PWM Signal

The EV charger communicates with the EV via one wire (and the earth connection of the supply is the return path). This communciation wire, called Communication Pilot, carries a signal which is either constant +12 Volt or a PWM signal. PWM stands for Pulse Width Modulation and is a square wave signal. The signal alternates between +12 Volt and -12 Volt with a frequency of 1 kHz. Via this communication signal the charger can detect the presence of the EV and the charger can mandate the maximum charge current an EV is allowed to consume. The EV can signal to the charger that it wants to receive power.


The EV Charger that sends the PWM signal to the EV can be in one of 3 states: No EV, EV Connected and EV Requesting power. These are respectivily state A, state B and state C (or D). There is also an error state called State E. This state is entered if the connected EV doesn’t provide the correct response.

State A

State A is the state where the Charger doesn’t detect the presence of the EV. The communication signal is 12 Volt (or 12 Volt PWM). As the EV is not connected, there is no current flowing through the communication wire and no votage drop detected by the charger.

State B

The transition to state B is achieved as soon as the charge cable connects the charger with the EV. The EV connects the communication pin to the earth pin via a resitor and a diode. The value of the resistor is 2740 Ohm and will cause a voltage drop to 9 volt. The diode will only allow the positive part of the PWM signal to pass. This is to make sure it really is an EV that is connected and not a child’s finger. Figure shows the circuit in the EV.

State C

At some point in time the EV wants to start the charging process. This can be directly after detecting state B, when the user locks the car or other configurations in the EV. Now the EV initiates State C. It does do that by decreasing the resistor value to 880 Ohm. The positive voltage of the PWM signal drops to 6 volts. The charger now will activate the supply by switching the contactors or relays in the charger. This makes the power available. When the EV has charged its batteries, it can transition back to State B. The charger will deactivate it’s contactors and the EV is disconnected from the mains supply. Thus, only in state C the cable and EV are powered.

Mode 3 and Mode 3 simplified charging

The state changes from A to B (by the user connecting the cable) and B to C (by the EV requesting power) are called the Mode 3 charging. However there is also Mode 3 simplified charging. This is if the EV transitions directly to state C. The circuit for that looks like this:

The EV skips state B and, by that, state B is not detected by the chargers. The Voltage on the communication wire drops from 12 Volt to 6 volt. While Mode 3 simplified is official described in the standard, not all chargers support it. Mode 3 simplified is often used by Light Electric Vehicles and adapters as it is more simple to implement.

Maximum current

The PWM signal must indicate the maximum power the EVSE is able to supply. This makes it possible for any EV to charge at any charge point (EVSE) without overloading the mains supply. The maximum current is coded in the duty cycle of the PWM signal. As we have seen, the communication signal alternates from +12 Volt to -12 Volt with a 1 Khz. This means it completes 1 cycle (of +12 Volt and – 12 Volt) in 1 millisecond (ms). However, the PWM signal can vary how long it will stay positive (+12 Volt) and negative (-12 Volt). The sum is always 1 ms. For example, if the PWM signal is 50/100 ms positive and 50/100 ms negative, the PWM ducty cycle is 50%. If the postive signal is only 17/100 ms, the negative signal will last 83/100 ms. Now the duty cycle is 17%. The charge current may vary between 6 ampère and 80 ampère and