Intro to Power Distribution

Power distribution is one of the most important systems in solar cars as it ensures that power gets to where it needs to safely.

In MSXII, we have 2 main DC buses, an LV bus for powering boards at around 12V and an HV bus for powering the motors and receiving power from the solar array at around 90 - 151 V.

LV Power

Our LV bus is powered from two sources, directly from the main battery pack through an isolated DC-DC converter and from a supplementary battery. Since our supplementary battery is necessary for starting the car and handling faults where it isn't safe to use the main battery, we try to avoid draining the supplemental battery as much as possible.

Thus, we define two types of LV rails:

• Protected: Powered off of the DC-DC when possible, but falls back to the supplementary battery when necessary
• Unprotected: Powered solely off of the DC-DC

Critical systems that must be active when the car is off or in a fault condition such as power distribution or the BMS are powered off of a protected rail. On the other hand, non-critical systems such as the cameras or motor controllers do not need to be active and are thus powered off of an unprotected rail. In our case, we enforce these rails in hardware, simplifying the firmware and ensuring that non-critical systems can't be powered from the supplementary battery by accident.

HV Power

Our HV bus comprises of the main battery pack, the solar array and associated MPPTs, and motors and motor controllers. We control the power distribution between these systems through the use of special high-voltage relays, allowing our LV boards to control the HV bus. Note that all of our relays are electromechanical normally-open (NO) relays so they fail open.

• Battery relay: Connects the battery pack to the rest of the HV bus
• Solar relay: Connects the MPPTs (and thus the array) to the rest of the HV bus
• Motor relays: Connects the motor controllers (and thus the motors) to the rest of the HV bus

For safety, each relay's coil is connected to a multi-pole relay that is powered through a latching kill switch. This means that if the kill switch is hit, all relays will immediately lose power and disconnect, isolating each HV system. If a fault is read over the CAN bus, the coils can be unpowered through the power distribution board, providing the same result. For additional safety, we expect all critical systems to continuously broadcast a heartbeat packet. If any system's heartbeat packet is missed more than once in a row, we consider that a fault condition and isolate the HV bus. This also protects us from possible instability in the CAN network, ensuring that we never miss a critical message.