What is Acceptance Rate and How Does It Affect Charging Speed?

Did you think your EV would charge faster than it does? Unfortunately this is a common complaint and it primarily stems from misleading marketing. While charging a plug-in vehicle seems straightforward, there are several factors that add complexity to the process. Aside from the confusion around Level 1, 2, and 3 charging speeds which you can learn about here, one of the key misunderstandings is that charging speed is entirely dependent on the Electric Vehicle Supply Equipment (ESVE).  EVSE (like the “JuiceBox” or “Tesla Wall Connector”) is often mis-referred to as the “charger”, which although used as an industry standard term, is not exactly correct. This terminology distinction is necessary because the device actually charging the car’s battery is an EV’s onboard charger.  EVSE simply supplies electricity to your car’s onboard charger which then converts the energy so the car battery can accept it.

(In the illustration below, the JuiceBox, Tesla Wall Connector, or any other Level 2 EVSE would be the “Control Device.”)

Provide an illustration of ESVE and an onboard charger to visualize the distinction.
Source: USDOE (2012), Plug-In Electric Vehicle Handbook for Public Charging Station Hosts. Illustration by Dean Armstrong, NREL

What is an Onboard Charger?

Onboard chargers are built into every plug-in vehicle by their manufacturer. They are necessary because batteries store energy as direct current (DC), but all homes supply power as alternating current (AC). This means when you plug your car in, AC power would be supplied to the battery, which it cannot accept. To solve this problem, car manufacturers build in onboard chargers. A car’s onboard charger accepts the AC power from the home and converts it to DC power, which can then be stored by the battery. Onboard charger specifications vary depending on the make, model, and options of the particular car. This is significant because different onboard chargers have different max acceptance rates. 

For example, a Porsche Panamera plug-in E-Hybrid comes standard with an onboard 3.6 kilowatt (kW) charger, however, for $1,230 more, the Porsche factory can upgrade the onboard charger to 7.2 kW. If plugged into a 32 amp EVSE that has a power output of 7.2 kW, the standard car would only charge at 3.6kW, taking about 3 hours to fully charge. However, if you plugged the upgraded model car into that same EVSE, it would charge at the full 7.2 kW capacity of the upgraded onboard charger, taking only 1.5 hours to charge. That said, if your EVSE delivers less power than your onboard charger’s max acceptance rate, your car’s charge rate will be limited to the EVSE’s delivery rate. 

Onboard charger acceptance rate is a topic that is often overlooked when setting up a charging system. Usually, if you look up fast EV home charging or ask your dealer for suggestions, they will recommend purchasing a Level 2 charger. Level 2 EV chargers are perfect for home use, but overlooking the onboard charger’s acceptance rate can result in overestimating how quickly your car will charge. Part of the reason for this disconnect likely stems from the different units used to describe onboard chargers’ acceptance rates compared to the power supplied by Level 2 chargers.

Kilowatts vs. Amperage

Onboard chargers’ acceptance rates are generally measured in kilowatts while Level 2 chargers’ power delivery are usually measured in amps. This adds a layer of confusion, but fortunately there is a fairly straightforward way to convert the amps to kWs. To determine kWs being delivered by the ESVE, we take the voltage the ESVE is connected to and multiply by the amperage of the ESVE, before dividing by 1000. The voltage will almost always be 240V for Level 2 chargers in the United States. This means we just need to know the amperage of the Level 2 charger to convert it to kWs. Let’s say we have a JuiceBox 40 as our ESVE with an amperage of 40 amps. The equation will look like this: 

240 V X 40 amps ÷ 1000 = 9.6 kW. 

With this knowledge we can easily compare the power delivered to the car with the car’s onboard charger’s acceptance rate. As long as the car’s acceptance rate is equal to or less than the ESVE’s supply rate, the car’s battery will charge at its max acceptance rate, which is as fast as possible. For example, a Nissan Leaf has an acceptance rate of 6.6kW meaning that the 9.6kW delivered by the JuiceBox 40 will allow the Leaf to charge as quickly as it is capable of. The higher capacity EVSE will not hurt the Leaf, as the car’s control system only asks the EVSE for the current that it can accept.

Planning for the Future

We believe in electrification of the future. It’s likely your next vehicle will be a plug-in hybrid or a fully electric vehicle. The installation cost of 40 amp chargers is generally not significantly more expensive than lower output Level 2 chargers, so we recommend going with higher amperage installations, even if your current vehicle’s acceptance rate is lower than the charger’s output. 

Still have questions? No problem, we’re here to help. Give us a call or send us an email, and we will make sure to get you and your vehicle’s needs taken care of.

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