# Wiring Li Ion cells in parallel to make a 3S8P pack

I'm making a 3S, 8P Li Ion battery pack with balance leads. While wiring, I was wondering if there's a difference between bridging the parallel cells versus not bridging them down the center:

simulate this circuit – Schematic created using CircuitLab

What would the impact on my circuit be both ways, if any?

• Providing the upper and lower cells are the same capacity there is no difference. If you mix battery capacities (and all cells tend to differ per batches even with the same designations) then you can end up with larger cell terminal voltage differences over time. A bridged configuration only needs one cell balancing circuit, whereas the serial strings with no bridge require multiple cell balance circuits ...so is more expensive. – Jack Creasey May 22 '17 at 19:12
• Thanks @JackCreasey! They all have the same capacity and initial V so I'll just bridge them – FloatingRock May 22 '17 at 19:17

For the first connection you will need a separate battery ballancing and protection circuit for each 8 parallel legs (strings).

Using the second will save a lot of electronics - you can use a single ballancer as long as it is powerful enough.

You should be very careful when connecting batteries like that. All batteries should be precharged to exactly the same voltage otherwise very large current will flow to equalize the battery state while connecting them.

The best thing about the second circuit is that you will have a longer battery pack life. Why?

Let's say you've got the following pack:

simulate this circuit – Schematic created using CircuitLab

After given period of usage the batteries will decrease their capacity one by one but not simultaneously. It is never simultaneously. A failed Laptop battery usualy has one or two failed cells and the others got life for dozens of cycles more. In a series connected battery pack the total capacity of the pack is equal to the capacity of the cell with the less capacity. Imagine that the first cells to fail (loose capacity) are Bat1 and Bat5 and their capcity has fallen to 30%, while others are still at 70%. With this connection Bat1 got its parallel support from Bat4 and Bat5 is supported by Bat2. This way your total battery pack capacity will fall to (70+30)/2 = 50%.

Now think about the first circuit - if Bat1 and Bat5 fall to 30% and there is no interconnection between the legs, each leg's capacity will be 30% and the total battery pack capacity will be (30+30)/2=30%.

• Except if the capacity of a cell drops, its terminal voltage will fall much faster at which point it will act as a load and discharge the cell in parallel with it. Not good. – Tom Carpenter May 22 '17 at 19:42
• @TomCarpenter - exactly right!. What is the best way to avoid that problem? Is there a best way? – zeffur May 22 '17 at 19:46
• @TomCarpenter your scenario has equal impact on both connections. Large self-discharging current of a Li-Ion cell happens but is less often than loosing capacity and it usualy happens when a cell loses its capacity to 10-15% or less. There is nothing to do then. This cell will disballance the whole pack even it is in series connection. – Todor Simeonov May 22 '17 at 20:08