Thursday, January 30, 2014

Constructing battery packs from prismatic cells

When the FSAE team decided to build an electric race car a few years ago, it seemed pretty likely that we could source our batteries from A123 Systems, an alumni company. They make very high quality lithium iron phosphate cells and battery packs. The packs we had hoped to get look like this:

It's a familiar story around here, but A123 went bankrupt and changed hands and we were never able to get any packs. However, we were able to get enough raw AMP20 cells for the pack we needed. A team member spent their thesis designing our custom pack based loosely on the A123 pack. I have somehow ended up on the battery sub-team and helped assemble one of our new modules. Bring on the photos.

CAD of a complete module. The cells are oriented vertically and all the tab attachments are at the top.

 We first made smaller packs of six cells in series. The box on the left measures the voltage of each cell and can charge them selectively to ensure they are balanced. We charged each mini pack to about 50%, enough verify that the cells were good, but not all the way full so there is less energy when assembling the larger pack.

 We assembled the pack on its side. The cell tabs slide into the black plastic retainer to keep them aligned. 

 A good while later... In this picture I'm bending one of the foil tabs slightly so it goes into the spacer nicely.

 All 28 cells stacked up. The end plates and threaded rods put pressure on the cells to keep them compressed per the datasheet.

Tightening down the clamp bars. 

The cell tabs (not visible) sit in the gaps to the right of the angled grey bars. Those bars are held in place by the long yellow bars above them which get screwed into the large yellow plate.

 All clamped down. The pack measures 92 volts (about half charged).

 A battery management board gets screwed to the top. It has spring loaded pins that contact the the aluminum clamp bars for monitoring cell voltage and balancing. The wires coming off of it connect to the vehicle computer, or in this case, a desktop computer.

 "Only" 100V at full charge, but if shorted the cells can push over 1000 amps of current, so DANGER is applicable.

Now that one module is complete we have two more to build. I'm counter boring the screw holes on the long clamp bars here. The profiles were cut on a waterjet from a larger plate. The material is a Kevlar reinforced plastic. I didn't know how difficult it would be to work with but it turned out to be pretty tame. 

All told there will be three modules connected in series. The final configuration will be 84s1p, 300 volts, and 6 kWh. The FSAE rules specify that we cannot pull more than 80 kW at any one time, which this configuration will easily handle. A123 has just sent us the first version of their battery pack design guide, which is pretty funny considering that we (and several others) have a completed pack already. However, it is a sign that the company is getting back to business and that their modules might be available to us in the future.

Until next time.