When you plug in several electrical heaters into sockets on the same circuit in your home, because they are connected in parallel, the overall resistance of the circuit is less than it would be for a single heater. The total current that flows would likely exceed the safe capacity of the circuit. At this point, a fuse or circuit breaker should interrupt the flow of electrical current.
Consider that an AC charger can charge an electrical vehicle with up to 22 kW and a rapid DC charger can charge with 175kW or more. The fact that you can still charge most EVs using a domestic socket suggests that Ohm’s law does not directly apply and some kind of regulation must take place.
Communication is needed
Every EV has an onboard charger which is used when charging an EV using an AC charger. It converts the AC power into DC for the EV’s battery management system so it can charge the batteries at the maximum current it supports. This suggests that for an EV and an EV charger to agree on how much current can flow, some form of communication is needed.
You would expect that today, when making the connection to an AC charger, information like vehicle ID, vehicle type, battery type, and current battery charge level would be the minimum information shared by the EV. And that the charging station would share its ID, its operational status and the charging currents the vehicle can choose. Yet the reality is quite different. In fact, an AC charger and an EV don’t communicate much more information than an AA battery does with its charger.
Limited information sharing
A typical European AC charger uses “Basic Signaling” communication with the CP, PP and PE pins in the type 2 connector. A resistance between the PP and PE pins indicates the maximum capacity of the cable that is used. When a cable is connected between an AC charger and an EV, a resistance value of 2740 Ω between the pins CP and PE exists. A drop in the resistance to 882 Ω signals that the EV is ready for charging. Hello Ohm’s law!
On the connection of an EV, the charging station provides a 1kHz square wave signal on the CP pin. The duty cycle of the signal (Pulse Width Modulation, or PWM) tells the EV the maximum current the EV can draw per phase. A duty cycle of 10% for example, means that the EV can draw 6 amperes at most, depending on the maximum cable capacity.
The CP signal remains active throughout the charging cycle. The duty cycle of the PWM signal enables smart charging. The EV changes its charging current as soon as the duty cycle changes. A 22kW smart charging station may charge one vehicle with the full 22kW but reduce the charging rate to 11kW when a second vehicle is connected. A home charger (wall box) may consider the current energy consumption in the home. The smart metering controls should maximize the charging current while avoiding overloading the home grid connection when other large consumers like a heat pump, induction oven, or air conditioning are also active. It should also allow for dynamic charging when PV panels generate surplus electricity during the day.
A downside to smart charging?
During an AC charging cycle, the onboard charger converts the power to DC, generating heat and resulting in a loss of energy. Research by the German automotive association ADAC has shown that energy losses range from ~6% to up to ~10 % when charging with 11kW. Depending on the brand and model of a vehicle, these losses can be double or even more when an EV is charged with reduced power. Onboard electronics are also active during a charging cycle. Charging with reduced power takes more time and results in a total higher consumption by these onboard electronics. The energy provided to the vehicle by the charger might show up as 22kWh on your bill, but the actual energy added to the car’s battery may only be 19kWh – and there’s no transparent display of the value of losses.
When operating a private AC charging infrastructure for multiple vehicles, it may therefore make sense to charge each vehicle with the optimal amount of power, while keeping within the power supply limit of the facility or site.
OCPP can help solve this
OCPP, or Online Charge Point Protocol, is a communication protocol that many wall boxes support. It is maintained by the Open Charge Alliance. Currently, Version 1.6J is the most widely used version. However, the latest version 2.01 adds significant features to support new use cases.
Provided a wall box supports the smart charging profile, an OCPP central system can manage charging profiles in charging stations. Charging profiles can define the maximum power for a new transaction. The charging profile for a running transaction can also be changed, dynamically increasing or decreasing the power that can be consumed by the EV.
OCPP also allows for remotely stopping or starting transactions for connected EVs. This allows you to charge EVs with optimized power, one vehicle after the other. It also enables you to prioritize the EVs of different people, e.g. those who need to leave earlier. A RFID card could be used at the wall box to communicate by OCPP to the central system to link a charge-point connector to a specific user and/or vehicle provided this information is available in a central system.
Using the zenon ocpp driver
The zenon OCPP driver is a proven enabler for a smart charging infrastructure for small to medium sites. With DC charging, the onboard charger is bypassed. This does, however, require high-level communication between the charger and the EV. For most vehicles using CCS connectors, this is based on Power Line Communication. For CHAde- MO or Tesla connectors, this is based on CAN. New AC charging stations supporting OCPP 2.01 and ISO 15118 make it possible to enable high-level communication with compatible EVs, to identify the vehicle, determine the battery state of charge, and offer support for new use cases like Vehicle to Grid (V2G). In V2G, the EV battery can be used to provide power to a facility, for example, when energy prices are high. It also supports Plug&Charge, where high-level communication enables the identification and authorization of charging transactions.
There is no doubt that the future of EVs and EV charging will be exciting. zenon OCPP driver will help you to drive it forward.
