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Logic puzzle.... how many bus bars?
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paulca:

--- Quote from: T3sl4co1l on July 04, 2023, 08:36:14 pm ---Misuse of tool: GPT has no reasoning.  The only way it could possibly answer such a question is if it were a regular topic of conversation, so that it could be trained on these or related phrases.

Is it really that hard to draw out a basic circuit diagram and count..?

Tim

--- End quote ---

ChatGPT did a very good job of answering it.  It was my lack of explaining the bus bars where terminal to terminal only.  It gave me 3 solutions which would have worked, the first two it put the packs in parallel, but the later it did something different, it's just it added up the bus bars wrong and suggested, at the end I only needed 7... when it had described at least 12.  This is a common problem with "interative" conversational engage ments with GPT.  it's errors and confusions compound and reoccur.

i did draw out the basic circuit in my head, it says I need 23 bus bars.  16 to parallel the pairs and 7 to series the pairs.

Maybe I'm wrong, maybe that is the simplist form, if I can't change the size of the bus bars.  If I had bus bars which were 2 terminals and some with 3 terminals long, then I would need 16 of less. 

Buying a sheet of copper is about £10.  However, buying a metal cutting bit and trying to cut them out with my jigsaw sounds like a recipe for disaster.  Buying a vice and an angle grinder and a drill press also sounds like a bit too far!

Ordering various bus bars from china and waiting for 2 weeks is seeming to be the cheapest way to get plated copper bars.

The alternative of adding a second BMS and running 2 packs is something I want to keep for the next expansion up.  8S2P per pack/bms.

Also I do have a bag of 6x6 lugs and 6AWG cable to make links.  The downside is, they tend to have a slightly higher resistance than a bare NPC bus bar.  Then again I'm setting the max current on these packs at 40A, so it's not like I'm pushing 200A through the interlinks and creating a ton of heat.

T3sl4co1l:
Would you not simply re-stack them using two 2-position bars and seven 4-position bars? ???

If you wish to use the existing links, then each 4-link can be made from three 2-links (assuming the bolts are long enough to join two, and you don't mind the slight increase in contact resistance), and any additional parts will have to be made.

If you need any arrangement other than everything in one line, or maybe a side by side (2-row) arrangement, geometry gets much more complicated, and, you might be better off with lugs and cables I suppose.

Tim
paulca:
I was hoping that ChatGPT would come up with some resource out there where they discuss these odd geometries.

There is, for example what they call, snake layout.  It puts 8 or 16 cells into series while leaving the remaining + and - beside each other for convenience.  Other layouts use square bus plates to common 4 adjacent terminals of 4 cells and then run the series links around the outside.  it was seeing these "kits" which made me realise there may be some more mathematically clever things I could do.  These kits where for bigger batteries and... the collection of cut and drilled bus plates, in bare unplated copper was £250!  As much as half my battery!

I'm not sure I need any of this, as you can see cabling is fairly messy and I have plenty of room.  The plan, by the way, is to tidy that up with cable ties.  A plastic sheet will fit over the cells and the BMS, breaker, etc. etc. will sit atop of that.  The box will be closed, but not sealed.  The whole thing exists in the garage where the only combustable item except the box it's in, is the wooden table it's sitting on.  Unless the flames get up to the roof joists, it'll be fine.  THe garage is detached, if it burns to the ground the house will be completely fine.

i have 16 bars.  I can create 8 pairs.  I just need to either measure and order 7 more for putting them in series, or I make 7 jumper links.

For space I think I need two rows of pairs.  Basically as they are in the photo, but rotate each odd cell and put 2 links to make them a pair.  Then starting in teh top left, series them left to right, down a row and right to left.  Leaving the + in the top left and the - in the bottom left.

It's just that bus bars are hard to find, and will most likely take 2 weeks from china.  I suppose I best stop talking rubbish and order some.
AndyBeez:
Dude, just draw ChatGPT a circuit diagram. I think it needs all the help it can get. Preferably the answer.

To me 8S2P means a pair of eight 29.6 volt series battery packs linked in parallel to double the amperage - as per the photo.

So agreed, 7 battery links (per pair) and 2 cross links to make a parallel makes 16 bus bars.

I think the issue is the possible mismatch between the capacities of the two packs. You need to consider the collective maximum capacity of the combined (paralleled) packs as being that of the lowest individual pack. This way the lower capacity pack is not over discharged - instead the higher capacity pack remains inside its discharge range.
Ian.M:
Its a fencepost problem!  (the posts being 2P cell pairs and the spans links)  Therefore the answer is NS-1 + two ends.

Your preferred folded layout is effectively two 4S2P packs in series so you need six new four hole bars + four of your existing two hole bars (links) + whatever jumper or bar you need to link the two 4S2P halves of the pack.



If you weren't folding the pack, (i.e. build as straight 8S2P) its seven four hole bars and only two two hole bars.

If you are a cheapskate and don't care about additional contact resistance, a four hole bar can be replaced by three two hole bars, and you'd have 10 of them left over from reconfiguring the packs in the photo, so could save three four hole bars.  :-\
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