One it was clear that I needed a custom step-up charge controller for my solar car project, I needed to plan out the design. Let’s address the MUST HAVE requirements first:
- Output voltage regulation is one important safeguard against overcharging the lithium-ion battery. Voltage regulation is also necessary to prevent damage to other vehicle electronic should the contactors open unexpectedly for any reason.
- Isolated high voltage output. While solar panels are commonly connected in arrays capable of generating 400v DC, it would be best practice to have the solar panels isolated from the high voltage drive battery. This proved to be a minor burden. The standard non-isolated step-up converter is not well suited for a 10:1 voltage ratio anyway. I used a full bridge converter which provided the necessary voltage gain and isolated output.
- Contactor control output. It seemed much easier to access the battery pack using the existing contactors instead of opening the pack and making a new connection inside. More on this in another update, or check the Real Solar Cars youtube channel. It would not work to simply turn the car on to charge. The Chevy Volt uses about 250w at idle. The 420w array on the car usually collects about 200w in realistic conditions. In any case, 250w is a lot of overhead. To minimize the overhead, it’s necessary to charge the car with all systems off except the contactors and battery management system. The battery management system on the Chevy Volt is enabled by a 12v signal on the same connector as the contactor signals.
- A CAN bus interface is needed to receive data from the vehicle’s factory battery management system.
The automotive environment adds unique challenges that are not often encountered in DIY projects:
- High and low temperatures were addressed by choosing automotive grade parts when possible. I also spent a lot of effort making the board operate efficiently to minimize the temperature rise. Multiple temperature sensors help ensure that the board can shut down before reaching damaging temperatures.
- Moisture may eventually lead to corrosion. This is likely to affect connectors first. So no unnecessary connectors. It may be desireable to apply conformal coat to the circuit board.
- Vibration is another reason to avoid unnecessary connectors.
- Serviceability. The circuit board needs to be removable for repairs or modifications. I ended up using a 40 pin IDE connector for this.
- Safety critical. It’s not likely that rouge data on the Chevy Volt high voltage management CAN bus would cause a collision. This bus is separate from the main CAN bus. What is more likely is getting stranded at the side of the road. The solar charging board should not impede any important vehicle operations. It must be possible to switch off the solar charger board and have the vehicle operate as before. This was accomplished by adding diodes which allow the vehicle’s contactor control signals to pass through. In the event that these diodes interfere with vehicle operation, the solar charger can be quickly disconnected and replaced with a bypass board. The bypass board connects the input signals directly to the output signals.
Charger board minimum required features:
- 400v DC regulated and programmable output voltage.
- 400v DC output to be isolated from solar panels and vehicle chassis.
- CAN bus and contactor control outputs for the vehicle interface.
Charger board design guidance:
- All important functionality incorporated on a single circuit board.
- Preference for vibration resistant SMT parts when possible.
- A single board with mostly SMT parts is preferable for mass manufacturing as well.
System design guidance:
- A connector to allow the solar charger to be quickly replaced with a bypass board.
- Pass-through diodes mounted on the vehicle side of the interface connector.
- A high quality highly flexible interface cable rated for water and oil.
The PCDB1910 board was designed to meet these minimum feature requirements.