Monday, March 26, 2012

Battery Pack design

Anyone who has followed my build has seen my previous battery plans. Initially, I had anticipated using 10Ah Headway cells. Then I switched to the blue 8Ah cells, since they yielded a lower internal resistance meaning less losses to heat and inefficiencies. Then, I switched to 8Ah cells with an even lower resistance, red 'high power cells'. 
The game plan was to mate 972 of these cells together, 9p/108s, for a nominal voltage of 345ish and 2000A bursts. 
I was almost certain the headways were the route I was going to take, however I decided against it in favor of some of the highly acclaimed A123 20Ah pouches. 

Alright, why? Because the cells are pretty awesome. Although their origins are unknown, they still boast some pretty impressive specifications. 

To get the same performance out of this battery pack, I will a 4s/416s setup, for 343v nominal, and 80Ah capable of 2000A bursts. 

Not only that, but the headway pack would weight about double what the A123 pack should weigh.

The only drawback to the A123 cells is their inability to be put into a pack easily. I originally designed a rather primitive pack, consisting of bus bars separating the cells, and the entire pack would be compressed via a threaded rod that would go through the cells. 

While this design would work, I had two issues;
1. The threaded rod would have to be some sort of nylon or ceramic. The nylon rod would stretch under heat, and the ceramic rod would be fragile.
2. The cost of the copper needed for the bus bars would be ridiculous. I calculated about $700 worth, using bars of 1/4" and 3/8" copper.

With that in mind, I went about designing another pack. In order to combat the expensive copper necessary for the first pack, I needed to design a pack that didn't rely on the cells connecting directly to each other, since that would call for thick copper to space the cells. I had always wanted to do something along the lines of the white zombie's pack, having the tabs bend over a copper bus bar, and then another bus bar being clamped down on these tabs. However, this wasn't practical using 4 cells in parallel. After a little finicking with my A123 test cells, I figured I would cheat, and treat two 20ah cells as one 40ah cell. Now I need 2 parallel (which is 4 cells), which would allow me to use the clamping system. 4 cells would need 8 slots, and I dont think the cell's tabs farthest from the central copper mounting bars would reach.

A couple of minutes in CAD got me this 

Simply by changing the design, I was able to reduce the cost of the copper significantly. With the new clamping design, I can use 1/8 copper bus bars, with more surface area. The total copper price for all eight packs will be around $223 (according to 

I then optimized the design, and entered some more accurate dimensions for the final battery pack file. 
I decided to make the pack with a 1/4" polycarbonate top, so that I can see the batteries. It also isn't much more expensive than other plastics. I wanted to use 3/8", however the cell tabs would not be long enough to go through the plastic slots, and attach to the copper bus bars. 
The sides of the box will be 3/8" on the short sides and 1/4" on the longer sides. There will be half an inch of space between the cells and the edges, so there is some room for error. Foam padding will be placed in these gaps to secure the cells. 

Here are some pictures of the final design

I still need to add some copper bus bars to the pack, but I just needed the box dimensions for now. 
The hole drilled through the center of the copper bars will hold them to the polycarbonate top plate, while two more holes will be drilled on the sides of the same bus bar to secure it to the larger bus bar which will hold 2 groups in series.

From there, I contacted a couple of engineering schools around where I live, and found a place to CNC the top plate and the individual sides of the battery box.

I still have more to update, I will update soon!


  1. Curious how you plan to keep the cells cool while charging? Packing them together like that will trap a bit of heat I'd think. Anyway, the project looks really cool and I'll be following.