June 13, 2004

Building the car: Assembling the Kokam battery stacks

We plan to install the cells flat on their sides in stacks. Since this is for a racecar capable of discharging and reggening at 600+ amps and since our home track is located in warm Florida, we design in pack cooling. So instead off stacking the cells directly on top of each other, we decide to create air spaces between each cell.

To do this, we decide to use 40 mil sheet PVC.
This is available from Home Depot where it is used as a shower liner.

40 mil PVC roll

Each battery tab looks like this before we start. We decide against tab welding since we want to retan the ability to easily switch out cells.

battery tab unmodified except for test hole
We plan to connect the cells by pressing the tab faces together using two pieces of 1/16 brass sheet held by two screws. To save space, we trim down the tabs to a little wider than 3/4 inch.

This still gives us a contact area of 3 square inches. The tab material is thin enough that scissors work fine. Then using a Roper Whitney No. 5 Jr Hand Punch, we make two 3/16 inch holes.

battery tab ready to be connected
We have been warned by Kokam to take care when handling the cell. Do not bend it. Do not put undue strain on the tabs.

To keep our PVC strips in place, we use a little mild glue.

gluing on the PVC

To get the air gap we want, we use 3 layers of 40 mil PVC.

three layers

The next cell goes on with the opposite face up. This puts the positive tab of the first cell to the negative tab of the second cell.

two cells stacked

Then line up the holes on the two tabs.

ready to connect two tabs

We put a dab of the weakest 'thread lock' on each screw. The black 14 gauge wire is for the BMS system.

tabs connected

Notice that the PVC strip also serves to insulate the tabs from the batteries above and below.

insulate the tabs

It is important to be careful that the PVC does not slip out and the tabs short. These are powerful batteries!

shorted tab
Depending on where the battery stack will sit, the air flow will be different. air flow for lower rear battery stack

This set up allows a straight thru air flow.

straight air flow

Here is a side view of a 'hybrid' air flow stack. The bottom section gets air from this side. The top cells get air from the front (left side in this picture).

side view battery stack-note air gaps

Here is a 16 cell stack ready to be put in the car.

16 cell stack
   

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