EV charging 107: Smart Charging

Smart charging is all about controlling the time and rate of charging

Introduction

Now that we have seen how different charging technologies work, the next thing to talk about is how to make charging smart. As I think of it, charging is a smart process in itself. The Battery Management System or BMS is the first smart person in the chain. We can call BMS as smart because it ensures that the charging happens based on the battery and the state at which battery is. That is if the battery is getting overheated during charging BMS will stop the charging. BMS will also control the rate of charging, a typical battery charging process will look like the curve in Figure 1. During the first phase of charging the current drawn will be high and then it subsequently reduces. This happens in two phases, one called a constant current (CC) phase when there is a high current and constant voltage (CV) phase when there is a lower current. The take away from this for us is that the charging current can change over the time of charging, and BMS will ensure that battery is protected. That is, it is possible to control the charging.

Figure 1: Illustration of battery charging (Picture Courtesy: lithium-batterycharger.com)

Smart Charging in Vehicle Grid Integration

Smart charging is all about controlling the time and rate of charging. When we talk about EV smart charging in general, we are not talking about the BMS. We are talking about how EV can be smartly connected to the grid. We all know that EV and its batteries are a very distinct kind of load, which can be controlled. That means we don't need to treat EV like any other appliance we have at home. That is the charging of an EV need not be equated to the way we operate our AC, or microwave, or TV currently. We should treat EVs as amazing equipment, which can be switched on and off remotely. We already saw that because of BMS we can also control the current it is taking, and thereby the power it is consuming at a particular time. We can club all of this under an umbrella called V1G.

Stretch your imagination a little and you can see that we can also treat EV as a power source to power your homes, or supply power back to the grid itself. Mind that both of these things are not easy. Yet, the first part of powering your home called V2H is much easier than powering the grid i.e. V2G. As vehicle grid integration is a complex subject in itself, perhaps I can write another article about it. The figure below will help in understanding these concepts.

Figure 2: Representation of Vehicle Grid Integration (picture courtesy: smartcharge.eu)

Language is the key to smartnes

What is it that you need to achieve this? Your EV is smart in itself, you need the charger, aka EVSE to be smart. Once your charger is smart you need to ensure that your charger can talk. There is a minimum of two entities that the charger needs to talk to achieve smart charging. One is the EV, and the second partner in this smart charging game is the often the electricity grid, where the electricity is coming from. It is also possible to do smart charging without involving the grid, which I am illustrating in an example in the subsequent section.

Lucky for us, these languages for electronic devices to talk are available and they are called communication protocols. I have already talked in the previous articles about the common languages for EV and EVSE to talk, these are PLC and CAN. Talking to the grid is the trickier part, though there protocols that help EV talk to the Grid. This part is called the backend of charging and is a complex ecosystem bound by a long list of protocols including OCPP, OSCP and Open ADR. Just to give you an idea of how complex the backend is I am adding Figure 3. If you want to know more, I can write about this in detail in upcoming articles.

Figure 3: Snapshot of protocols associated with backend of charging (Picture courtesy: V2G clarity.com)

How smart do you want to be?

The level of smartness is something we need to comprehend well. Once we are quite clear that dumb charging is not acceptable, we need to match the requirements with costs. The simpler thumb rule is the smarter you want to be, the costlier it is going to get. The simplest form of control is ON/OFF control, which is like a remote switch.

The next logical step is to reduce the charging rate. There are numerous possibilities within this step, which constitutes multiple strategies you can use for smart charging. One strategy is explained with an example below

Coordinated charging:

Imagine charging of electric cars in a parking lot of a mall, where there are many chargers. The mall wants to provide charging, but they also don't want to make an additional investment to upgrade the building electrical system to provide charging. Naturally, they also don't want a very high electricity bill due to demand charges from the charging. So they decide they want to restrict the maximum charging power to 22 kW.

When the first car comes in being the first it can get charged at 22 kW. When the next car comes to charge, both of them need to share this 22 kW. Easiest share is a divide in half, that is each car gets 11 kW each. But the most suited one would be to see which car has the most depleted battery. It makes logical sense for that car to get higher power. We already saw in Figure 1, as batteries get charged their current requirement reduces also.

That is if the first car has a state of charge of 60% and the second car has a state of charge of 40% when they arrive, the best-suited share here is that the first car gets 8.8 kW and the second car gets 13.2 kW. The second consideration here would be how long the car is parked because that determines the time available for charging. In this case, the car which would be there for the shortest time should get charged faster and should get higher power.

As the number of cars increase, and we factor in other parameters, the smart charging problem gets complex. It is impossible to do this without algorithms and controllers. But this is a charging that was achieved without the EVSE talking to the grid but with control within the mall itself. This type of smart charging is called User Managed Charging. A useful video to explain this is below

What should smart charging do ?

It is important to determine the goals of smart charging. There are two important goals for smart charging in India in my opinion. They are peak management and ensuring green charging.

Peak Management

EV charging is a problem if the charging demand will arise when the electricity grid is experiencing a peak load. This causes problems in the operation of the grid. Just like in the case of the mall, the grid operator also might not want to invest to upgrade the network to meet the EV charging load. The simplest solution is to ensure that charging happens when the grid is experiencing low demand. So the smart charging strategy needed here is to shift the charging when there is a low load and stop the charging at peak.

Up to an extent, this is achievable with price signals alone, i.e. increase the cost of power during peak, and reduce the cost of power at minimum load. This strategy can be achieved with Time of Use rates and is a passive charging control strategy. But when the number of EVs increase, we need an active charging control strategy, wherein guaranteed results can be brought in. To achieve this we might need aggregators and demand response. This is a very interesting area of research for me, and we will definitely talk about this in detail in other articles. A simple illustration of this is in Figure 4.

Figure 4: Smart charging for peak management (picture courtesy: Flickr)

Green charging

The last thing for this article, which is also perhaps of utmost importance to India, is to ensure that we are not using coal based electricity to charge our electric vehicles. This kind of defeats the purpose of using electric vehicles to fight emissions and air pollution. So we need to charge our EVs when the sun is shining. As shown in figure 5, in the case of India, this means moving our charging to day time when we have a lot of solar power, than follow the trends in other countries where most of the charging happens at night.

As the Solar generation is variable, for the best possible solution charging has to follow the generation. Of course, there are many intermediate solutions in this case which include reducing cost of power during excess solar generation, adding battery storage to the system, or using the grid as storage, feeding excess solar energy to the grid with net metering, etc. Just like in the case of peak management there is only a limit up to which cost incentive will work to ensure green charging. We will also cover Solar EV charging in detail in upcoming articles.

I am ending this one with a 'food for thought'. When planning for a future grid with highly variable renewables, EVs can be an asset for grid management also. In that way, EV can actually help in addition of more and more Solar to the grid. Future looks a little technically challenging but bright, ain't it?

Link to other articles in the series

Link to Part 1: EV Charging 101

Link to Part 2: EV Charging 102

Link to Part 3: EV charging 103

Link to Part 4: EV Charging 104

Link to Part 5: EV Charging 105

Link to Part 6: EV Charging 106

Note: I am an electrical and electronics engineer and not an expert in EV or batteries. However, my current area of research is EV charging, and my auto-didactic tendencies are lately on overdrive with my focused reading. The article is a summary of my notes from multiple sources. A fellow EV enthusiast may find it useful, though caution is advised. As this article intended to help me in improving my writing skills, please do let me know how I am doing in the comments section.