to fit a pack into is a triangle. 11Sx36P.

Is it ok to make sub-packs of 1Sx36P and then series them together? It would be easy to shape the sub-pack into a triangle as they are all parallel connections. And then I would need to nest the 11 sub-packs to make a triangle..........

Click to expand...

I have made a few illustrations. The sub-pack is 36 cells in parallel.



Here is the sub-pack assembled



and here is four sub-packs joined in series to make a 4Sx36P pack.



Again, the motivation for doing this is to be able to configure a larger pack that is 11Sx36P in to triangular space. Making a large pack with the complicated network of metal strips is too scary for me to tackle. So I am trying to break the problem into manageable chunks.

I envision the strip pieces to be copper of some width and thickness such that the resistance of the 4 cells in parallel at say 8A each is small. The gauge of the copper cable will be sized to support 300A for about 6 feet.

I think there has to be a fuse (maybe 50A) between the positive cable and each of the strips. This way if a cell goes hot it only cuts out 4 cells in the sub-pack.

I need input on this? I think the key is to minimize battery drain differential in the 4 parallel group. Maybe leads out both ends is necessary? I like the idea of two 6 gauge cables running on both sides top and bottom. Two 4 gauge would be beefy, and heavy. thoughts?

on a side note, why is nickel more common for strips? is it the spot welding? sheet thickness? It certainly isn't conductivity or cost.

Thank you!

mkp007 said:

on a side note, why is nickel more common for strips? is it the spot welding? sheet thickness? It certainly isn't conductivity or cost.

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bare copper corrodes, so its often plated with nickle.
plain copper bars cannot be welded to the nickle plated cells (or tabs)
so either nickle plated copper (best) or nickle strips (common) are used.

if you were not welding them, copper can be used, but then you would want to coat it so it doesnt corrode.. some people do this with solder, or with a silver compound.... the silver coating is more common with larger EV bus bars, than smaller ebike packs.

When a Copper bus or strip is coated with Nickel then welded to an 18650 cell, is the Copper welded to the cell or just the Nickel coating? Maybe this should be better asked in the ultimate suppository for welding 18650 cells.

In the drawings above, I see groups of 4 cells paralleled together then those groups are paralleled again with looks to be thick bus bars to form 36P. I assume that the strips on the 4 cell groups would be welded to the cells and the strips would then be soldered to the bus bars. How much current can be carried by that strip of material joining the groups of four cells? I am asking because it is a known fact here that anything above 0.3 mm thick Nickel gets quite difficult to weld unless you have access to some very expensive welding equipment.