Parallel protection 18650 pack

[jvens]

Hi guys,

I am building a serie of 10S20P batteries. I use copper strips for that, with nickel in the middle, so I can spot weld it decent to my cells.
In the end, a BMS will be attached, that is connected to each serie.

Now, I have been reading about the tesla fuse thing.. I see some people (that don't have welding opportunities) connect all of the cells with this kind of wires, to copper strips, laying on top of the packs.

My question is, IF you are able to weld, as I am, would it be a good idea to build just one wired fuse, per parallel string? In my case, 10S20P = 20 wires..?

So I can weld the whole pack efficient and safe, and fuse each string just with one fuse..? Or is it not neccesairy?

I was thinking, if one cell is broken, and you have everyhing connected parallel, than your currents are rising extremely for the other strings, right?

 

[LuboN]

Either fuse each cell or don´t bother at all... It is there to prevent parallel cells discharge to damaged (shorted) cell in the group.

 

[spinningmagnets]

I agree, one fuse per cell is best.

 

[jvens]

But why..?

If one cell in the middle of the serie will break, the curent will rise, correct?
So if you have one fuse at the end of the string, it will burn down right?

If I am wrong, what are you thinking of not using them at all? I have designed several packs without fuse wires, and didn't see problems here..

Or are all packs with special designed copper+nickel strips still dangerous?

 

[izeman]

If one cell in the middle of the serie will break, the curent will rise, correct?
So if you have one fuse at the end of the string, it will burn down right?

No. Imagine on cell completely fails. Goes to zero volts. Has extremely high resistance. And is still fully connected to all the other batteries in this block. It may be disconnected from the big battery, but will still be able to (in the best case) destroy the whole parallel string.

 

[jvens]

Hmm, what is the best idea in this case..?
I had the idea to use copper strips and just weld them on the cells. A bms keeps an eye on every serie cells. Why do I see the tesla fuse not so much.. is it safe to keep them away, with a good bms on board?


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[izeman]

That's a good question. Normally ebike packs are NOT fused at all. I don't know why. Maybe because the energy stored is relatively small compared to electric car batteries? So in case something goes wrong the fire is not THAT big??
It would make sense to fuse EVERY SINGLE cell. A BMS is still a must in my opinion, because otherwise it's hard to even notice when something goes wrong, and it's a last wall of safety.
I think it's a economical thing not to fure ebike batteries. It would add additional cost and complexity.
My best bet is to use high quality geniune cells and just weld them together as ALL big manufacturers do it.