Moroso Motorsports Park Dec 30-31, 2006

Preparing for the race

With the 2005 Florida Autocross Championship under our belts, it is time to refocus on Road Racing. The last time an electric car has won a sanctioned closed circuit road race against gas cars was 1896 . What do we need to do, to be the first EV in 101 years to do this?

   
1)
We need to make our batteries reliable under the extraordinary stresses of racing.
     
 
Our Kokam Lithium Polymer batteries are amazing.
 
They are miles ahead of every other battery we have looked at.
 
There are no other large capacity batteries that can deliver the power they deliver.
 
They would be very reliable if we would use them within the limits that Kokam designed them for.
 
These last two years of racing has taught us that, for our racing requirements, we are pushing the limits of the 70 amp-hrs SLPB 60460330 cells. The cells are rated for 5C continuous duty. That is a solid 350 amps. Peak discharge can be as high as 10C, 700 amps. The Electric Imp's racing requirements is over 600 amps practically continuously. When the motors are not demanding 600 amps, they are trying to return up to 400 amp back into the batteries from regenerative braking.
 

old cell stack when new

 

The cells might have held up to this abuse if we were able to keep them sufficiently cool. Our idea of separating the cells using thin strips of PVC sheet did not take into account the flexibility of the cells. The battery covers compressed the cells which flexed down and sealed off all the airflow. Running in the Florida heat, the cells were getting above the 70 degrees Celsius limit and failing.


Kokam had already developed an answer to our race requirements with their RC airplane batteries. Some of the RC high lift sailplanes were happy with the highest capacity cells. They did not need much power but they wanted to fly as long as possible. While others, such as the RC Helicopter fliers, were willing to trade off some capacity for higher power. So Kokam tailored their batteries to create the High Energy line for maximum capacity, the High Power line for higher currents and the Ultra High Power cells for very high currents.

Kokam decided to do the same with their Large Capacity cells. The math according to Kokam's catalogue, page 14 middle graph, looks like this:

 

The High Energy cell will delivery pretty much it's rated capacity at the 1C discharge rate (70 amps for the 70 amp-hr cell, 100 amps for the 100 amp-hr cell). Up to 3C, it will continue to deliver over 97% of it's rated capacity but once the discharge is pushed out to 5C, the cell can only deliver about 78% of rated capacity.

The High Power cell will deliver it's rated capacity at 0.5C. At 1C, it will provide 95% of rated capacity. But at 10C, it will still provide 82% of capacity. That is more capacity at double the HE discharge rate.

The Ultra High Power cells take this further. They can deliver 96% of their rated capacity at 15C. The largest cell is 12 amp-hrs, which is too small for the Electric Imp.

 

This is not the only improvement that Kokam has incorporated into their new cell. They are now confident that the cells will last over 800, 100% discharge cycles. Testing has indicated even better results are possible. See Kokam's catalogue, page 15 upper right corner.

 

Kokam also suggests we move up to the 100 amp-hr cells. This will give us greater capacity: 35.1 kWhrs vs. 24.6 kWhrs at the cost of some extra weight: 408 lbs vs. 565 lbs. Since at full power we are using about 1 kilowatt-hour per mile on the race track (compared to .2 - .3 kWhr on the street), the extra capacity will allow us to run more power for longer.

The 100 amp-hr cells are rated 5C continuous, 8C pulse. Our goal of 600 amps is only 100 amps beyond the cells continuous rating. This should mean greater reliability.

 

The final step towards making the cells reliable under the stresses of racing is improving the cooling.

 

We stand the cells up and improve the air flow. Details of the changes are in work history ["Car installation changes", Oct. 2006 -part 4].

We add temperature sensors to keep an eye on how well our improvements work.

We also make changes to the front and rear body work to improve cooling ["Improving the air flow to the battery pack", Nov. 2006 -part 1].

   
2)
We need to improve our ability to charge the pack at the race track.
     
  The Kokam pack can charge in half an hour if we could find a way to provide 200 amps at 395 volts DC.
  Our ManizintaMicro PFC 50 charger which is sized to take advantage of the largest common AC outlet (50 amp 240 volt outlet) puts out about 30 amps at 395 volts.
 
The biggest working electrical outlet at Moroso, 30 amps at 120 volts, has problems suppling enough power to allow the charger to run at even 6 amps at 395 volts.
     
 

The most practical solution is getting a large generator to power the PFC 50. This means at least a 12.5 kW generator, but knowing we will need the generator to operate at a constant high load in the Florida heat, we opt for a 15 kW Generator.
 
15kW Generator
 


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Battery Beach Burnout Jan 2006
Final Prep Dec 30-31, 2006 Moroso

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