October 26, 2006

Replacing the pack with new High Power Kokam cells

Part 4: Car installation changes


What makes ProEV unique, is our experience racing Lithium powered electric race cars. We apply this knowledge in redesigning our setup, to make the car a better race car.

Top on our list is to improve the cooling of the cells. Our original method for cooling the cells consisted of separating the cells with strips of PVC sheeting. This looked good out of the car -see Work History, June 13, 2004 ("Assembling the Kokam battery stacks"). However, when we installed the stacks in the car and tightened down the battery covers, the cells were pressed down and blocked the air passages. This significantly reduced the air flow over the batteries.

We looked at a number of ideas including liquid cooling for the new pack, but the simplest was suggested by the very clever engineers at AC Propulsion. It is to put Coroplast plastic sheeting between the cells.

 

Coroplast is easy to work with and very light. It consists of two sheets of plastic separated by plastic ribs. This is the same idea as our PVC sheeting but the solid sheet and the ribs keep the cell from sagging.

Coroplast battery sandwich
Our original installation had the cells on their sides. This gave us the lowest possible Center of Gravity (CG) which helps to make a better handling race car. Since the battery trays have a three inch lip, it was almost impossible to get good airflow to the lowest cells in the stack.
Old stack

 

We consider standing the cells up. This means the tray's three inch lip blocks clean airflow to the base of the cells, but assures that all cells get ample airflow over the rest of their surface.

An additional benefit is that now, changing out a damaged cell, will no longer require removing the cells above it in the stack. Instead a damaged cell can be disconnected from adjacent cells then easily slid up and out. This means quicker, easier repairs.

There is a slight cost in CG. We plan to have four stacks of 24 cells. With the Coroplast spacers, the cells form a block 11.4 inches wide and 12.7 inches high. If we had mounted the cells on their sides, the pack Center of Gravity would have been at 5.7 inches form the bottom of the car. Instead. with the cells mounted vertically, the pack center of gravity will in fact be 6.35 inches off the floor. That is 565 lbs. raised .65 of an inch.

 

Another decision is to relocate the controllers. They were mounted on top of the rear battery stacks. This meant that when we had a problem with either rear stack, we had to remove the controller to reach the stack. This was at least a two hour job.

Controller first mounting

 

We decided to move the controllers to under the driver's seat. They each weight 75 lbs. Their CG moved from 7.5 inches above the floor down to 1.5 inches above the floor but the driver's CG (130 lbs) was raised by 2 inches.

Designing a car involves compromise. We will give up a little handling to get better battery cooling and easier servicing.

 
 

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Oct 26,2006 Part 3
Oct 26, 2006 Part 5

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