October 26, 2006
Replacing the pack with new High Power Kokam cells
Part 1- Choosing the new pack

From our racing we have learned that at full power the Electric Imp uses around 1,000 watt-hours per mile.

For our next goal; to win Sports Car Club of America's road races and the Florida Region Championship, we needed to be able to run a thirty mile race at full power. It is clear that our old pack did not have enough capacity.

Here are the figures:


We should also modify this value for the high discharge rate.

Based on our testing we know that a straight discharge at 700 amps (10C) would give us 84% of the rated capacity. So a conservative estimate would be:


We might do better than this, since in racing the discharge is not constant and there are burst of recharging from regenerative braking.

We consulted with Kokam. They suggested we try their new High Power Lithium Polymer cells in the 100 amp-hr capacity. Greater capacity and slightly higher pack voltage.


Here is the math:

The High Power cells have slightly less energy density than the High Energy cells but they lose less capacity under high discharge. We will need 700 amps but with the larger capacity cells this is 7C rather than 10C. Kokam says around 86% at 700 amps.



These cells are rated for 500 amps continuous duty and 800 amps pulse. This means we could discharge them (the pack) at 500 amps and not have the cells get above their maximum operating temperature. The cells can deliver the 700 amps we require but the effectiveness of our cooling system will determine how long we can run at full power.

These are the batteries to do the job.

And as an additional bonus, the new cells, Kokam tells us, have even better cycle life. They are conservatively rated for >800 100% discharge cycles. Graphs at Kokam's internet site give indications that they might last many more cycles.

Kokam's Depth of Discharge (DOD) graphs.  


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June 22, 2004
Oct 26, 2006 Part 2


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