Modular 18650 battery

I was curious; is there a reason why all the 18650 packs I see spot welded together are constructed as single monolithic units? Is there a reason why I should not weld them into more manageable units akin to lipo brick sizes, and then wire them together? The upside to this approach would be that if one unit did fail, it would be easier to diagnose and replace. I thought I'd ask if there were any downsides; its a relatively new method of constructing batteries for me and I'm not confident that I know everything there is to know.

Thanks in advance

To me, that sounds like a good idea. I if you have a spot welder, you just do the entire pack with it, since it would be faster.

It would be nice if they sold 18650 'modules' like 5 to 10 parallel cells spot welded together with tabs. Then you could simply solder the modules together to get whatever voltage you want.

Thats exactly what I had in mind, 1s10p or 1s20p blocks that I could just connect in series with xt90 connectors. If my BMS sees a bad cell group I want to be able to get it out of there without hauling a 20kg battery out of my bike and cutting a whole cell group out of the middle in order to find the one dud cell. Im not even sure I would know how to cut it out once its all glued together, so I'd rather not glue the groups together in the first place if its all the same to keep them separate. My fear was that perhaps people are constructing them as solid 1 piece batteries because of mechanical concerns about the 18650 format, or something like that.

i would guess it's mostly due to buit-in BMS. If you have a built in BMS, charging gets much simpler. I build my own packs into small bricks, but its time consuming to charge each pack individually. I am contemplating on switching to built-in BMS and ditch my icharger, but I would lose the flexibility to combine multiple packs for range or power requirement (or at least lose the convenience of charging the battery mounted on the bike, and not have to take it out).

I have several batteries built like that. It works great.

More wires and connectors means more chance for problems. That being said I will be, over the winter building another 18650 pack myself. This one will be modular. Haven't figured the final design yet, it will probably change, but it will be something like this.

warrah said:

I was curious; is there a reason why all the 18650 packs I see spot welded together are constructed as single monolithic units? Is there a reason why I should not weld them into more manageable units akin to lipo brick sizes, and then wire them together? The upside to this approach would be that if one unit did fail, it would be easier to diagnose and replace. I thought I'd ask if there were any downsides; its a relatively new method of constructing batteries for me and I'm not confident that I know everything there is to know.

Thanks in advance

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All commercial lithium cells are spot welded to minimize heat to the cell. The flat nickel stips spot weld easily and are compact interconnects as well. Since manufacturers know the shape and size of their product, they do one automated spot weld cycle so it ends up a monolith.

But an individual can make them in any electrical and physical arrangement that is needed. If you want to fit in the triangle, you can certainly do that by assembling your bunch of cells that way. Or you can mimic a LIPO pack, whatever. It's like stacking LEGO blocks.

I'm making a replacement battery now that has to fit in the existing shell. It looked like I could fit about 52 cells in the shell. The target voltage was 24v. I had to scratch my head a bit to find an arrangement that worked. I ended up with 7 modules of 7 cells, all I could fit in. There are 6 blocks of 7 cells, and seventh module fits into a leftover scrap of space in a less regular layout. I'm including a BMS from China via ebay. This will make a nominal 24v 14Ah pack for an existing ebike. I have a Chinese ebike with leaking lipo packs that is next, and it will be a completely different layout.

So there are no restrictions. With 18650 cells you can of course build any size you need, larger or smaller, in odd shapes, so they are extremely convenient.