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The Challenges of AC and DC Charging May Be Slowing EV Adoption

October 21, 2020 by Steve Arar

Some of the roadblocks to EV adoption lies at the core of AC and DC charging technology.

We recently discussed how one of the main problems with widespread EV adoption is the "charging deserts" in city infrastructures. While many companies are working around this shortage with solutions like pop-up charging hubs and charging robots, some of the challenges of EV adoption exist in AC and DC charging at its core. 

In this article, we’ll look at AC and DC charging techniques and examine some of the technological challenges we face when designing a high-power charger.

 

The Charger Power Capacity 

Two different technologies for recharging an EV are demonstrated in the following figure.

 

AC and DC charging differences

AC and DC charging differences. Image used courtesy of Texas Instruments
 

The AC charger has limited power (less than about 22 kW) and needs a longer time period to fully charge the car. The charging time can reach up to about 12 hours depending on the power level of the charger and the battery characteristics. As shown in the figure, AC charging relies on onboard circuitry to create a high DC voltage from the commonly-available AC sources. 

A DC charger, on the other hand, uses off-board circuitry to generate a high DC voltage (300–700 V) that is directly applied to the vehicle’s battery management system (BMS). The power level of a DC charger can range from about 25 kW to 350 kW. This significantly reduces the charging time.

Note that the power capacity of a charger determines the rate at which energy is delivered to the battery and consequently the time it takes to charge the battery. The following table compares the charging time of AC (level 1 and 2) and DC chargers for a given battery.

 

Comparison of AC and DC charging times

Comparison of AC and DC charging times. Image used courtesy of Texas Instruments
 

Grid Peak Power Limitation

Feeding multiple high-power charging piles directly from the electric grid can increase the peak power delivered by the grid to an unacceptable level. An article from Analog Devices on energy storage systems that can boost EV's fast-charger infrastructure explains that when you simultaneously charge five typical EVs in 15 minutes, you can increase the peak power delivered by the grid to more than 1 MW.

 

Infrastructure integrating renewables, EV charging, and energy storage

Infrastructure integrating renewables, EV charging, and energy storage. Image used courtesy of Analog Devices

 

The grid should deliver this power for 15 minutes. With this in mind, city planners might invest in improving the grid so that it can provide high power levels. But instead of investing in the grid infrastructure itself, developers can use the power that is locally generated from renewable sources such as solar and wind. This will reduce the peak power that the grid should provide.

Unfortunately, the energy generated by renewable sources is intermittent. Hence, we need energy storage systems that act like large batteries to store the locally-generated energy and use it when charging EVs. 

 

Managing the Generated Heat

Several power conversion systems are required to implement the charging station that is depicted above. Considering the high power levels that we are dealing with, we have to reduce losses as much as possible. For example, with a 350-kW charger, a 1% loss in efficiency is equivalent to 3.5 kW of power dissipation.

This power dissipates as heat and increases the system temperature. Without an efficient heat management mechanism, the generated heat can damage the system. That’s why we need to design the system using highly-efficient components. For example, using SiC MOSFETs rather than silicon IGBTs can considerably increase the system efficiency.

In addition, we will probably need liquid cooling to manage the heat generated by the cables, connectors, and circuitry.

 

The Need for a Reliable Battery Management System  

When we discuss a unified infrastructure that integrates renewables, energy storage, and EV charging, we need accurate information about the battery state of charge (SOC) and state of health (SOH). This information allows us to increase the battery lifetime by about 30% by avoiding battery overcharging or overdischarging.

The importance of this lifetime improvement becomes apparent when we note that almost half of the system's overall cost is related to the battery. 

 

Conclusion

Without an efficient charging infrastructure, EV uptake may be a slow-rolling process. Depending on the city structure, either home charging or public charging could become the desired charging solution. With fast chargers, we face several technical challenges such as the grid peak power limitation, heat management, and the need for a reliable battery management system.  

7 Comments
  • P
    pvsheridan October 23, 2020

    Appreciate the article, and appreciate that you implicitly admit that so-called “renewables” cannot possibly energize electrification of the ground transportation fleet…no even close.

    Like. Reply
  • P
    pvsheridan October 23, 2020

    You’re censoring this post?  Why?  Let’s try again:  Appreciate the article, and appreciate that you implicitly admit that so-called “renewables” cannot possibly energize electrification of the ground transportation fleet…not even close.

    Like. Reply
  • M
    MisterBill2 October 23, 2020

    With a charging rate of 300KW the required infrastructure ti support a place like the one where I buy my gasoline would be interesting, since there are 8 lines leading to sets of two pumps each line. And many days there are lines of cars at all 8 places. So that would equate to 16 cars charging at a time.  JUST THINK, 16 X 300 kw IS A LOT!!! And there is far less than a minute to refill any energy storage system between cars, so the benefit of storing energy between customers is not present. And there is another similar station about a mile away.
    So how is that much power delivery arrangement going to magically arrive? Our municipal government will certainly not pay for it, and the capital expense will be far more than the costof putting tanks into the ground, , and the generating system to produce that much power will be rather large and expensive.
    Now do the math and see how big any renewable source of that much power will be.  This is why I am a skeptic.

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    • M
      MisterBill2 October 23, 2020
      Local power generation does not make sense, given the amount of power that quick recharging takes. The numbers are brutal at crushing dreams, that is the pain that those afflicted with seeing reality suffer frequently. It is a pain that those who are not aware of reality never feel.
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  • J
    jculp October 23, 2020

    Today, the gas stations are busy because no one has a gasoline dispenser at home or at work.  Most suburbanites will do 90% of their charging at home or work.  City dwellers usually drive fewer miles than suburbanites.  The only challenge that I see for total EV adoption is the high demand for fast-charging on holidays along the highway corridors.

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  • B
    bobwareham October 27, 2020

    Why don’t all EV have solar built into the roof so they are constant trickle charging the battery as soon as it becomes daylight the charger would start working and off at dusk and the cost would be minimal or a flexible panel fitted onto the roof?

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    • C
      chrisbaron October 27, 2020
      Nissan Leaf battery is 62kWh. Solar radiation is ~1kW per sq. m peak. At 25% solar panel efficiency that is 250W per sq. m. Say 4sqm per roof (no sun roof) or 1kW. So 62kWh would take 62hrs to charge fully or ~2.5 24hr days but since it is dark roughly half each day we are looking at 5 days to charge the battery once. But of course the sun is not at maximum location all the time so say double for the sun moving. So 10 days to charge your battery. 1 full 24 hr day will only be able to charge about 10% of your battery. That's why they don't put solar panels on the roofs of electric cars. Physics is an unforgiving bitch. Sorry.
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