One lesser known component of electric vehicles (EVs) is the onboard charger. It does not even have its own Wikipedia page. It is embedded in the general battery charger page.  Even though the onboard charger cannot claim the battery’s fame it has a very important role in the vehicle. It plays a crucial role to understand charging speed and costs. This article aims to show you why and how.

Let’s start with some context about the power grid and batteries:

The power grid operates at different AC voltage levels. Transformers are used to convert high voltage into lower voltage or vice versa, while converters are employed to change AC into DC and vice versa, as well as to convert DC from one voltage level to another. It’s important to note that not all converters can handle conversions in both directions.

Electric Vehicle (EV) batteries operate on DC. Therefore, to recharge the battery, the AC grid power must be converted into DC at some point. This conversion can occur either within the vehicle or at the charging station. The onboard charger is responsible for converting AC grid power into battery-usable DC power. This process is known as AC charging. Conversely, when AC is converted within a charging station, it is referred to as DC charging.

The diagram below illustrates the various vehicle components.

Do not be confused by the DC/DC converter, the other blue box in the vehicle above. It is not used to recharge the battery but rather to convert a high DC voltage level (200V to 800V) into a lower DC voltage level (48V to 12V) for auxiliary equipment such as lights, window motors, etc.

In a typical high-power charging (HPC) station, a transformer brings a high AC grid voltage level (20kV) down to a lower AC level (400V), and a charging station converter converts AC to DC power usable directly by the EV.

For someone in charge of designing a fleet charging strategy, it is important to understand the speed of charging that can be achieved and whether it is better to charge with DC or AC. Speed in the electricity sector is achieved by increasing power levels. Let’s have a look at how they vary in EVs as well as charging stations and their cost implications.

Power and Price Levels

Onboard chargers have varying power levels from 1.8 kW (USA), 2.4, 3.7, 7.4, 11, 22 to 44 kW. (Note: The USA has a lower power level due to its 120V household voltage, whereas Europe has 230V.) Most EVs contain onboard chargers with a maximum of 7 and 11 kW. The Renault Zoe was one of the few that had a 43 kW AC onboard charger. Tesla typically offers 22 kW onboard chargers.

The same vehicle model can have different onboard chargers depending on the chosen battery size. This applies to both AC and DC charging. For example, a small VW ID.3 battery has only a 7 kW AC and 50 kW DC charging capacity, whereas the larger versions offer 11 kW AC and 100 kW DC.

Higher onboard charger power levels come with higher costs. Prices start at a few hundred euros for the lowest power levels and can exceed 2000 euros. These figures represent only the cost of the charger, not the charging pole, but they help explain why EV OEMs often choose lower power levels to save costs. AC charging stations with Type 2 plugs are priced between 500 to 1500 euros, depending on power levels up to 22 kW and additional features like load management, apps, or connectivity. Less common 44 kW stations can cost over 3000 euros.

Within DC charging, power levels vary, starting at low power 24 kW, with common levels being 50, 150, and 350 kW. Note that while some charging stations can offer 350 kW, most vehicles today cannot yet charge at that power level, typically maxing out around 120 kW. DC charging station prices for 24 kW models start at 10,000 euros.

CharIN, the organization responsible for the most common CCS standard, explains how it intends to achieve different power levels in the future, including 8 kW low power charging, in its position paper.

Bidirectional Charging

Another term often discussed is bidirectional charging or V2G (Vehicle-to-Grid). This feature would allow the battery to earn extra revenue and help stabilize the power grid. But what is the relationship between bidirectional charging and the onboard charger?

As the name suggests, the onboard charger in most vehicles today can only charge the battery and not draw energy from it. Currently, except for the Hyundai Ioniq 5, no other models can discharge the battery with AC. VW's big announcement at its Power Day earlier this year was that its EVs will have bidirectional charging capabilities starting in 2022, and we expect most EVs to have this functionality. For DC Chademo charging, discharging has been possible for a few years already.

Discussion: Systemic Point of View

How does the onboard charger affect efficient charging from a systemic point of view, and how should this be applied in practice?

Efficient charging means that the EV or its battery is charged and discharged in a way that benefits grid stability, is synchronized with renewable power production, and achieves lower costs. The key parameters are the available power that can be drawn from the vehicles and at what cost. Ideally, we would have very high power available all the time at minimal costs.

As vehicles draw energy from charging poles, even if the onboard charger has a high nominal power, using a low power charging pole will limit the charging speed to the lowest system power. For example, if the vehicle can charge at 22 kW, but the AC charging pole can only deliver 11 kW, the maximum power will be limited to 11 kW—in other words, the weakest link determines the speed. Therefore, it's important not to undersize or oversize charging poles based on the vehicle’s onboard charger.

Knowing the driven kilometers and the energy drawn from the battery will allow us to calculate the time needed to recharge the battery. This information is crucial for creating an efficient charging and discharging schedule.

Should AC (Onboard Charging) or DC Charging Be Preferred?

For installing charging infrastructure at a private company site, DC is an order of magnitude more expensive from a capital expenditure (capex) point of view because it involves more power electronics in the charging station. For charging at public stations, the rule of thumb is that the higher the power level (i.e., speed of charge), the higher the electricity costs the operator will charge you. Depending on your mobility needs, it might not always be useful to charge as fast as possible.

We see onboard chargers converging towards 7.4 and 11 kW, with fewer models offering 22 kW. So, a safe bet is to install 22 kW AC charging stations, but 11 kW will probably cover most of your needs. Additionally, try to choose vehicles with the highest onboard charger power available, ideally 22 kW, but not less than 7 kW. If you have specific needs to charge much faster at certain times, it might be worth considering higher power DC charging stations, but not AC, given that most vehicles will not be able to exceed 11 kW. For these high power needs, the site's connection point power becomes a crucial factor, but this will be covered in another article.

If you have a project you’d like us to help with to make charging as efficient as possible, feel free to reach out to us at RiDERgy.