Charging the electric vehicle traction battery is done in two ways, one with the raw AC source from the grid by using a portable (or onboard) charging device. The second method is by means of a external charger which directly charges the vehicle's battery (without the help of the onboard charger, if any) by a sophisticated DC charging equipment which usually handles a larger charging power (so aptly called as DC Fast Chargers).

However, in this article we focus on the charging using the raw AC source, which is also a device that follows certain protocols for safety & stability of everyone and everything around it. This is usually how it is done with the case for EV's which has a fixed onboard charger and a charging port.

The charging process is uses a Low-Level Communication that is achieved by means of Pulse Width Modulation (PWM) signals. The PWM is used for low-level communication between the electric vehicle (EV) and the electric vehicle supply equipment (EVSE) . The signal voltages alternates between two defined levels.

Signal Voltage Status Status Description
+12 V State A No EV connected to the EVSE
+9 V State B EV connected to the EVSE, but not ready for charging
+6 V State C Connected and ready for charging, ventilation is not required
+3 V State D Connected, ready for charging and ventilation is required
+0 V State E Electrical short to earth on the controller of the EVSE, no power supply
-12 V State F EVSE is unavailable

The EVSE specifies the maximum charging current for the EV via the duty cycle. The PWM signal is applied to the circuit of control pilot. The standard IEC 61851-1 is defining the meaning of the applicable duty cycle values.

Duty Cycle Status Status Description
Duty cycle < 3 % No charging allowed
3 % ≤ duty cycle ≤ 7 % Force high-level communication protocol according to ISO 15118 or DIN 70121
7 % < duty cycle< 8 % No charging allowed
8 % ≤ duty cycle< 10 % Max. current consumption for AC charging is 6 A
10 % ≤ duty cycle ≤ 85 % Available current = duty cycle * 0.6 A
85 % < duty cycle ≤ 96 % Available current = (duty cycle - 64) * 2.5 A
96 % < duty cycle ≤ 97 % Max. current consumption for AC charging is 80 A
Duty cycle > 97 % No charging allowed

The duty cycle method allows the vehicle to gain continuous access to electricity to charge the battery, without overloading the grid connectivity.