Solar Car Electrical System Knowledge Base

Owned by Titus Chow
Last updated: Aug 28, 2024 by Aden Chu

Outline

This section covers the basics of how a solar car's electrical system works and how we've approached these challenges. This includes:

  • System communication
  • Power distribution
  • Extracting energy from the sun
  • Energy storage and keeping it safe
  • Making the car move
  • Driver interaction and usability
  • Data collection

Intro

As an inherently electric vehicle, a solar car can be thought of as an extremely efficient electric car that charges its battery pack through energy collected from the sun instead of requiring the use of an external charger. Since the solar array is relatively small and solar cells have limited efficiency, the resulting goal is to minimize energy use during normal operation.

A solar car's electrical system involves a number of different sub-systems that are each responsible for a different task.

  • Solar
    • Photovoltaic (PV) array: Collects energy from the sun and is our primary source of power
    • Maximum Power Point Tracker (MPPT): Maximizes the array's output and matches the battery voltage
  • Motor System
    • Motors: Generally special high-efficiency direct-drive brushless DC (BLDC) motors; responsible for actually moving the car
    • Motor Controllers: Generates the electrical pulses in the correct order required to drive the motors
    • Precharge Controllers: Precharges and discharges the capacitors in the motor controllers to prevent arcing and electric shocks
  • HV Battery
    • Battery Pack: Stores and outputs energy; extremely dangerous if not handled properly
    • Battery Management System (BMS): Monitors the voltage, temperature, and current of the battery pack, ensuring that it stays within safe operating ranges
  • Power Distribution: Handles low-voltage and high-voltage power rails; responsible for ensuring power is distributed properly and safely
    • Low Voltage (LV): Power that's used for most boards. Typically 12V DC
    • High Voltage (HV): Power from the solar array and battery pack. Used to drive the motors through the motor controllers. For MSXII/MSXIV, HV is between 90 V and 151.2 V. 
  • Driver Controls
    • Lights: Handles all lights in and on the car
    • Driver Input: Reads switches, buttons, and pedals and notifies the appropriate system
    • Driver Display: Displays information for the driver such as speed and battery status
  • Controller Area Network (CAN) bus: Automotive communication protocol; the network that systems communicate with each other over
  • Telemetry: Collects and transmits data from the car to an external receiver for remote monitoring

Solar cars tend to use special high-efficiency motors that drive the wheels directly, without an axle or gearbox. Since we don't need a gearbox and can have multiple motors, we can drive each wheel individually, minimizing losses. Typically, these are BLDC motors. In general, motors essentially generate a magnetic field using an alternate current through a coil and spins the permanent magnets located on the rotor. As a result, motor controllers are required to generate the pulses that drive the electromagnets. As an added bonus, we can use something called regenerative braking, which harvests power from the motors to slow the car down. This power is then sent back to the car, allowing us to slow the car down with fewer losses compared to friction-based braking.

When more power is produced than the car is consuming, any excess power is stored in the battery pack. The biggest concern when it comes to the batteries is safety. We need to keep track of voltage, temperature, and current and act appropriately if anything ever leaves its safe operating range. Another concern is mechanical stability, ensuring that our pack will remain safe even in a worst-case scenario.

In normal cars, there is a 12V lead-acid battery that is used to start the engine and power general electronics within the car. The same concept is shared in a solar car, where we have a supplementary (aux) battery that is used to start the car and switch over to the main battery pack. There are a number of high-voltage contactors that control power to the battery pack, motors, and solar array. These contactors are used to ensure that systems are connected in the correct order and that all high-voltage systems are disconnected in the case of a battery protection system fault. 

In order for a solar car to be useful, the driver needs to be able to interact with it. There is always some way for the driver to control the direction and speed of the car and other essentials for operating a car, such as turn signals and the horn. The driver also needs a way to see important information such as speed and battery status.

All of these systems need to communicate somehow, and although there a number of different options, CAN is most frequently used. CAN is an automotive communications protocol used in nearly every single car nowadays. Designed specifically for automotive use, CAN is a multi-master fault-tolerant protocol, using a differential signal and built-in CRC for robustness.

Finally, the car produces and processes a lot of data. That data is collected and transmitted to an external receiver, allowing us to monitor the car without plugging into it. In addition, we can process the data more efficiently and look at long-term trends to optimize driving habits.