Charging Pile Control Board Design for Outdoor Use
A shared charging pile is a networked power outlet with a meter, a payment interface and a safety chain. Its control board has to be robust, because it is installed outdoors and expected to run for years without attention.
What the Control Board Does
The board identifies the user, authorises a charging session, controls the contactor that connects the outlet, measures the energy delivered and reports the result to a server.
It also monitors the environment and the state of the equipment, and it has to disconnect the outlet when a fault is detected. Those functions continue to be required even when the network is unavailable.
The Power Path
The contactor connects the mains to the socket under the control of the board, and it is rated for the full current of the outlet. The measurement of that current is used for metering and for protection.
The metering has to be accurate, because it is the basis of a transaction. A dedicated metering device with its own calibration is often used rather than a shunt on the control board, and the interface between them is part of the design.
Measurement and Metering
Voltage and current are measured continuously, and the energy is accumulated in the meter. The accumulated value has to survive a power failure, which means it is stored in non volatile memory at a defined interval.
The accuracy requirement comes from the regulation for a billing instrument rather than from the engineering preference. The metering device and its calibration are certified as a unit, which is why it is usually a module rather than a circuit.
Identification and Payment
The user is identified with a radio card, a QR code or an application on a phone. Each method has a different interface, and the board may support several of them.
The card reader is a module with its own antenna, and its placement is a design decision. A reader mounted behind metal will not work, and one placed beside a switching supply will read unreliably.
Communication
The pile reports to a server over a mobile network, an Ethernet link or a local mesh. The choice follows from the site, and the board has to provide the interface and the power for the module.
The protocol defines the messages and the states. A defined data model is what allows the equipment to be integrated with a management system rather than reverse engineered, and it should be agreed before the hardware is designed.
Environmental Protection
The enclosure is outdoors, so the board sees a wide temperature range, humidity and, in some locations, salt. Conformal coating and a sealed enclosure are the first measures, followed by a layout that keeps the sensitive circuits away from the edges.
Condensation is the more common problem than rain. A product that is sealed against water but not against breathing will collect moisture inside as the temperature changes, which is why a membrane vent is often fitted.
Surge and Lightning Protection
An outdoor product with a mains connection and a data connection is exposed to surges on both. A varistor and a spark gap on the mains side and a protective device on the data side are the usual arrangement.
The layout of the protection matters as much as the devices. The transient current has to be diverted before it reaches the sensitive circuit, which means the protection is placed at the entry and its return path is short.
Safety Chain
Protection covers earth leakage, overcurrent, over temperature and the state of the enclosure. A residual current device and a contactor with a positive action are required, and they are monitored rather than assumed.
An emergency stop and an isolation switch are part of the installation rather than the board, but the board has to detect their state and report it. The distinction between the functions has to be clear in the documentation.
Power Supply for the Electronics
The control electronics run from an isolated supply derived from the mains, and the interface to the outside world is isolated as well. The isolation is a safety requirement and also a means of breaking ground loops.
The supply has to start reliably at the lowest mains voltage and survive the surges that reach it. The converter layout and its protection are part of the same design problem as the rest of the board.
Thermal and Mechanical Design
The contactor, the supply and the metering device all produce heat in a sealed box. The heat has to reach the enclosure, and the metal parts of the installation are usually the path.
The connectors are large and the cables are stiff, so the board has to be supported against the load they apply. A terminal that is soldered to the board and pulled by a cable will crack unless the mechanical design takes the force.
Testing and Certification
Testing covers the metering accuracy, the isolation, the leakage current, the behaviour of the safety chain and the communication. The environmental tests cover the temperature range and the humidity.
The certification for a billing device is separate from the electrical safety certification, and the two have different test houses. Planning both in the schedule is part of the project rather than an administrative detail.
Firmware and Updates
The product will be in the field for years, so the firmware has to be updatable, either over the network or locally. The boot loader and the update mechanism are part of the design and not an afterthought.
The update has to be safe: a failure in the middle must leave the equipment able to recover. A dual image with a rollback is the usual approach, and the current carrying circuits are unaffected by it but the memory and the processor are not.
Process Control and Verification
On a design of this kind, surge is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
Process Control and Verification
On a design of this kind, surge is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
FAQ
Why is the meter a separate module? Because a billing instrument is certified as a unit with its own calibration. Integrating the measurement into the control board would move that certification onto the whole product.
How is the energy total protected against a power failure? By storing it in non volatile memory at a defined interval and on the detection of a power loss.
Why is condensation a bigger risk than rain? Because a sealed enclosure still breathes as the temperature changes, and the moisture that enters condenses on the coldest surface, which is often the board.
