Category Archives: Technical

Folding a battery tray

Trev’s main tub structure was made by folding and bonding honeycomb boards. Our new battery tray, which will fit under the floor of the car, was made using the same technique.

For the tub we used aluminium foil honeycomb. The battery tray is made from cheaper polyproplyene honeycomb, as shown in the images below.

The first step is to fold an edge. Secure the honeycomb sheet to a table with old lead acid batteries, and clamp a square steel tube along the fold line. To form a right angle fold in 10 mm thick honeycomb, heat a 10 × π / 2 = 16 mm strip using hot air guns.

Use hot air guns to heat the fold line.

When the polypropylene softens, fold and clamp the sheet. The softened honeycomb will crush, and the outside edge of the fold  will have a nice 10 mm radius.

Fold and clamp. This photo shows the second fold.

The fold will retain its shape when the honeycomb cools, after a few minutes.

Next, fold up the front of the tray. For a fold angle α (in radians) and a honeycomb thickness t, the strip that is heated and crushed has width α t.

Design the fold.

Fold the front edge of the tray.

Our honeycomb sheet was not long enough to include the rear end of the tray, so a separate rear panel was simply glued on. At this stage our 2300 × 540 × 80 mm tray had a mass of 2.4 kg.

Next, apply two layers of Kevlar to the inside of the tray. The first layer has the fibres at ±45° to the tray, to resist twisting. The second layer has the fibres at 0° and 90° to the tray, to resist bending. Kevlar is more expensive than fibreglass, but gives a low-mass structure with the stiffness and toughness we require.

Apply Kevlar to the inside of the tray.

Go home. The next day, when the resin has cured, trim the excess Kevlar, turn the tray over (you can use the same four old batteries to support it), and apply Kevlar to the outside of the tray.

Apply Kevlar to the outside of the tray.

A day later, trim the excess Kevlar and the tray is ready for a battery. The mass of the tray is 5.3 kg.

The (almost) finished tray.

Advertisements

Low mass, low energy

Using tonnes of machinery to move one or two people around a city seems ridiculous. But how important is low mass?

Many years ago, before we started designing Trev, I was driving a Solectria electric car to work when my path became blocked by a large “SUV” that had broken down in the middle of an intersection. Pushing the SUV to the side of the road made me realise just how much energy is required to move these massive machines. My electric car was better because it was using energy generated from clean, renewable sources. But it still needed a lot of it.

I sometimes demonstrate the importance of low mass by tying a small child to Trev and another to a conventional car, then asking them to race across the yard. Trev moves, the other car doesn’t. It usually takes five kids towing a conventional car to keep up with one towing Trev. A large SUV took ten kids.

In 2008 I had two high-school students investigate the relationship between vehicle mass and CO2 emissions. They used emissions data from the 2007 Australian Green Vehicle Guide and looked up vehicle masses from manufacturer’s web sites. The results are shown in the following graph.

CO2 emissions vs vehicle mass

The red dots show CO2 emissions for petrol cars, the blue dots for diesel, and the green dots for hybrids. The trends are obvious—halving the mass halves the emissions. The reduction is not entirely due to the reduction in mass; lower mass cars are generally smaller, with smaller engines and less aerodynamic drag. But mass is the dominant factor.

The graph also shows that for a given car mass (and size), there is wide variation in emissions. Improving vehicle technologies is one way to reduce emissions, but choosing an appropriate vehicle with low emissions can be a lot more effective.

Trev has a mass of just over 300 kg, and so it takes a lot less energy to push it along the road. In 2007 we drove Trev from Darwin to Adelaide, cruising at 80-90 km/h. We used 187 kWh of electricity, worth $33, to drive 3000 km. Petrol costs for a conventional car would be ten times this amount.

Electric cars are coming. Most will be based on conventional cars, and so will be heavy and require a lot of energy. A 1300 kg electric car with a range of 120 km might have 200 kg of batteries. Trev has the same range with only 45 kg of batteries. (We are going to increase this to 80 kg for Zero Race, because the charging points are up to 250 km apart.) Reducing the mass of a car is a very effective way of reducing the energy required to move the car, and the amount of materials required to build the car.

Trev. Not only does it use clean energy, it also uses a lot less energy.

Does it get hot in there?

We always get a lot of interest when we have Trev on display. The questions we were asked at our recent day in Rundle Mall were typical. Here are some of them.

Does it get hot in there?

In October 2007, two UniSA students drove Trev from Darwin to Adelaide in ambient temperatures around 35°C. Both survived. One of them has come back for more, and will be driving Trev for part of its tour around the world.

Like any car, Trev can get hot. When it is moving, air flowing through the car from an inlet in the front provides some relief. We also had a small fan in the car for our trip across the Australian outback. (We took it away from the driver to provide additional cooling for the motor controller, but we will put it back.)

The air conditioning systems used in conventional cars are not suitable for low-energy vehicles—they use more power than Trev driving at 100 km/h. We are still looking for efficient, effective ways to keep the driver comfortable.

How fast does it go?

Trev was designed to fit in with normal urban traffic, including on freeways. It has a top speed of over 100 km/h, and accelerates smoothly up to 100 km/h in around 10 seconds.

How far will it go?

When we drove from Darwin to Adelaide, we could travel up to 120 km at 90 km/h before we had to stop and recharge. Recharging took about an hour, and we were able to travel about 500 km per day. For Zero Race, we are increasing the range to over 250 km so that we don’t have to stop so often. For urban use, however, a range of 100-150 km is plenty.

Where are the solar panels?

Our experience with solar racing cars inspired us to build Trev—if you can drive across Australia without using fossil fuels, you should be able to drive to work and back. But if you have a photovoltaic panel, it will be more effective on the roof of your house than on the roof of your car. Trev is a pure electric car, and can be recharged using clean electricity from solar, wind or other renewable energy sources.

What type of battery does it use?

We are using lithium ion polymer cells. There are thirty-six large cells connected in series, giving a battery voltage around 130 V. The estimated life of the batteries is 250000 km.

How safe is it?

You will be less vulnerable in Trev than on a bicycle or motorcycle. Trev is also less ‘aggressive’ towards other road users than most conventional cars. The occupants sit within a rigid tub structure that will provide some protection during a crash. But heavy vehicles will have a greater impact on Trev than Trev will have on them.

Did you see where my husband went?

No.

Our next appearance will be at the final stage of the Tour Down Under. Come and see us, and ask your own questions.

A big battery in a small car

One of the main upgrades to Trev is to increase the range of the car from 120 km per charge to over 250 km per charge. To do this, we have to fit a big battery into a small car.

Actually, the battery will not be that big. Currently, Trev has a 45 kg battery. We need to increase the battery mass to about 85 kg. The mass of the car with the larger battery will still be under 400 kg.

Choice of battery type is critical. We will use large lithium ion polymer cells, similar to those in our current battery, only larger. The advantages of lithium ion polymer are:

  • they have high energy per kilogram (about 160 Wh/kg)
  • they have high energy per volume
  • the large, flat cells are easy to fit into tight spaces.

With most batteries there is a trade-off between energy capacity and power. However, high energy cells can deliver more than enough power for our low-mass car.

Last week we lifted the car up onto trestles to check that there was enough room beneath the car for a large battery. There was.

Design for the new battery box.

The diagram shows the structural tub chassis in green, and the proposed new battery box in white. The tub chassis is built from boards with an aluminium honeycomb core and fibreglass skins, with kevlar reinforcement on the floor. For the battery box, we are considering using polypropylene honeycomb with kevlar skins, which will be even lighter.

The entire high-voltage system—battery, fuses, battery management system, contactors and motor controller—will fit into the battery box. The motor controller is taller than the battery, but will stick up through the main floor under the rear seat. This layout will allow very simple wiring, from battery to motor controller to motor.

To accommodate  the battery box, we will raise the car by about 60 mm. This will also allow us to increase the movement on our suspension so we can cope with rough roads. We will continue to use double wishbone suspension at the front, but will lengthen the suspension arms by putting the lower pivots on the underside of the main floor, where the battery box narrows near the front.

This design improves the overall simplicity of the car, by putting all of the high voltage components together in a box that can be easily separated from the rest of the car.

Now all we have to do is complete the detailed design and engineering analysis, then build it.

Technical work begins

Last night, the beginnings of the Team Trev Technical Team met at the UniSA workshop to start preparing Trev for the long drive. Here is what we looked like:

The start of a technical team at the end of our first meeting.

Between us, we have helped design and build racing cars, road cars, solar racing cars, land yachts, pedal-prix vehicles, sailplanes, electric cars, and Trev.

There are a few more people eager to help with the technical work who could not make it to our initial meeting, but will be joining us in the coming weeks. Meanwhile, others are working hard on administration, fundraising and logistics.

You can follow our progress here at teamtrev.com, or on our Google Group.

Preparing Trev for a long trip

Trev was designed for the city—short trips on good roads, carrying one or  sometimes two people. In Trev’s home city of Adelaide, 98% of cars travel less than 100 km each day and the average daily distance travelled is just 32 km.

Driving around the world will be a challenge. Zero Race will compress three years of commuting into 80 days. Each day, Trev will have to travel up 250 km—without stopping—before lunch. After lunch: another 200-250 km.

Trev travelled about 500 km each day of the 2007 World Solar Challenge, but could travel only 80-120 km before we had to stop and recharge. To travel 250 km without stopping, we will have to increase the energy capacity of Trev’s battery from 5.3 kWh to about 13 kWh. This will increase the mass of the car by about 50 kg. Even so, Trev will still weigh less than half the weight of a small conventional car, and still be a lot more efficient.

The new battery will be fitted under the floor.

There are many other tasks required to prepare Trev for a long trip, including:

  • fitting a new brushless motor and motor controller
  • improving the brakes and suspension
  • improving the canopy hinge and latches
  • getting Trev registered for on-road use.

We are still looking for volunteers to help with this work. Even if you can’t join us in the workshop, you can participate by contributing to discussions on the Team Trev Google Group and on Trevipedia.