Getting it Working pt.2 – Battery Driven Car

Featured, General / Sunday, February 24th, 2019

Invigorated by our quick process and early completion of our 1 February deadline, we felt confident that we could have our car driving under battery power within weeks. Unfortunately, fate (or the weather) reared its ugly head as snowmageddon reigned in Seattle, with several feet aggregate of snow dumped on the city in just two weeks. We lost five precious school days and a weekend of work time due to the impassable roads. It became clear that we would not be able to make a 15 February deadline, however, with a lot of effort and a four day weekend we were able to reach our deadline by 18 February.

Progress was feverish and the hours grueling, however, by the 18th, we were test driving the car.

15 February Deadline – A Battery Powered Drivable Car

Our 15 February deadline consisted of the following criteria, all of which would need to be completed by the 18th if we were to race.

  • A new and permanent aluminum floor riveted to the chassis
  • A temporary seat for testing bolted to the new floor and chassis
  • Temporary mounts for our five-point harness welded to the chassis
  • Five-point harness attached and functional
  • Roll bar welded to chassis in compliance with competition rules
  • Crush zone tubes welded to upper and lower halves of chassis
  • Pedals for motor and brakes attached within chassis and adjustable for drivers of differing heights
  • Housing for battery box designed, built, and attached to car
  • Battery box designed and built to competition specifications.
  • Battery transferred to battery box and secured for transport.
  • Battery box mounted within enclosure attached to car
  • Auxiliary battery and motor controller attached within proper enclosures and wired.
  • Brakes attached and connected to pedals

All these and more would be completed in just four days, from Friday night to Monday morning.

Ok, first things first: a floor. Plywood just wouldn’t cut it for the final car. Though easy to work with, it lacked the sturdiness and resilience required for such prolonged use in harsh conditions. Not to mention the small problem of splinters in very unpleasant places for the driver that plywood caused. We replaced the plywood floor with a thin sheet-aluminum one which was quickly riveted on after painting the bottom of the chassis to avoid corrosion.

Charles the riveter

This floor solution has proved far more durable and resilient than the previous prototype plywood floor.

We are currently in search of a permanent driver seat solution that meets our requirements of size, comfortability, sturdiness, and an eight-point mounting system (see more here) however, in the meantime we are using a surplus school chair for preliminary testing. This was attached to the chassis along with hard points for the five point harness which were welded to the steel skeleton.

Test fitting the seat and harness attachment points

In short order after the mounting of the seat, the roll bar and harness were also attached in their semi-permanant locations for testing. The two crush zone lengths were also connected to the upper chassis during this period.

Car with roll bar, harness, and crush zone attached.

The most important and challenging milestone to reach for 18 February was the battery box and the mounting armature. The battery box must be airtight while allowing for airflow through fans in order to cool cells, be sturdy yet light, and hold the batteries tight enough to hold them in a vertical orientation and protect the batter in the unlikely event of a roll over.

Although early designs for this system involved metal boxes of various types with ratcheting straps and epoxy to keep the battery in place. But as ever, we found that the simplest solution was the best. we settled upon a box made of an aviation grade composite honeycomb sheet which had been donated to us which would be clamped together in order to hold the battery in place.

The battery being prepared to be put in its box with a foam padding to compress it

The battery would be put on a honeycomb plate and aligned to one edge with the other edge having an aluminum bracket. These would keep the battery secure laterally. A honeycomb lid would then be fabricated by connecting honeycomb sheets with treated fiberglass tape and epoxy, creating a strong bond. This lid would be placed atop the plate and clamped on either side with aluminum supports which would prevent any movement vertically.

Battery box lid under construction
Fiberglass tape joint

Airflow through the battery box would be provided by only exhaust ventilation fans so as to maintain the negative pressure required in the rules.

A test of the fan’s air moving capabilities

After an analysis of the center of gravity of the car, it was determined that, although not the most structurally ideal position, placing the battery behind the outrigger wheel would be both the best place to put the heavy battery in order to balance the car in case of a high speed turn, and the safest place in the unlikely event of a battery fire. In order to accommodate this, a steel enclosure was cantilevered behind the outrigger wheel, preloaded so that the battery would rest at a roughly level attitude.

The completed outrigger assembly including battery box and battery

As can be seen in the above picture, after mounting the battery with temporary methods, the motor controller was mounted within the same battery enclosure. A large and tempting red battery emergency disconnect switch was also mounted outside the car within reach of onlookers as per the safety requirements of the competition. An extremely temporary mudflap was also fashioned out of a plastic board in order to prevent mud and water from being throw into the battery box through the air intake.

We next completed the basic and preliminary wiring that would be required for our cockpit including an ignition key and a switch panel which included a motor reverse switch.

Prototype control switch panel for testing purposes

The last step before we could take the Lean Green Driving Machine out under its own power was the small matter of being able to stop and start the thing. For this, we employed disk brakes (read more here) connected to levers on a foot actuated mounting bar as well as regenerative braking from the motor. The throttle was mounted to the same adjustable bar as the brakes.

With that, we drove the car out of the garage under its own power for the first time.

The car rolling out for the first time under its own power

You can see a video of our initial test here:

Despite losing a week to snow, we were able to accomplish all our 15 February goals and more by 18 February, and have begun testing our vehicle and making adjustments and even further progress in finalizing many of the prototype elements still in the car.

14 February: Beginning of long weekend
18 February: End of the long weekend

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