The hidden importance of onboard chargers for EVs

11 July 2021

One lesser known component of electric vehicles (EVs) is the onboard charger. It does not even have its own Wikipedia page, being 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.

Onboard charger, courtesy of https://visic-tech.com/
Onboard charger, courtesy of https://visic-tech.com/

Context

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

The power grid works at different AC voltage levels. Transformers are used to transform high voltage into lower voltage or vice versa. Converters are used to convert AC into DC and vice versa as well as DC into DC from one voltage level to the next. Not all converters can convert both ways. (EV) batteries work on DC. So, to recharge the battery the AC grid power needs to be converted into DC at some point. This is either done in the vehicle or at the charging station. The onboard charger, in the diagram located in the blue box on the right, is used to convert AC grid power into battery usable DC power. This type of charging is known as AC charging. When AC is converted within a charging station it is known as DC charging.

The diagram below shows the different vehicle components.

EV Components, illustration by Joshua Bauer, NREL 64702.
EV Components, illustration by Joshua Bauer, NREL 64702.

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 (48 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) into a lower AC level (400V), a charging station converter converts AC to DC power usable by the EV directly.

For someone in charge of designing a fleet charging strategy, it is important to understand the speed of charging that can be achieved as well as if 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 USA has a lower power level as it has 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 having a 43kW AC onboard charger. Tesla has typically 22 kW.

The same vehicle model can have different onboard chargers depending on the chosen battery size. This is valid both for AC as for DC, e.g. a small VW ID 3 battery has only a 7kW AC and 50 kW DC charging possibility whereas the bigger ones 11 kW and 100 kW.

With a higher onboard charger power come higher costs. Prices start at a few hundred euros for the lowest power levels and can go above 2000 euros. This is not the price of the charging pole but just useful to understand why EV OEMs choose to save costs by choosing lower power levels. 

AC charging stations with type 2 plugs have prices of 500 to 1500 euros depending on power levels up to 22kW and additional features like a type of load management, an app or connectivity. For less common 44kW stations, prices can go beyond 3000 euros.

Within DC charging there are different power levels. Starting at low power 24 kW, to the common 50, 150 and 350 kW. Note that charging stations can do 350 kW but vehicles do not yet have the ability to charge at that power level. Most vehicles achieve around 120kW maximum today. DC Prices for 24 kW charging stations 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 among others 8 kW low power charging in its position paper. 

Another term often discussed is bidirectional charging or V2G. This feature would allow the battery to earn extra revenue and help keep the power grid stable. What is the relationship between bidirectional charging and the onboard charger?

Bidirectional charging

As the name suggests the onboard charger can only charge the battery and not yet draw energy from it in most vehicles today. Currently, except the Hyundai Ioniq 5 no other models can discharge the battery with AC to our knowledge. VW's big announcement at its Power Day earlier this year was that its EVs will have bidirectional charging capabilities starting in 2022. 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 used in practice?

Efficient charging means that the EV or its battery is charged and discharged to benefit grid stability, in sync with renewable power production and achieving lower costs. For this the key parameters are the available power that can be drawn from the vehicles, at what costs. Ideally, we would have a 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, by using a low power charging pole, it will only charge at the lowest system power. For example, if the vehicle can charge at 22 kW, as most AC charging poles can do 11kW, the maximal power is limited to 11kW, in other words by the weakest link. So, it is important not to undersize or oversize charging poles depending on the vehicle’s onboard charger.

Knowing the driven kilometers, with that the energy drawn from the battery, will allow us to know how much time is needed to get the battery charged again. This is crucial information to then create an efficient charging and discharging schedule.

Should AC aka 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 capex point of view as it contains more power electronics in the charging station. 

For charging at public stations, the rule of thumb is the higher the power level ie speed of charge the higher the electricity costs the operator will charge you. Depending on your mobility needs it might be useful not to charge as fast as possible.

We see onboard chargers converging towards 7.4 and 11kW and fewer having 22 kW. So, a safe bet is to install 22 kW AC charging stations but 11 kW will probably be able to cover your needs. Also, try to get vehicles with the highest onboard charger power, 22kW, otherwise 11kW, but not less than 7 kW. If you have specific needs to charge much faster at some points in time, it might be useful to think about higher power DC charging stations but not AC, given that most vehicles will not be able to give more that 11 kW. For these high power needs, the site’s connection point power becomes a crucial variable. 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.

If you want to understand the history behind onboard chargers and AC charging stations, this article in German is a great reference.