July 10, 2004

The story so far...

ProEV's Kokam battery powered Electric Imp is sitting at Moroso Motorsports Park on the night before a Sports Car Club of America race. There are a million reasons that we cannot run tomorrow including that our battery pack is less than ¼ charged and our charger is dead (Note to self: When installing a one way connecting Anderson plug make sure the one way is the right way).

But since we are here, we decide to see if we can get the car through tech inspection. The Sports Car Club of America is happy to have electric cars race at their club races as long as the region's Executive approves and the car passes the applicable standard safety rules. We had gotten the approval of the Florida Regional Executive before we started the project and had emailed the Florida region Scrutineer that we would be at this race.

The Tech inspector has not seen the email and is a little surprised to find himself faced with scrutinizing an electric racecar. He checks with SCCA National. When they raise no objection, he is willing. A racecar is a racecar. Skip the section about the fuel cell (racing gas tank, not the other type). Don't need an oil catch tank. Rules for the exhaust pipe-not applicable.

He checks the roll cage. The scatter shield over the driveline. The race seat and 6 point seat belts. He also asks some relevant questions like how the batteries are secured and whether the pack is isolated. Then he gives us a logbook and approves the car. We are cleared to race. The Korean TV crew pull him away for an interview.

It will be a short race for us if we don't get the batteries charged, but help is on the way. I have been keeping fellow EV list member and local EAA organizer Shawn Waggoner up to date on our efforts. When I told him that there was no way we were running because we had killed our PFC50, he thought he might have a solution. A friend of his, Matt Graham, had just got a brand new PFC20. Maybe we could use it. He would check.

Sure enough, Shawn and Matt turn up at the track. They whipped out their Fluke meters (they had way-cooler Multi-meters than ours), plugged in the PFC20 and soon have our pack charging. We experiment a little and managed to get a steady 5 amps from the trackside outlet.

Charging is going to take a while but in a great display of EV fellowship, they leave the charger in our care despite just meeting us for the first time and knowing we blew up our last charger! Thank you Matt and Shawn!

We have used 14.2 kw-hrs from a 22.8 kw-hr pack. In theory, the pack is 38% full. This does not take into account any pseudo Puerkets effect, but let's call it 26 amp-hrs left.

The charger computer control is not set up, so we can't leave the charger on overnight. We charge for an hour and a half before the track closes and another two hours before qualifying in the morning. Best case, we have 43.5 amp-hrs in the battery or about 62%.

Another way of checking state of charge. Resting pack voltage is 333 volts or about 3.784 volts per cell. If 3.0 volts is empty and 4.2 volts is full: .784/1.2 = 65%. The two methods of guesstimation come pretty close to agreeing.

I drive the car to pit lane and wait for the start of the SPU (Super Production Under 2 liter) qualifying session. The crew checks tire pressures and switches on the data recording. No need to warm up the motor. The track goes green and out I go.

There are a number of cars ahead of me. They swerve back and forth warming up their tires. I have no trouble keeping up with them. In the first corner, the cars ahead of me seem awfully tentative. The Imp corners well and even going easy on the throttle, I am up against the bumper of the car ahead of me.

I ease off and accelerate again. Hard brake and another corner. Enough warmup. My tires are sticking already. Let's go!

I come out of turn 4 hard and pass before 5. Hard out of turn 6 and pass another two cars before the chicane. The road ahead of me is clear.

Through 9 and 10, I am at racing speed, though my line is not clean and I am conservative on the throttle. Out of the corner, I check my distance to the car behind me. There is little difference in acceleration. I might have a little edge.

The speed climbs over 100 MPH but car stops accelerating. Is that all it's got? I watch behind me and my marker car is rapidly closing.

I brake and turn into turn 2. Direct drive means no need to think about shifting. The throttle application is amazingly smooth. You know exactly how much torque to expect. I find it easy to bring the car right to edge. A smooth four-wheel drift from the apex to the outside berm. The gap behind me reopens.

Brake hard for the 3 and 4 complex. The gap increases. I brake hard for turn 5 making use of the regen pedal and the brake pedal. The car decelerates nicely. Turn and back on the throttle. A little lift and full throttle out of turn 6 and....... the clunk of the contactors dropping out.

I check my mirrors and move off the racing line. Shift lever to neutral. Front and rear motor switches to 'off'. Front motor to 'on' and press the start button. Rear motor to 'on' and press start button.

I accelerate hard again and the contactors drop out again. I pull off. Reset. Then wait for a long break in traffic and gently drive to the pits.

In the paddock, we put the car on charge. The only data system we have up on the car is the 'slow trace' mode that the Siemens inverter allows. We have it set to record throttle position, regen throttle position, amps, voltage, torque, and RPM. Despite some odd timing data, looking at this information is extremely helpful.

Our maximum amps were around 630 amps which at one point we held for 13.5 seconds. During that time voltage dropped immediately to 276 volts and over the next 13 seconds to 260 volts. This was the section for turn 4 to turn 5. The rpm went from 3750 to 4950. Torque started at 286 lbf-ft and dropped off to 192.

We look at the data from when the contactors drop out. The voltage had hit the minimum we had specified with Parameter 420 (Vbat_UnderVolt). It is set to 248 volt. Parameter 421 (Vbat_Min) is set to 250 volts.

In theory, the inverter should cut back on the current out when VBat_Min is reached. This should keep the voltage above Vbat_UnderVolt. If you hit Vbat_UnderVolt, the contactors open, shutting off power, to protect the battery.

In reality, it is necessary to set the two parameters further apart than 2 volts, or the battery pack hits Vbat_UnderVolt before the inverter can react to Vbat_Min.

We decide to set Vbat_Min to 264 volts (3 volts per cell). That gives us a 16 volt cushion. If the voltage goes too low, we will lose some power, but not shut down.

We take a look at what had happened on the main straight. The current was around 600 amps and the torque around 243 lbf-ft until we hit 5600 rpm. Then the torque drops to 0 and the current to around 17 amps. The RPM holds at 5600 rpm.

"Is there a rev limiter?" our crew chief asks.

We check the software (and check with Victor of Metricmind.com ). There are 4 rev limiter parameters (380-383). A 'soft' rev limiter and a 'hard' rev limiter for clockwise rotation. A 'soft' rev limiter and a 'hard' rev limiter for counter clockwise rotation. The 'hard' limit is set for 5600 rpm. We change this to 11,000 rpm. The motor is rated to 12,000.

Next we look at regen currents. Our braking set up uses two pedals. The brake pedal is in the normal position. The far left pedal, traditionally the clutch, is the regen pedal. It uses a stock hydraulic master cylinder with an electronic pressure sensor taking the place of a potentiometer.

The regen current was set to 140. The trace from turn 5 shows 140 amps. Voltage climbs from 269 under 175 amps out to 300 volts with 150 amps in. 65 lbf-ft of torque retardation.

We consult with Dr. Kim, Vice President of Kokam Batteries. 2C (140 amps) is the max they want us to charge the battery with. He does not think that 4C (280 amps) for very short bursts will do any harm but no higher until we build more experience with the batteries. We reset parameters 431 and 433 to 140 amps on both motors (240 amps max total).

The whole track session used 4.208 kw-hrs. Total travel was 2 laps or 4.5 miles. I estimate that we spent almost a lap at speed. That means 2.25 miles at around .500 kw-hrs per mile or 1.125 kw-hrs and 3.083 kw-hrs used by 1 hot lap. This means 1.37 kw-hr per mile. This is a much larger than we expected.

We decide not to run the short race this afternoon. We need to charge as much as possible and then do a few clean laps to get a betterunderstanding of the car.

The pack charges until evening. We look forward to tomorrow.

To be continued..

 

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