March 25, 26, 27, 2005

.Moroso Pre-race

It's the schedule for the Flamingo Stakes that is a killer.

Each day features a 15 minute morning qualifying session and a 14 lap (31.5 mile) race in the afternoon.
These are the longest races we expect to compete in this year.

Since our only track experience with the car is two aborted laps (see Moroso July, 2004), starting with the longest races will be challenging!

Preparing the car, we concentrate on improvements that will increase our range.

Our biggest problem is the difference in state of charge (SOC) between cells.

The Kokam Lithium Polymer cells are set up and charged in series.

That means all the cells are charged together and that charging must stop when the most charged cell is full. When the most charged cell is full, the least charged cell in the Imp is still around 3.75 volts or 50% charged.

Since we must stop driving the car when the least charged cell is empty, our range would be the 50% of theoretical because that is all the charge we have in the lowest charged cell.

Our Battery Management System (BMS) uses a computer to control relays to allow us to connect to and read each cell. Since we can connect to each cell, we can charge each cell individually.

We decide to use an isolated DC/DC converter as a charger. This means, we can take the power from the whole pack and put power into the weakest cell. Our Siemens inverters give us 360 volt to 12 volt and we choose a Vicor VI-200-EX to take the 12 volts to 4.2 volts.

The Vicor set up is limited to 30 amps which is the current carrying capacity limit of our relays as well.

Based on these friendly charts in the application manual, figure 1, we expect that even at current limit, as long as the cell is above 2 volts, the Vicor will give us our 30 amps.

This does not turn out to be true.

The Vicor dc/dc converter only gives us a few amps. Vicor tech support says we need to design a current limiting circuit. They send us an example, but it will require extensive modification and testing which we do not have time for.

Luckily we have a plan B.

If we can't boost the lower cells, how about discharging the higher cells?

We wire a couple of large resistors onto a board and instruct the software to connect the highest voltage cell.

We play around with discharge time and soon the process is pretty much automated:

- Put pack on charge.
- Read voltage of all the cells.
- Discharge the highest cell.
- Read voltage of all the cells.
- Discharge the highest cell.

At the end of charge, all the cells are about the same voltage.

Besides all the regular maintenance, we make two other fairly major changes to the car.

> We add a Lexan sheet behind the driver's seat to close off the rear of the car. Since we are required to race with the windows open, we hope this will reduce airflow into the car and improve range.

> We also install front and rear anti-roll bars.
If we can't use full engine power, then we better be able to make it up in the corners!



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March 13, 2005
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