If you're thinking about energy consumption, the efficiency of the vehicle is likely the first thing that comes to mind. Vehicle window stickers contain a large section detailing fuel economy, emissions, and savings compared to the average vehicle.

Although the window stickers for EVs also include these details, one significant detail is missing: the charging efficiency. 

What is charging efficiency?

Think of it this way. If you go to a gas station and pay for a gallon of gas, you will get exactly one gallon from the pump and that whole gallon will wind up in your gas tank (provided you don’t spill any on your shoe). Unlike gasoline vehicles, EVs do not receive all of the energy that is purchased. You might pay for 100 kWh from the grid or a charging station, but only see 90 kWh hit your battery. This discrepancy is due to charging losses, which is energy that is lost on the way from the outlet (or charger) to your battery. It means that more energy is drawn from the electric source than the battery actually receives. 

Charging Components 

To understand these losses, we must first understand the major components of the charging system in an EV.

1. On-board charger (OBC) - This is used to convert AC power into DC, which is the only type of energy EV batteries can store. The conversion results in heat, which is lost to the environment (waste). Modern EV batteries include liquid-cooling to reduce these losses. The on-board charger is most efficient at higher currents. 
2. Charger and power output - Higher power (rate of energy) generates higher heat, which means higher energy losses. Increasing cable thickness is one way to reduce this waste. 
3. Charging cable - Energy is lost flowing through the cable due to resistance from electrical components. Shorter cables can help reduce these losses. Importantly, the cable must be designed for a particular charging speed (or exceed it) to minimize loss.
4. Battery - Delivered electrical energy is converted into chemical energy in the battery. This conversion produces heat waste. Modern EVs come with thermal management systems to reduce energy loss when the battery is changing temperatures.  

Most of the charging loss happens in the process of converting AC power to DC via the OBC. So, is DC charging significantly more efficient? Simply put - yes, but it is not without drawbacks, namely the reduction of health for the battery with exposure to high currents and the higher cost associated with fast charging. We go into it more below.

Optimization of Charging Efficiency

Voltage

Charging voltage, or the force of electric current, is typically broken into three categories. For a more in-depth review of charging basics, please see our review of charging at home and on the road. 

1. Level 1: 120V - standard home outlet
2. Level 2: 240V - typically used for home appliances (e.g. dryer, oven)
3. Level 3: 400V - commercial/industrial use

Determining the current (I), or the amount of electric charge flowing per second, is based on the voltage of the power source (V) and the internal resistance (R) of the system. To get power in watts (or kilowatts), you multiply the voltage and the amperage.

For example, a typical home outlet contains a 120V source with a 15 amp breaker. Circuit breakers should not be loaded to beyond 80% of their maximum current rating, so in this case, charging at 12A is the limit. To calculate the power, we multiply the current by the voltage, yielding 1.44kW.

This calculation shows that we consume 1.44 kWh of energy each hour, but do we actually add 1.44 kWh of energy to the battery? Not quite.

A study from 2016 revealed for Level 1 charging, efficiency increases as the current increases (Kieldsen, 2016). Using a rate of 8A or 12A resulted in efficiencies of 75% and 80%, respectively. In our previous example, this means you are paying for (and using) 1.44 kWh of energy each hour, but the vehicle is only receiving 1.15 kWh.

What happens when we increase the voltage from 120V to 240V?

A report from 2014 indicated that, on average, Level 2 charging was 89.4% efficient, while efficiency at Level 1 was 83.8% (Sears, 2014). A study from 2017 confirmed this data, indicating an average efficiency of 80% for Level 1, and greater than 90% for Level 2 (Apostolaki-Iosifidou, 2017). In fact, increasing the current further improved Level 2 charging, improving efficiency from ~93% to ~95% [Table 1]. 

Is DC fast charging efficient?

This brings us to DC fast charging. Although far fewer studies exist compared to Level 1 and Level 2 charging, most studies indicate efficiencies above 90%. A 2018 study out of Europe demonstrated 93% efficiency at ambient temperatures (Trentadue, 2018). Similarly, a 2015 study out of Korea demonstrated overall charging efficiency ranging from 85-89% (Genovese, 2015). Additionally, they measured the efficiency of the charger itself, demonstrating 92.6% efficiency at 43 and 50 kW. In contrast, 16 and 22 kW chargers (Level 2) had an efficiency of 91.6-92.2%. The high efficiency of DC fast charging should be no surprise,  considering that it bypasses the on-board charger, which is one of the most inefficient components of the whole charging system.

How does state of charge affect charging efficiency?

If you’ve ever used a fast charger, you’re probably aware that the charge rate drops precipitously as the state of charge (SoC) increases. That is, you add energy more slowly as your battery gets more full. The chart below shows how DC charging slows as your battery fills up, while AC charging remains pretty consistent across most states of charge.

But is there also a relationship between charge efficiency and SoC? 

Yes - several studies indicate that as SoC increases, not only does the rate of charge decrease, the charging efficiency also decreases, which is more pronounced once above 80% SoC. However, this relationship is also dependent on the power of the charger used. 

One study revealed that charging losses doubled between 80-100% SoC when compared to the losses associated with charging from 20-80% SoC (Kostopoulos, 2019). The charging rate used for this study was 22 kW, which lines up well with others studies indicating 85-90% efficiency. Furthermore, when efficiency is examined as a function of power and SoC, we can see that there appears to be a Goldilocks zone for charging, where we supply enough current, but not too much (Genovese, 2015). In other words, too much current can also reduce efficiency. For the cars used in these studies, namely the BMW i3 and the Nissan Leaf, that number is ~43 kW. Further studies are warranted for charge rates above 50 kW, but it’s likely that the battery size and maximum charge rate are key factors when determining optimal charging power.

How does temperature affect charging efficiency?

Just as the efficiency of an EV drops in extreme temperatures, charging efficiency also drops at very low or very high temperatures. When charging at low temperatures, some of the charging energy must go towards warming the battery. These are not power losses per se, but additional power consumption to charge the same amount. One way to improve this is by preconditioning the battery.

One study using a 22kW wall box at 73F indicated 81-89% efficiency (Kostopoulos, 2020). This represents a temperature in which EVs receive the maximum charging power supported by the on-board charger since no battery management system is needed. In this study, the EV was charged from 0% to 100%, so the charging efficiency began quite high (~89%), and dropped as the SoC hit 60% or more. 

Lastly, one study compared charging efficiencies using a 50kW charger at 77F and -13F. They showed that charging efficiency at near ambient temperatures exceeded 90%, but dropped to under 40% in the extreme cold (Trentadue, 2018). Therefore, caution should be used when charging in extreme temperatures, particularly when using fast charging, as the energy waste and costs are high.

Does charger quality matter for charging efficiency?

Regardless of the brand, an EVSE is made of many components, any one of which may be more or less efficient than similar components in a similar charger. So, the “efficiency” of the transfer of energy from the grid all the way to the battery is determined by the components along the way. A typical Level 2 home charger operates in the range of about 83-94% efficiency grid-to-battery depending on the specific charger. Therefore, when choosing an EVSE for your home charging needs, it is important to consider build quality and construction, which typically maximize charging efficiency. 

Conclusions

As EVs continue to gain ground in the transportation sector, it is crucial for us to have an in-depth understanding of the optimal charging procedure, for both increased lifespan of batteries and reduced environmental waste. Excess consumption of electrical energy during charging is both financially wasteful and creates unnecessary load on the grid. 

Currently, it is near impossible for consumers to directly compare the charging efficiency between different EVs. The manufacturer specifies the battery capacity and the drivable range that can be achieved, but the amount of energy needed to fully charge the battery is not included. To this end, a German automobile club known as ADAC has implemented a requirement for EV manufacturers to include charging losses to the general technical information provided. These details will assist consumers with making more environmentally-conscious choices, and also provide insight into the proper charging procedure.

More studies are needed to compare different charging rates across EV platforms, particularly those that can utilize DC fast (50-250kW) and ultra-fast (250-400kW) charging. The industry is heavily pressured to increase range and decrease charging times, both of which can have a significant impact on energy use if not handled appropriately.

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Written by Brandon August, a lifelong explorer of all things academic. After obtaining an undergraduate physics degree and a doctoral degree in biomedical, he began to explore various professional fields in health and wellness, rideshare work, freelance writing, and day trading. On the recreational side, he has always been involved in the automotive field, owning various vehicles across the years. After a recent move to California, he entered the EV space, purchasing both a Chevrolet Bolt EV and a Bolt EUV for his household.