The eBaja Electric Conversion

Benefits

Converting a racing vehicle from gas to electric is a worthwhile undertaking. With the conversion to electric, a driver has near instant torque as opposed to a gradual torque curve from engines using gas. This allows drivers to have a faster take-off speed at the start of the race and for speeding back up after a slowdown on the track. Those extra few seconds saved from the final time makes all the difference in competitive offroad racing. Additionally, with the battery installed in the back of the buggy, pit crews are able to easily get to the most important part of the buggy's electrical system. With plug attachments, crewmembers will be able to swap the battery out for a spare quickly and efficiently.


Project Requirements




System Design

The electrical system is comprised of the battery, which powers the motor controller and auxiliary system. In between the battery and motor controller is the contactor, which acts as a relay for the "ignition" of the system (a simple keyed switch that will connect the contactor and turn on the controller). The motor controller then takes the direct current coming from the battery and converts it into alternating current to be used by the electric motor. The speed of the motor - and thus the current from the battery - is controlled by the throttle. There is also a "transmission" labelled "F/N/R" or "Forward/Neutral/Reverse" for short that is a simple switch that tells the motor controller which direction to spin the shaft of the motor. In the case of the neutral position, the motor controller does not move the motor even when the throttle is pressed down. Finally, on a separate line connected by a DC to DC step down converter, the aux system pulls 12V from the main line to power various lights such as headlights, taillights, and signals.


Hardware Used


The work horse of the buggy's electrical system is the battery pack. Comprised of 10 Nissan Leaf battery packs, each comprised of 4 LiPo cells (a parallel pair of two cells in series). It outputs 78V to the system which is used by the controller that drives the electric motor and is used by various auxiliary subsystems.


LiPo batteries need active management when charging and discharging; as cells are not chemically identical, there are small variations of the rated voltage for each cell. Without this management, one cell could be overcharged trying to charge a cell further down the series. When a cell is overcharged, damage is done to the cell which can lead to bloating, or worse, ignition of the cell which can cascade to destroy more cells in the pack. In order to balance the cells, a Battery Management System (BMS) is attached to each battery pack to monitor the voltages of each cell. If a cell is full, the BMS draws power from that cell to charge other cells thus preserving the battery pack's lifespan.


In order to charge such a large battery, one needs a higher voltage than the battery to store energy into it. Due to this, the battery requires a DC power supply to charge to its full 78V rating.


In order to control the amount of speed the motor runs at, a motor controller is necessary in the system. Additionally, the electric motor is powered through three-phase power, so the controller converts the DC current from the battery into a 3-phase AC power signal to be used for the motor. The controller has a throttle to control the amount of current the controller pulls from the battery and thus controls the speed of the motor, and it has an ignition switch to toggle power to the controller. Also, to control the drive mode of the motor (forward, reverse, and neutral), there is a toggle switch for the three drive modes. This switch, however, will be replaced with a larger switch so that it may be easier for a driver with gloved hands to operate.


The electric motor is the endpoint of our system. Power is fed to the motor through the controller, speed is controlled by the position of the throttle lever arm, and this mechanical power is transferred to the drivetrain of the buggy.


The Intelligent DC-DC Converter is the bridge between the main power line supplied by the battery and the auxiliary system that must run on 12V. It takes the high 78V from the line and decreases that voltage to a useable 12V for the headlights, taillights, and turn signals. Without this component, many of the components of the auxiliary system will be destroyed with a direct connection to the power line.


The plugs that connect the power from the battery to the system need to be rather large because the cables needed to handle the current pushed to the rest of the system need to be large. These cable plugs are great tools to make connecting and disconnecting the battery easier, and without them one would need to unscrew the bolts from the battery's terminals which introduces chances of arcing and injury to the person swapping out the battery. These plugs also require a larger pull force to disconnect them, making them a safer option for the application of offroad racing.