SARCC Race Day 12/31/06

Qualifying, 9:54 AM

James and I make a number of decisions. The first is to replace the tires. The old tires are shot.

We consider pulling the racing brake pads and trying a set of street pads but it might make braking worst. The stock Impreza is lighter and has less horsepower. We are overwhelming the brakes. We decide we will have to live with what we have.

The Inverter DAQ computers hiccupped and we have no data, so we can not tell what caused the power loss in yesterday's race. We used 17.928 kWhrs out of a worst case available 30.05 kWhrs (35.150 kWhrs pack, 95% charged, 90% Puerkets due for steady 300 amp discharge). This means the pack should have 40% charge remaining.

The voltage came in at 348 volts or 3.663 volts per cell which, using the study we did with the 70 amp-hr Non High Power cells, indicates around 30%. Even at 30% charge, with a 300 amps current, voltage sag should not be enough to reach our 285 minimum voltage.

It seems more probable that the motors got too hot again and the inverters cut current to keep them cool. With the bad brakes, the regen had to work harder. I suspect this added to the heat. We review data from qualifying and see that we came very close to the motor heat limits in that session.

The battery temperatures were less than during yesterday's qualifying session at around 47.5 C. This makes sense as we were running 100 amps lower discharge currents. We remove the cloth we have to filter dust from the radiator. We need better air flow through the radiator for cooling the motors.

We charge for 2 hours and 4 minutes. The pack is at 385 volts. 95% charged. Our charge current was < 30 amps. Which means we have returned less than 60 amp-hrs. This matches our kWhrs used calculations (59.8 amp-hrs) better than our State of Charge based on the 70 amp- hr cell study information (70 amp-hrs). It indicates the 70 amp-hr study is pessimistic for the new High Powered cells.

We look at the data. We used 19.281 kWhrs but the voltage never sagged below 332V even at the 400 amp current levels. Yesterday's power problem was hot motors, not soft batteries. In just 8 laps, the motor temps had climbed to 157 degrees C, just short of their 175 degrees C cut off point. Another lap and we might have lost power.

The crew gets to work on cleaning up the airflow going into the battery boxes. Less air restrictions, more air through the radiator, cooler motors.

We also decide to try more camber and shock pressure in the rear to try and improve rear tire bite for turn 6.

Battery temperatures showed some variation. On cell # 1, there are three temperature sensors. The one near the positive cell terminal read 10 degrees C hotter than sensor located at the middle of the cell and 15 degrees hotter than the sensor at one of the lower corners. The maximum temperature is 55 degrees C at the positive terminal. This pattern is repeated on the other cells.

We use .952 kWhrs per mile.

We charge for 3 hours and 10 minutes. It is a hot day and the charger goes into thermal limit. Even with a big fan blowing into the trunk, the charge current drops to around 12 amps.