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I'm putting together an 18650 battery pack for an e-bike. The BMS is specified for 10s 36v 40a. I want to avoid soldering, spot-welding, or an end cap kit if possible.

I thought I would make each group of 10 cells in series to reach 36v, and then those groups would be in parallel. This would allow me to just use dual tubes of 5x2 cells a piece to reach 36v.

But from what I'm reading now, my understanding is that a BMS does not work like this. I should actually connect each group in parallel to reach desired capacity, and then connect the groups together in series.

Assuming this is correct, is there a way to make the pack in a similar way as I was planning before, with tubes instead of a brick?

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  • \$\begingroup\$ Isn't the tube method just another name for the end-cap method? \$\endgroup\$ – DKNguyen Nov 25 '19 at 23:27
  • \$\begingroup\$ I don't think so. The end-caps that I'm referring to are the vruzend kits that let you just bolt on connectors to the snap together caps. It would still end up as a brick, instead of a tube. \$\endgroup\$ – FallenSpaces Nov 25 '19 at 23:35
  • \$\begingroup\$ "what I'm reading now, my understanding is that a BMS does not work like this." - it can, but you would need a separate BMS for each series string. \$\endgroup\$ – Bruce Abbott Nov 26 '19 at 2:20
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Constructing battery packs without hard mechanical connections is usually a very bad idea.
Or worse.

In some cases the environment may not be hostile BUT an ebike is not such an environment. 40A is at least 10S2P for any 18650s.
(If you know of any manufacturer rated 40A discharge 18650s please point me to them - I'd be genuinely interested).

So <= 20A / cell in 2P. Maybe 6.7A or 5A in 3P and 4P arrangements (deep ending on cells used). The odds of all those connections remaining good in an ebike environment is minimal.

Spot welding / tabbing is cheap and easy.
A DIY spot welder is easy and cheap compared to all the other input required.

Soldering is doable with fine bridge wires BUT a bad idea.
LiIon cells should not be soldered.
Fine wires plus luck skill and speed may allow you to 'get away with it' Or not.


Soldering:

Soldering is a VERY bad idea - especially if you are not already skilled in the art of soldering. Heating the end of a cell tends to break the electrical connection and heat elements of the interior that should not be heated. While some people report good results this is by no means certain. A "quick dab" to keep temperature rise low is a good way to create terrible joints.


Spot welding:

There are numerous DIY spot welder descriptions on web.
This can be done using a battery as an energy source, either directly, or by adding a large capacitor.
A usually available method is to use a microwave over transformer. Instructions are given in each case, but the basic method is to remove the high voltage secondary and add a few turns of thick wire as low voltage secondary. Some people build elaborate mechanical electrode assemblies but something very simple indeed will suffice.

Here are many googlabet search links to DIY microwave transformer spot welders and here are many images - each linked to a related page.

enter image description here

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  • \$\begingroup\$ I've been soldering packs for decades and never had a failure, but it does some require skill to get a good joint without cooking the battery, so I don't recommend it. (I use desoldering braid saturated with solder to make the connections). \$\endgroup\$ – Bruce Abbott Nov 26 '19 at 2:37
  • \$\begingroup\$ Wouldn't I need a high cca battery with posts for spot welding? I only have a couple 12v SLA motocross batteries that have tab terminals. I have everything I need for soldering, but I've never soldered anything before, and I'm definitely worried that I would mess something up. I did notice that there are ways to do it that could make things easier though, such as tinning the braid/cable first, and then just hitting the top of it to solder in place. I suppose that would depend on the size of the braid, and you probably couldnt do it with tabbing or thinner wire. \$\endgroup\$ – FallenSpaces Nov 26 '19 at 6:40

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