Logic puzzle....  how many bus bars? - Page 1
Forum section: electronics / general

author: paulca
author_rank: Super Contributor
timestamp: July 04, 2023, 02:41:40 pm
content: Having had chat GPT fail at this spectacularly. I thought test you guys out.to create a battery with 8S2P cell configuration, how many bus bars is the minimum required?As this is a lithium pack, the cells need to be in parallel first, then the parallel groups put in series. This is the part I could not get ChatGPT to understand.The obvious answer is, you need 2 busbars for each cell pair to put them into parallel so if you have 8 pairs you need 16 bus bars. Then additionally to put the 8 groups into series you need another 7 bus bars.Part of me however is saying ,"There has to be a mathematical trick to this, if you lay them up a specific way you might save ordering a few bus bars."What say ye?

author: Nusa
author_rank: Super Contributor
timestamp: July 04, 2023, 02:52:21 pm
content: In fact it's quite common to lay out battery packs to share bus bars. 1S = 2 bus bars. 2S = 3 bus bars. nS = n+1 bus bars. 8S = 9 bus bars. The parallel aspect doesn't change things, it just makes the bus bars bigger.

author: paulca
author_rank: Super Contributor
timestamp: July 04, 2023, 03:12:19 pm
content: Quote from: Nusa on July 04, 2023, 02:52:21 pmTrue. What if I added that you already have single link bus bars (the majority of) and the cost of buying/fabbing common bars shared between many cells (like 7 arge ones) is excessive (£150+) for copper and .... you'd have to have them fabbed for you if you want them plated.i can buy these bus bars, but either a 2-4 week wait from china or £5 a pair locally.

author: paulca
author_rank: Super Contributor
timestamp: July 04, 2023, 03:14:11 pm
content: Alternative to buying more bus bars include:Buy a second BMS and put the packs in parallel. cost ~£100 + shipping wait time from china.Fabricate the missing "bus bars" from 6AWG wire and crimp lugs.

author: Nusa
author_rank: Super Contributor
timestamp: July 04, 2023, 03:57:24 pm
content: Sounds like where you went wrong is failing to adequately describe the physical aspects of the batteries you had in mind. Still haven't, really, but crimp lugs suggest you're talking about large cells with screw terminals.The problem as originally described makes one think of 18650-like packs, which typically have cells spot-welded to bus bar plates or the equivalent made up of nickel straps. The spot welder for this kind of task is in the $50 range these days. I've put together a fair number of tool battery packs. (The very first one I ever did was soldered, and it works. But that's not recommended, and I knew it.)

author: paulca
author_rank: Super Contributor
timestamp: July 04, 2023, 04:56:13 pm
content: 2 8S packs. I want to make one from them. I have 16 bus bars.Top pack is the new one, switch I swapped in to test/condition/balance/assess it. Basically so when I come to make parallel pairs I can match lower capacity cells with higher capacity ones and capacity balance the pack better.These are cheap, possibly second hand 105Ah (with a pinch) cells. Minimum datasheet capacity is 95Ah and even that is optimistic. I'd say some are more like 88Ah and some 95Ah, but none 105Ah. But again, the were cheap. Like £100 per kWh shipped, £260 for 2.4-2.6kWh. i'll take it.

author: paulca
author_rank: Super Contributor
timestamp: July 04, 2023, 04:59:16 pm
content: I need another 7 bus bars and some clever tetris I think.

author: paulca
author_rank: Super Contributor
timestamp: July 04, 2023, 05:05:14 pm
content: The option ChatGPT continually gave me was to just connect those end + and - together. 2 large connects and I'm done.Of course in doing that i then need a second BMS to maintain the other 8 cells. The BMS I have and none i have seen will manage parallel packs. Why would they when they can just say, "Buy a second one!"

author: themadhippy
author_rank: Super Contributor
timestamp: July 04, 2023, 05:13:29 pm
content: Quoteat least one more than you ordered

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2023, 08:36:14 pm
content: 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 reallythathard to draw out a basic circuit diagram and count..?Tim

author: paulca
author_rank: Super Contributor
timestamp: July 05, 2023, 10:33:03 am
content: Quote from: T3sl4co1l on July 04, 2023, 08:36:14 pmChatGPT 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2023, 10:47:08 am
content: 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

author: paulca
author_rank: Super Contributor
timestamp: July 05, 2023, 12:01:22 pm
content: 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.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: July 05, 2023, 12:19:09 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: July 05, 2023, 12:41:12 pm
content: 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.

author: paulca
author_rank: Super Contributor
timestamp: July 06, 2023, 08:48:08 am
content: Found a local supplier with the right bus bars. £5 for 4. got 8. Should arrive tomorrow. Kinda means I need to hurry up and capacity order these cells so I can match pairs.Note, I'm not bothering with actual capacity test. I'm top balancing the pack with a nice long absorb time and then I'm going to run it flat to LVC or close to.The lowest cell a that point will be matched with the highest cell from the other pack and so on.

author: EPAIII
author_rank: Super Contributor
timestamp: July 10, 2023, 11:15:31 am
content: Exactly.Quote from: T3sl4co1l on July 05, 2023, 10:47:08 am

author: paulca
author_rank: Super Contributor
timestamp: July 10, 2023, 11:38:29 am
content: Quote from: EPAIII on July 10, 2023, 11:15:31 amOk. Now find me those of the right size.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 10, 2023, 11:52:02 am
content: Quote from: paulca on July 10, 2023, 11:38:29 amNow, a missive reaction to the one-word reply -- 100% deserved, I get that. But, why use an impossible request to do it..? (You've given no dimensions, nor available / acceptable sources from which to shop or fab such parts; I'm certain the point is, you don't expect anyone to find anything. Maybe this is a common use of phrasing in some areas that I'm just not aware of.)Tim

author: paulca
author_rank: Super Contributor
timestamp: July 11, 2023, 08:58:03 am
content: I honestly thought there might be a mathematical shortcut to not need 23 2 hole bars.My original did lack a few details, mostly because of "hopping domains". Hopping from domain of DIY solar power to the far wider, more generic electronics domain/audience. I forgot there needed to be a lot more specifics as people wont be familiar that cells come in prismatic form and traditionally bus bars are 2 hole, terminaL to terminal only.Part of the reason for limiting the number needed is because nickle plated copper bus bars not not cheap or even easy to find! They ship 1 per cell with the cells.Suggestions to use 4 hole bars or 3 hole bars, would be fine if I had a band-saw, a drill press and a nickel electro-plating bath.Most of this was established about a week ago in the thread. So people agreeing with previous points already rejected was starting to get annoying.

author: Ian.M
author_rank: Super Contributor
timestamp: July 11, 2023, 12:02:26 pm
content: What's wrong with tin plated busbars? A PITA to make from copper flat bar stock with just hand tools, (as ideally you'd use a drill press to get the holes straight), but you can cut to length with an ordinary hacksaw, and tin them yourself with solder, paste flux, a blowtorch and a cotton moleskin rag (or a wad of cotton wool) smeared with flux to wipe with.

author: paulca
author_rank: Super Contributor
timestamp: July 11, 2023, 01:47:36 pm
content: The bus bars I ordered turned out to have the wrong sized holes.Having discovered how tricky it is to ream them with a cheap (yellow painted) dril bit was, I had to order a hand reamer.

author: Ian.M
author_rank: Super Contributor
timestamp: July 11, 2023, 02:08:01 pm
content: Copper is soft and grabby. Ideally you should stone the front of the cutting edge to a neutral rake to reduce the drill's tendency to self-feed. Use a sharp HSS drill bit, with a bright or TiN surface finish, not black oxide (the chips tend to stick), and keep it wet with a suitable cutting fluid. WD40 will do at a pinch.If you don't have a drill press, a reamer was probably the best choice.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2023, 04:03:14 pm
content: Not only that, it's the contrast between relatively hard nickel plating and soft smooshy copper underneath. Maybe the plating isn't enough to do anything once you're through, but it might make it difficult to start.I'd be fine with plain copper, myself; but I don't know what if any battery standards are, if particular platings are recommended, or required. Some anti-ox grease on the mating face should deal with things well enough, I would think. Unless of course the problem is interfacing to a different metal (the batteries aren't aluminum or something are they?), but even then copper isn't incompatible with too many things I think.Ah well; definitely enough money in there, it's worth doing right.Tim

author: paulca
author_rank: Super Contributor
timestamp: July 11, 2023, 05:41:29 pm
content: Unplated copper is fine, you can scotch bright the hell out of it, get it nice a pink and shining before matting the busbars to the terminals. However. Copper will not stay that way, it will oxidise which at very least means you have to repolish them when you move/rearrange them. There are other issues with copper on aluminium terminals, although I understand? that is only really an issue if they get wet... which is unlikely.The only drill bits I have are cheap "yellow" pretend titanium coated. The flutes are blunt, hell half the tips calm blunt out of the packet! I'm fairly sure if you did have a drill press, these bits would not cut a chip out of a surface, they would simply skate on it and get hot.Tring to ream one and the bit self fed... or grabbed and tried to break my wrist. I did persist until the bit spun freely and moved it around like a reemer until it started to smell. (drill + hot metal). There was swarth on the floor and it had worked.I thought it would be better to clamp all 8 toegher with some mole grips and having put all 8 on the bit already, I clamped them and tried to ream them. Nearly broke my wrist twice and when I did get the bit to spin it only reamed half of them.The hand reamer ... if it isn't a blunt unfaced peice of shiz should be fairly slow, but more reliable.

author: themadhippy
author_rank: Super Contributor
timestamp: July 11, 2023, 06:01:14 pm
content: For drilling stuff like copper or stainless i find my trusty ole sds drill is the ideal tool,low speed and plenty of torque,just remember to turn the hammer off.

author: paulca
author_rank: Super Contributor
timestamp: July 12, 2023, 05:34:47 pm
content: Hand reamer did the trick. Although it luckily fitted the drill chuck or I'd have been there all night. The reamer was not "unfaced" it was (originally) extremely sharp and so it just peeled a bit of copper out and jammed too hard to turn.Plan B. In the drill on high speed, small squirt of lithium grease (the only thing I had handy) and it reamed them out in seconds.Pack built... which unsuprising turned out to be 6x4 way links (each made from 3x2 hole bars) and the remaining 2 way links.Turns out I could have saved 3 bus bars.The fold in the pack was too long of the bus bars, so I put 2x 6AWG wire links across with 6x10 lugs. So those - - and + + junctions are un-needed as the series connection is connected via both cells.The other end, the BMS has 2x negative lugs, 10AWG by 2. This I could have connected one of those to a - on each of the final pair.I'm sure I have a bunch of resistence caused by stacking the bus bars, but ... it's 100 Amp classed battery which will probably never see more than 50 Amps and it's now finished. I'd estimate the "working capacity" including losses in the inverter is about 4kWh. To be tested.The next battery I build will be bigger, more likely 300 Amp class and I'll have to plan ahead and budget proper bus barring.