18650 battery pack. Current distribution

Question

I'm creating a 6P20S Li-ion pack from 18650 cells. Here is the design

I am are using 8 by 0.2 mm nickel strips rated for 6.5 A. With 6 parallel branches that's 6*6.5 = around 40 Amps. But since we doubled nickel strips we get 80 A flowing in and out of the battery.

My question is about the current accumulation on each terminal of the battery which contains 6 cells in parallel. I'm thinking that the 80 A current going to the battery cable will have to pass through a single point on the battery which can currently only handle 13 A (1 parallel branch)

So I'm not sure if we should have 6 wires connected to each parallel cell to distribute the current before connecting to the main battery cable. Or could we just triple or quadruple up on the nickel strips at the terminals.

Thanks!

Answer 1

I'm thinking that the 80A current going to the battery cable will have to pass through a single point on the battery which can currently only handle 13A (1 parallel branch)

Depending on where you connect the output leads, that could be correct. Even if you connect it to one of the inner cells the current going through straps in adjoining cells will exceed their rating, since it will be the sum of cell currents in that leg.

So I'm not sure if we should have 6 wires connected to each parallel cell to distribute the current before connecting to the main battery cable.

Yes, that is the best solution. Each wire only has to handle ~13 A so it can be a thinner gauge, which is easier to handle than a single wire rated for 80 A. If the wires are all made the same length then they should distribute the current more evenly between cells.

Answer 2

First, I see that 80 A is a relative current of about 4.5 C. That continuous current would discharge the battery in only 13 minutes. Most Li-ion cells would degrade quickly if treated that way. The current handling of the strips would be a relatively small problem compared to the short life of the battery.

I will therefore assume that the 80 A is peak, not continuous. In that case, the current handling of the strips is less of an issue. What matters to their heat generation is the average current, not the peak current. With that consideration, you may see that doubling up the strips may not be necessary.

What I would pay attention to, instead, is the connections to the two ends of the battery. That is where a careless arrangement might force one cell to work harder than its neighbors. There are two approaches:

• Use a hefty copper bar in parallel with the strips to distribute the current evenly to the 6 cells at the end.
• Use 6 individual wires of the same length, one for each end cell, joined at a single point away from the cells.