To distribute low voltage power inside the accumulator, monitor the pack, control the critical elements of the safety shutdown system, and get the resulting data onto the vehicle network.
Everything inside the accumulator container that is not high voltage. The Battery Management System monitors cell voltage through voltage taps and cell temperature through thermistors at the negative terminals. An Insulation Monitoring Device watches isolation between the tractive system and the chassis. Safety shutdown latching holds the shutdown state once a fault occurs. A precharge controller brings the DC bus up through a resistor before the main contactors close, so the inverter's DC bus capacitance does not draw an inrush that welds a contactor. The subsystem also owns the accumulator sub-harnesses, grounding, the voltage tap fuse board, and the interfaces to the tractive system active light, ready-to-move light and voltage indicator.
Design robust safety systems first. On a first-year EV program the low voltage side is what gets the car through electrical technical inspection, and a car that does not pass does not score. Star grounding over chassis grounding. Chassis return is simpler and uses fewer wires, but it creates ground loops that show up as CAN errors and sensor glitches. Star grounding isolates signal paths, gives cleaner CAN and sensor behaviour, and can be validated; the cost is planning effort and more wire runs. 1. List every low voltage function inside the container and what it must interface with. 2. Decide grounding architecture (star chosen) and lay out the returns. 3. Design the boards, with conformal coating selected against the requirement (acrylic is easier to rework, silicone is more vibration resistant and more temperature tolerant but costs more and is harder to remove). 4. Design HV monitoring: optocoupler (smaller, cheaper, digital, needs conformal coating) against voltage transducer (no conformal coating needed, larger, more expensive, analog). 5. Build the sub-harnesses, test in simulated conditions before the pack is live.
- Ensure reliable function of team-designed electronics inside the accumulator. - Interface accurately with the harness and components to provide control and power. - Design robust safety systems that pass scrutineering. - Fuse the voltage taps. They were unfused, which is a known rules and safety gap. - Find temperature sensing connectors that are less bulky and easier to separate.
- Voltage taps are not fused. - Temperature sensing connectors are bulky and difficult to separate. - High voltage noise interfering with low voltage sensors, showing up as CAN dropout and resets. Star grounding and EMI-shielded HV cable are the countermeasures. - Isolation requirements on PCBs, and the trace spacing rules between low and high voltage. Conformal coating relaxes the required distances.
Altium Designer for PCB design. Orion BMS 2 software for pack monitoring configuration. Bench testing in simulated conditions before pack integration.