Battery pack build

So, I just got a new toy! A spot welder, and some 3500mAh 18650 cells and nickel strip. This is going to end up being a long stick, because it’s going inside a acrylic tube and I’m making another light-up cane.

I’m debating if I want to do 4s2p, or 8p, assuming I can fit all 8 cells in vertically along with controller, voltage buck (or boost) to 5v for the LED strips, BMS/PCB, fuse, power switch & charging port.

For 4s2p configuration, I got a BMS that can do balancing, and I’m trying to see if I should do that, or just do like the last one, all cells in parallel and keep it simple and easy. My reasoning to go for 4s2p is I can put in more power and charger quicker for the same amps (I’m figuring a 2a over a 3a rated small JST type connector). Fully charging might be a challenge at some points in time, if I want to use it multiple nights in a row in 8p, since I can only do 4.2v.

My questions on 4s2p configuration, is what AWG should the individual cell/balance line lines be? Can I get away with 24awg, to keep the extra wiring along the cells very small, and tight to the cells? Or do I actually need to go up to 20awg?

And also, I’d do 2p and put those cell groups into series, right? Thinking about physical configuration, I can put 2 cells (A) + to +, fold over into vertical orientation, then the next group is in (B) - to - configuration, and then I join A to B on A’s - to B’s +, and continue. And of course solder onto the nickel strip the balancing wire between each group. That should work out? I’ll have very short nickel strips, and I get the feeling I should pre-bend them to get the crease where I want, so after welding they’ll fold where I want, the way I want.

What’s your BMS balance current? If it’s more than a few hundred mA, I’d be shocked. Most of the small ones are something like 50mA, for which 24AWG is more than enough. If you can’t find it, go see what the balance resistors are rated, and you can math it out at ~4.2V across them.

Correct. Once you’ve got 2P welded together, they’re effectively a single cell, chemically - they’ll always be at the same voltage. Any modern lithium pack will be parallel groups in series. The only time I’ve seen the opposite was older BionX packs, but they used a kind of screwball chemistry anyway that isn’t relevant anymore (spinel LiMn, it was legitimately self balancing).

I’m not sure what you’re proposing here, actually. If they’re going to be a single “8 cell long” layout, it’s actually rather difficult to do that with what you want, since you’d need quite a bit more than balancing wire running between the non-facing ends. Can you diagram it out somehow? Doing a 2P4S pack in a single line is going to be tricky, and have a lot of potentially exciting points in it.

But, yes, you should probably pre-bend the strips if you care about exact bend points.

So I got, since they physically will fit into the tube well.

Yeah, going to be tricky. Basically a long stick. And yes, I’ll have to run a larger gauge up the entire length for either the Pos or Neg, probably solid core 18awg, since it doesn’t need to flex at all. What I used for v1. But in this instance, I don’t need to to run 2, just 1, since the other end I can have a short length to tie into the boost/buck boards to get the 5v I need.

Hm, and as I go to diagram it out, it’ll be more complicated than I was imagining, since I’ll need the balancing wires.

I was thinking this:

A-==++==- B+==–==+ C-==++==- D+==–==+

However, unless on B & D I add a wire from one + end to the other, I can’t add the balancing lead to it. Which I’m not sure that’s a good thing to do or not, since it’d make a (very!) tiny impedance difference between the 2 cells for balancing. Probably doable & OK, but not sure.

I could, I suppose, do a strip spot welded from the middle of each pair (e.g. all of them are like A or C above), and run it down the side of one cell to where it’d connect to the next pair, and well insulate (kaptan tape and 3D printed shield?) separating the 2 battery groups so no dead-short can possibly happen.

I got these cells, so even at only 2p I expect the 5-6a@5v peaks well below the discharge limits, so I don’t think a relatively long strip to put them in series would have significant appreciable negatives on voltage/heat.

It’s probably OK to do that, with the low currents you’re drawing. But it’s going to be a complex enough build, and I’d probably avoid it for those reasons, personally. At least do it with the batteries below 30% SoC, so if they short, they won’t get too exciting.

As long as the voltage can equalize at rest, it’s fine if there are some minor differences from wire length when running.

Sounds like I should do a bit of discharge, they came to me ~3.4v. Not too much more, but a bit more.

And might be a bit complicated, but doesn’t seem too complicated. More I need to have it well planned out, with everything ready to go, along with the heatshrink ready to go. And having measured to be sure it’ll all fit in first, before I start spot welding anything. And probably spot weld a few bits of nickel strips together for some practice, and to find probably what “gear” (why is it that name anyways?) to probably use to start at.

If you drain them to ~3.0V/cell, they’ll be well below half and should lack the energy to do anything more than be a hot nuisance if shorted.

And, yes, you need to plan it out for sure, with a build like this.

Welding strips together is different from welding to a battery terminal. Short answer is “You should start high, and turn it down if the weld is really too hot.” You’ll be in a high vibration environment (walking stick), and my general guideline is that if I can’t put my finger on the weld immediately without burning myself, it’s too hot. Otherwise, if you’re doing several welds, just let it cool down between welds - perks of spot welding. As long as you can (somewhat uncomfortably, perhaps) touch the terminals, you’re not overheating them welding.

1 Like