General Principle of Charging
Charging is a process of forcing electric current into a drained battery. It could also be defined as restoring energy to a cell or battery pack. A device that performs charging is called a charger. The charging process includes restoring, stabilizing and termination. Restoring is a process of allowing an electric current to flow to a drained battery or cell to restore its energy. Stabilizing refers to a process of controlling the amount of current such as allowing a higher current during the start and gradually decreasing when near full charge. Termination is the process of cutting off the current flow when the battery or cell is already fully charged in order to avoid damaging the battery or cell. Putting these three processes in charging makes it a safe charging. The general principle of charging is the foundation on how does an EV charger works.
There are DIY charging practices wherein undermining the mentioned processes. This is called an unsafe charging practice. This practice must be avoided to prevent accident. Below is a comparison of safe and unsafe charging.
How Does an EV Charger Works in General?
An EV charger is designed to charge an electric vehicle. It follows specific standards that are stringent than the usual charging process. It has specific protocols for handshaking and communications to the electric vehicle. The charging process is more complicated than the usual charging process.
When an EV charger is plugged to an electric vehicle, the proximity pilot (PP) provides an analog signal. The electric vehicle will detect the signal and it is preventing itself from moving away.
Aside from the proximity pilot, another important signal involved in the handshaking is the control pilot (CP). Through the control pilot signal (CP), the electric vehicle can determine what type of charger connected to it, whether AC or DC.
There are two classifications of EV charger. One is AC and the other one is DC. AC EV charger refers to a charging station that the output is still AC. For this charger, the actual charging process is performed inside the electric vehicle through the onboard charger (OBC). A DC charger on the other hand refers to an EV charger that output is already DC. The charging process is not anymore performed by the onboard charger but by the DC charger itself. Read
here for more discussion about EV charger classification. Through the control pilot signal also, the electric vehicle will determine what kind of communication it will use to talk to the EV charger.
A control pilot signal (CP), is a square wave signal generated by the EV charger. It has a +/-12V level and a frequency of 1kHz. If the charger is AC, the control pilot signal carries the charger’s maximum current capacity through the duty cycle. The charging state is also indicated by the control pilot signal.
For a DC EV charger, the control pilot duty cycle is fixed at around 5%. This is an indication for the electric vehicle that the charger connected is going to use a high level communication (or digital communication) such as DIN SPEC or ISO15118. The charging state is still indicated though the level of the control pilot.
AC Charger Key Components
AC charger is not a real charger but can be compared to an adapter such as used in the mobile phones, tablets and laptops. These devices have constant output voltage. The actual charging is performed inside the equipment (mobile phone, tablet or laptop itself).
An AC charger is the same. It has an input voltage in the form of AC and the output voltage still in terms of AC. The actual charging process is performed inside the electric vehicle through the onboard charger (OBC).
AC charger is a simple one and consist only of few components only.
DC Charger Key Components
DC charger is a real charger. Its output is directly useable by the electric vehicle battery. Thus, its diagram is way complex compared to the AC charger. The main difference is the rectification and the DCDC conversion and charging process. Rectification is the process of converting an AC to pulsating DC. After rectification, an active front-end is installed. It serves two purposes. The first one is to ensure the voltage and current will in phase in order to mitigate harmonics that can cause many problems as well as to increase the power factor of the power section. The second purpose is to make the pulsating DC to a near linear DC (free from huge ripples) so that it can be useable by the next stage DCDC converter. This DCDC converter is electrically isolated and do perform the charging process.
The Role of the Proximity Pilot (PP) on How Does an EV Charger Works
The key function of the proximity pilot is to indicate the presence of the EV charger connector to the electric vehicle. When this presence is established, the vehicle is prevented from moving away. The Proximity signal has three levels.
When the charging cable is not connected to the electric vehicle inlet, the Proximity signal read by the electric vehicle is the voltage divider of the resistance 330 and 2.7k ohms with respect to 5V. This results to 4.5V level. See the section in red in below illustration.
- How the 4.5V is Derived
Prox signal (no charging cable connected) = ( 5 x 2.7k ) / ( 2.7k + 330 ) = 4.45 V (rounded off to 4.5V)
When the charging cord is connected to the vehicle inlet but the latch (S1) is pressed, the series combination of the 150 ohms and 330 ohms resistances in the EV charger connector will form a parallel connection to the 2.7k ohm in the vehicle inlet. This will result to Prox signal level of 3V.
- How the 3V is Derived
Prox signal (charging cable connected, S1 is pressed ) = [5 x (2.7k x (150+330))/(2.7k + (150+330)] / [(2.7k x (150+330))/(2.7k + (150+330)) +330] = 3V
When the charging cable is connected and the latch (S1) is not pressed, the bottom 330-ohm resistor in the charging cable is shorted, the proximity level will become 1.5V.
- How the 1.5V is Derived
Prox signal (charging cable connected, S1 is not pressed) = 5 x [(150×2.7k)/(150+2.7k)] / [{(150×2.7k)/(150+2.7k)} + 330] = 1.5V
What is the Role of Control Pilot (CP) on How Does an EV Charger Works
If there is one signal that can define how does an EV charger works, it is the control pilot. The control pilot is very important. It is the main signal for doing charging for AC chargers while still important in DC charger, though not anymore, the main signal for communication during the charging process.
The control pilot signal is generated by the electric vehicle charger (not the electric vehicle). It is a square wave signal with +/-12V signal amplitudes with a frequency of 1kHz.
Control Pilot Duty Cycle
During the start of the charging process (AC or DC charger), the pilot signal generated by the electric vehicle charger will give the electric vehicle the idea of what is the type of charger connected into it. If the pilot signal duty cycle is 5% (or very close to it), the charger connected is a DC charger or a charger that requires a high level communication (or digital communication). If the duty cycle is within 8% to 97%, the charger connected is an AC charger.
Below is a table summary of duty cycle and corresponding current to be drawn by the electric vehicle as well as if charging is allowed or not. This is based from IEC61851-1 definitions.
Based on the above table, the maximum current capacity of an AC charger is only 80A. For DC charging (or for off board charger that requires digital communication), the applicable current calculations in the table are not applicable as the duty cycle is fixed to around 5%.
Control Pilot Amplitude
The Control pilot amplitude denotes the state of charging. It could be state A, B, C, D, E or F. Each of this has meaning. It is useful for both AC and DC electric vehicle charger.
State A is when there is no charger connected to a vehicle. During this time, the pilot positive amplitude is +12V, while its negative amplitude is not characterized (N/A) as measured in the point of measurement illustrated in the diagram below.
State B is when a charger is connected to an electric vehicle but it is not charging yet (ready state). The positive amplitude of the pilot signal is +9V while the negative amplitude is -12V.
State C occurs when the charging is started. The amplitude of the positive signal is +6V while -12V for the negative amplitude.
State D also indicates a charging session but this is specific for an EV that requires ventilation during the charging process. There are batteries that may generate gas by product during charging. Such as lead acid batteries, it will generate hydrogen gas a byproduct. When hydrogen gas accumulates into a closed area, it may result to explosion. At state D, the amplitude of the positive pilot signal is +3V while still -12V for the negative pilot signal.
State E and F are associated to errors. Below is a table summary of the pilot states.
Both the proximity pilot and control pilot signals are measured with respect to the ground or protective earth (PE) that is connected to the vehicle chassis.
How Does an EV Charger Works: The AC Charging Process
The charging process of an AC charger is not complicated as the DC charger. Both J1772 and IEC61851-1 discuss this in details, but below process flow is an own simplified version so that easy to understand by everybody.
Disconnected Phase
The charging cable is not connected. The proximity signal is 4.5V. The pilot signal level is +12V. This is at state A.
Connected Phase
A charging cable is connected to the vehicle inlet. The pilot signal at this time is +9V. The proximity signal is 1.5V. This is at state B.
Initialization Phase
During this time, the EVSE (electric vehicle supply equipment or electric vehicle charger) will tell the electric vehicle what type of communication to use. If it is not a digital communication, the EVSE will proceed in telling its maximum current capacity. These happen through the control pilot duty cycle. This is still happening at state B. Once the electric vehicle acknowledges the EVSE information, it will proceed to charging phase.
If there is issue during the initialization, it will go back to connected phase, unless the driver will pull out the charging cable to go back to the disconnected phase.
Charging Phase
Control pilot signal will become +6V or +3V during this time. The electric vehicle will close its internal contactor. The EVSE will close its relay as well. Current will flow from the charger to the vehicle.
If there is error or fault during the process, the charging will stop and go back to connected phase, unless the driver will pull out the charging cable to go back to the disconnected phase.
How Does an EV Charger Works: The DC Charging Process
As mentioned, DC charging process is more complicated that AC charging process. There is a detailed explanation about this in IEC61851-23. For easier understanding, it summarizes in below process flow (this is an own short interpretation).
Unmated
EVSE charging cord is not connected to an EV.
Mated
Control pilot enters state B right away. EV cannot move since proximity signal is detected already. This time, the SLAC process starts. SLAC means signal level attenuation characterization. This is part of a power line communication (PLC) process.
Initialization
After a successful SLAC process, the PLC communication is established. This time, operating limits has been exchanged between the charger and EV. At this time, the DC voltage in the charger output is monitored. If it is higher than 60V, the charging process is terminated. The charging process will also terminate if there are other incompatibility between the charger and EV.
Cable Check
This time, EV changes the pilot state from B to C or D and set the EV status as ready. After connector locked has confirmed, EVSE starts checking the HV system isolation and continuously reports isolation state. The charger will check if the system isolation resistance is above 100 kΩ. If the isolation check is successful, EVSE will indicate status “Valid” and changes to “Ready” with cable check response.
Pre Charging
The electric vehicle will send a pre charge request to the charger. The requested current is not higher than 2A and with a particular voltage level. The charger will respond by adapting the DC output voltage within the tolerance range and ensuring the current not to exceed 2A.
Afterwards, the electric vehicle will going to close its disconnecting device, once the deviation of the DC output voltage from the EV battery voltage is less than 20V.
Then the EV will send a power delivery request to enable the charger output. As a respond, the charger will send a message saying it is ready to do energy transfer. The EV will finalize and set the current request to start the energy transfer.
Charge
This is the formal charging stage. The EV and the charger are exchanging messages during this time. Both are monitoring important parameters. If there are issues, charging will be halted.
Once the battery is almost full, EV will decrease its current request. The EVSE will follow.
Power Down
This time, the charging session is about to be done. It starts by the EV requesting the charger to disable its output. When the current is less than 1A, EV will open its disconnecting device. The EVSE will disable its output too. The pilot state will return to state B. The EV will only unlock the connector when the voltage is less than 60V. Then go back to unmated stage.
Communication Options
There are two communication options between the charging stations or charger and the electric vehicle. First is “Low-level communication” and second one is the “High level communication”.
In low-level communication, the parameters present are only the maximum current and the charging state. J1772 is an example of a low-level communication. This is often used in AC chargers, though there are few AC chargers that uses high level communications already.
In high level communication, there are several parameters that are considered or monitored such as compatibility, charging sequence, physical limits, energy demand, charging schedules and sales tariffs and authorization and payments. An example of high-level communication is ISO15118 and DIN SPEC 70121. High level communication is primarily used in DC chargers. The particular high level communication used is PLC or power line communication.
Charging Connector Options
There are several options for charging connectors. Every country has its own connector standard. For instance, in the US, for AC chargers, they use Type 1 connector. For DC charging, they use CCS1, NACS (J3400) and Tesla specific connector.
In Europe, for AC chargers, they use Type 2. For DC, they use CCS2. In China, they uses the GBT connector. In Japan, the flagship connector is CHAdeMO.
CCS stands for Combined Charging System. With respect to the vehicle inlet, it is a combination of an AC and DC connector. This means that with a single inlet, both the AC and DC charging connectors can fit.
International Standards in EV Charging
Conductive Charging
- IEC 61851: Electric Vehicle Conductive Charging System
This standard covers the general requirements (61851-1), EMC for offboard charger (61851-21-2), AC EV charging station (61851-22) and DC EV charging (61851-23).
The standard does not cover non-conductive charging or wireless charging.
Communication Standard
- ISO 15118: Secure Communication Interface
This standard includes the general information and use case definition (15118-1), network and application protocol requirements (15118-2) and physical and datalink layer requirements (15118-3).
The key content of this standard are Plug and Charge (PnC), smart charging, vehicle to grid and secure communication.
- DIN SPEC 70121
Electromobility – Digital communication between a d.c. EV charging station and an electric vehicle for control of d.c. charging in the Combined Charging System.
- DIN SPEC 70122
Electromobility – Conformance tests for digital communication between a d.c. EV charging station and an electric vehicle for control of d.c. charging in the Combined Charging System.
Safety Standard
For safety standard, the following are commonly used:
- IEC61851 part 1, 22 and 23 (for Europe markets)
- UL 2231, UL2202 and UL50E (for US markets)
- NEC Article 625 (for US markets)
Emission and Immunity Standard
- IEC61851-21-2 (for Europe markets)
- FCC Part 15 (for US markets)
Charging Station and CSMS Interoperability
- OCPP (the latest version is 2.0.1)
What Is OCPP?
OCPP stands for Open Charge Point Protocol. It is maintained by Open Charge Alliance in Netherlands. It is a free and open-source protocol.
How does an EV charger works is not only limited to the physical structure and mechanisms. A virtual mechanism is also important such OCPP. OCPP is a communication protocol used between an EV charging station and charging station management system or commonly called as backend. The main objective of OCPP is to foster interoperability between charging equipment and backend management system by different vendors.
Authentication Methods in EV Charging
Common authentication methods are:
- Card based
- It supports credit card or debit card
- RFID
- NFC
- This is near field communication
- App based authentication
- Vehicle authentication
- Autocharge – this is hassle-free authentication method that uses vehicle MAC address and OCPP.
- Plug and Charge (PnC) – another hassle-free authentication method that uses public and private key. This is secure than Autocharge. This method is supported by ISO15118.