Colloquially, it’s often called a charger, and you’ll find them labeled that way by news organizations, media companies, and in conversation. But what most people call EV chargers or charging docks aren’t chargers at all! It’s actually electric vehicle supply equipment, or EVSE. This equipment has its own important role and works with an EV’s charger to fuel the battery. So, where is the real charger and what does EVSE do anyway?

Bank of Tesla superchargers

The real charger is a piece of hardware inside the vehicle. The onboard charger takes alternating current (AC) power and converts it into direct current (DC) power. AC power is the type of power you find in your home, making the long journey from the power plant to your outlets. DC power provides steady, consistent voltage, making it ideal for electronics and batteries, including EV batteries. The onboard charger of an electric vehicle receives AC power through the charge port and converts it into DC power that the EV battery can use and store. The charger also regulates voltage, communicates with the charging equipment, and monitors the battery. Vehicle manufacturers have their own unique charging hardware and software. Some makers even have apps that allow EV owners to view battery health and adjust the charger’s settings.

Schematic of an EV with on-board charger highlighted
image from

EVSE plays a different role. As you might guess from the name, electric vehicle supply equipment is what supplies electricity to the vehicle. It’s the cord you plug into the charging port, but it also has important safety features. EVSE is a smart adaptor that communicates with the vehicle charger, determining the maximum amount of current that can be supplied through the charging cable and the maximum current the onboard charger can receive. EVSE can sense if the cable isn’t connected properly and prevent power from flowing through it, reducing shock risk. The equipment can also detect hardware faults and disconnect the power, preventing battery damage, electrical shorts, and fires.

To fully understand the relationship between the onboard charger and EVSE, we need to talk about volts, amps, kilowatts, and kilowatt hours. Let’s compare the flow of electricity through an EVSE cable to the battery to water flowing through a hose that you’re trying to use to fill a bucket. In this metaphor, 

  • Voltage is like water pressure - a measure of the force that the water exerts 
  • Amps represent how much water can flow through the pipe at once
  • The hose's “amp rating” is the maximum water flow that it can hold without bursting. On the same token, your vehicle’s amp rating is the maximum amount of current that can safely be delivered to your battery
  • The combination of volts (pressure) and amps (volume) make kilowatts, which measures of the the speed at which power is delivered to your battery. In our metaphor, it’s the measure of how quickly water fills your bucket
  • On the other hand, a kilowatt hour is a measure of capacity, or how much water the bucket can hold
Stock image of a hose filling a bucket

Different electric vehicles have different amp ratings and battery capacities, which is why it’s so important for the onboard charger and the EVSE to communicate. The onboard charger is the main limiting factor of charging speed. It determines and regulates how many volts and amps the battery can accept. The EVSE monitors the power output from the source of electricity and tells the onboard charger how much power it can safely pull from that source. Without the EVSE, the charger might try to pull too much power through the cable, or it might not charge as efficiently as it could. By working together, the onboard charger and EVSE allow the vehicle to charge at the safest maximum speed.

You may already know there are three levels of electric vehicle charging. There are also four types of EVSE that correspond to different charging levels. 

  • Mode 1 is basically an extension cord with no communication and control between the vehicle and the source of power. Because Mode 1 lacks safety features, it’s prohibited in the U.S. 
  • Mode 2 is the EVSE that comes with a new electric vehicle and is used for Level 1 charging. It has shock protection and can plug into standard 120V outlets.
  • Mode 3 uses a dedicated charging station that is permanently connected to an AC power supply. This mode uses 240V outlets, the kind you might install in your home to enable Level 2 charging, or the dedicated Level 2 EVSE commonly found at public charging stations.
  • Mode 4 is the fastest EVSE, the kind used for DC Fast charging, also called Level 3 charging. It actually by-passes the onboard charger to deliver DC current directly to the battery. Like Mode 3, Mode 4 EVSE requires a dedicated power supply. Right now, you can only find Mode 4 EVSE at charging stations.

For each charging level and modes 2-4, the EVSE checks what’s coming out of the power supply, checks what the onboard charger can handle, and allows the appropriate amount to flow through. Even if the power supply could deliver more kilowatts, the onboard charger cannot accept more power than it is rated for, and the EVSE guarantees the correct rate of charge.

This is especially important for Level 3 charging, which by-passes the onboard charger and works directly with the vehicle's battery management system (BMS). Peak Level 3 charging speeds range from 50kW to 250kW - or more! No matter how much power the supply can deliver, the EVSE and BMS work together to ensure the vehicle battery only receives as much power as it can handle, protecting the battery from damage. That’s why you should know how many kilowatts your vehicle can utilize, and why it’s impolite to take up a public charging spot that exceeds your vehicle’s maximum.

Now you know the difference between an electric vehicle charger and electric vehicle supply equipment. Honestly, most people will still call EVSE “chargers” or “charging docks,” and you shouldn’t feel bad if you’re one of them. There’s a reason we have colloquial terms and technical terms. But hopefully you know a little more about EV charging and the intricate communication that goes on between the onboard charger and the EVSE.

In addition to helping you make informed decisions around vehicle charging, you can use this knowledge to impress EV purists, or to be the most fun person at any party.