Archive for the ‘Battery’ Category

With the arrival of our Agni 95 series permanent magnet motor we began testing to see its performance at low currents. Nominally it should be able to handle 400 Amps and deliver 12 to 16 kW.

With a max efficiency of 90% at around 200 A the at least 80% of the range of operation will lie between 0 and 300 A.

Because such high voltages and currents are required to run the motor it can be quite difficult to test without a complete electric system that can sustain the required power. Furthermore, the battery system is not built yet so testing has so far only been completed on the small scale.

However, there is good news- it works. Three “no-load” tests have been performed to test the basic functionality of the motor controller in unison with the Agni motor.
Test 1 – 62 VDC, 2-3 A

Test 2 – 62 VDC, 2-3 A running at equivalent speed of 5.5 MPH

Test 3 – 4 12 VDC LiPo batteries in series. 48 V, 25 A

All three test showed that the motor and motor controller produced linear and smooth operation across its current range. The next steps will be to construct the battery system and run the motor at its full potential.

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With the arrival of the batteries, the beginning of the end for the electric system has started. The first design step for the accumulator was to arrange a battery array capable of powering our electric motor. The following photo displays this.

The batteries are arranged as six packs in series that are in four parallel sections.

The Zippy Flightmax 5S batteries consist of five 3.7 V Lithium Polymer batteries stacked together and wired in series, for a total voltage of 18.5 V. Four of these packs in series produce 74 Volts, well within the necessary range for the motor controller.

To produce the proper current, 6 of these 74V strings will be placed in parallel. The nominal current capacity of the system will be 30 Ah. The maximum continuous current rating of the Flightmax batteries is 25C, and in this configuration yields a maximum continuous current of 750 amps.

Usage of the batteries at this level is generally not recommended and greatly reduces battery life. Approximately one half this value, or 375 amps, is recommended for those wishing to maintain capacity for 100s of cycles. The Agni motor chosen can use at maximum 400 amps.

Battery casing is required to support the structure of the accumulator in addition to providing electrical isolation.

These acrylic cases will provide for these needs. Inside them 12 battery packs will be placed. They will be connected via solid copper bus bars capable of handling past 400 amps. After the system is set up it will be tested with the battery management system in addition to the charging system. In our next post we will discuss what the mechanical engineering team has done to fabricate a chassis from scratch.

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After much heated debate over the choice of powertrain, the hybrid design was finally chosen. Initially, it was thought that it was best to use a series design considering the specialties of our group members. With five electrical engineers to three mechanical engineers it made sense to make a design based on … electricity. While the parallel design is a mostly mechanically based design, it makes the most sense to us considering we are competing in a race.

The parallel design is most efficient at high speeds. It also allows us the most instantaneous power as there is a electric motor and ICE coupled mechanically. This is going to be a problem for our MEs because in addition to suspension, braking, and chassis now they have to deal with the powertrain, but the EEs are just going to have to step up and work in areas outside their experience.

Below is a diagram of the power flow in our parallel hybrid design contained within our chassis.

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Hybrid Powertrains

While petroleum fueled transportation has been a great solution for a long time, citing petroleum shortages, supply instability, high prices, and pollution, hybrid powertrain designs are becoming much more popular. Each type of design incorporates regenerative braking. This is done by using electric motors to absorb a vehicles motion to generate electricity. This electricity can later be used to propel the vehicle. However, there are several further variation between hybrid powertrains that each have their advantages and disadvantages.

The three general types of hybrid designs are

  • Series
  • Parallel
  • Series-Parallel

    1996 – 1999 GM EV1,  Diesel-Electric Trains

    A series hybrid drivetrain is a drivetrain in which two power sources feed a single powerplant (electric motor) that propels the vehicle. Most commonly, a combustion engine will drive a generator which will charge a battery. The battery then drives the electric motor. This setup mechanically decouples the engine from the wheels. The motor can be used as motive power source or as a generator while braking.

    Advantages of this design include that the engine can be run at any point in its speed-torque range. This allows it to run at its most efficient point at all times. In addition, there is much less complexity in the drive train as it is completely controlled by electric motors. There is no need for a drive shaft or differentials as each wheel could be potentially driven by its own motor.

    Disadvantages of this design include inefficiencies introduced by the conversion of energy from chemical (petroleum fuel) to electric (battery) to mechanical (wheels). Also, the generator attached to the ICE adds additional weight and cost. A larger electric motor must also be chosen because the mechanical energy required to move the vehicle depends solely on electric power.


    Honda Hybrids

    Parallel hybrids couple mechanical and electrical sources mechanically. This can be done many ways. Many automakers choose to couple the power sources through a differential. Thus the torque or speed from each system can be added.

    Advantages of this system include weight savings that are offered by the elimination of the generator. Also, a much simpler power converter can be used as there are not as many paths for electric power to flow. At high constant speeds this design acts similarly to a series platform because the engine can run near its top efficiency. A smaller battery can be used

    Disadvantages include that in stop-and-go driving the parallel system becomes much less efficient because the engine will not be able to run at its top efficiency. This type of design also requires a much more robust engine that includes a more robust transmission.


    Toyota Prius, Ford Escape

    A series-parallel hybrid combines the efficiencies and complexities of both types of systems. The ICE and electric motor are combines both mechanically and electrically.

    Advantages of this system are that it can act like both a series or parallel system depending on the driving conditions. At high speeds the system will function in parallel, routing power from the engine directly to the wheels, where the engine can run near its highest efficiency. In slow or stop-and-go conditions the system will run in series, routing power from the engine to the generator to the motors, allowing it to run at its highest efficiency. Therefore, this is the most fuel efficient design.

    Disadvantages of this system include a higher complexity and cost. An generator is required in addition to a mechanical coupling system such as a differential.

    Thanks to M. Ehsani and Yimin Gao and their text “Hybrid Trivetrains”

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