Home > News > So how much power does a Level-II charge really draw?

People stop me in the street asking: “So how much power does a Level-II charge really draw?”

The answer is: It depends…

Abstracted from Wikipedia:

  • The SAE J1772 (the Charge Station Plug /  EV Receptacle) signaling protocol steps are:
    • the charge station signals the availability of AC power
    • the EV detects the plug being plugged in via a proximity circuit
    • (thus the vehicle can prevent driving away while connected)
    • the control pilot functions begin:
    • the charge station detects the electric vehicle
    • charge station indicates readiness to supply energy
    • EV ventilation requirements are determined
    • charge station current capacity is provided to EV (as explained below)
    • the EV requests energy flow
    • the EV and charge station continuously monitor continuity of a safety ground
    • the charge continues until the EV determines that adequate charge has been reached (80% or 100%)
    • the charge may be interrupted (and power in the plug turned off) by disconnecting the plug from the EV

The amount of current used to charge is determined both by the vehicle’s maximum acceptable current (Amps) and by the charge station’s maximum available current.

The charging station puts 12 volts on the Contact Pilot (CP) and the Proximity Pilot (PP).

The charging station sends a 1000 Hz square wave on the contact pilot that is connected back to the protected earth on the side of the vehicle by means of a resistor and a diode (voltage range ±12 V ±0,4 V). The live wires of public charging stations are always dead if the CP-PP circuit is open.

If the circuit is closed then the charging station can also test the protective ground to be functional.

The vehicle can request a charging state by setting a resistor – using 2700 Ω a Mode 3 compatible vehicle is announced (“vehicle detected”) which does not require charging. Switching to 880 Ω the vehicle is “ready” to be charged and switching to 240 Ω the vehicle requests “with ventilation” charging which does not have an effect outdoors but the charging current will be switched off indoors if no ventilation is available.

The charging station signals the maximum current that is available via a Pulse Width Modulation (PWM) signal:

  • a 16% PWM is a 10 Amps maximum,
  • a 25% PWM is a 16 Amps maximum,
  • a 50% PWM is a 32 Amps maximum and
  • a 90% PWM is a fast charge option.

And so, from the EV we get:

and from the Charge Station we get:

And from the combination of the needs and capacities of the EV and the capacity of the charge station we get a negotiated settlement on the Amps. The amount power is, of course, the current in Amps times the voltage (240).

So, for the sake of argument, a 5 hour charge at a conservative 30Amps would take

30*240*5=  36 KiloWattHours.

at 60 Amps we get:

60*240*5 = 72 KiloWattHours

Driving on the flat highway takes between 10 and 20 KiloWatts (hills much more). If you drive for an hour, that becomes 10 to 20 KiloWattHours (KWHr). In the end the charge KiloWattHours must equal the used KiloWattHours.

Also means that the $7 charge at the Kona Airport is a bargain! (@ $.42/KWHr(retail) * 36KWHr = $15)

Doug
PS: Some of this data is a bit old, so your mileage may vary…

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