EV Charging 104

What is DC charging?

Remember all batteries are DC. Hence technically all battery charging is DC charging. To make it more clear, we can call it as AC/DC charging, implying you have to convert the grid power into DC at a suitable Voltage and Current that the battery can accept. If you have been reading my previous articles, you know by now that the Voltage will be decided by the battery pack Voltage, and speed of charge will depend on Current.

However, in the case of electric vehicle batteries, the cases where DC directly flows into the vehicles is called DC charging. Two ways to do that is using an off board charger or on board converter. So the answer to the question above is rather clear. Still, its worth a shot to look into the EV drive train and experience the distinction between the charger and converter.

On-board Converter

First the on-board converter. It is part of the drive train, and this is what connects the battery with rest of the components. EV specialists correct me if I am wrong, the way I understand it, in few design cases, the output from the AC/DC rectifier which is part of charger is also fed into the converter. Its important to remember that the On-board converter is bi-directional, which implies it can transfer power both ways.

Also remember that the on-board converter is typically a high power equipment as it is designed to provide the energy needed for the electric motor which helps the vehicle accelerate and drive. This part is important as it provides a pathway to charge the battery at a very fast rate. The on-board charger for the Mahindra e-Verito is 1 kW, where as the electric motor is rated 31 kW. So the on-board converter, which is designed for the motor, and should be able to supply 31 kW. Think of it, if the car were to be charged using the on-board converter it is possible to charge 30 times faster than the on-board charger. That's not to say that its the best practice, but that is the fastest way to charge electric vehicles now. It also opens a possibility to send energy in the vehicle back to the grid (V2G). But remember grid is AC and the DC power needs to be converted to proper sinusoidal AC before feeding in, just as in the case of a Solar Inverter.

This charging is called Flash Charging. Quoting my favourite example for buses, the TOSA project in Geneva, where 600 kW chargers charge batteries for a few seconds, and 400 kW chargers charge the batteries for a few minutes using the on-board converters. The best part about this charging is that is devoid of complex communication protocols between the charger and electric vehicle, which is needed for the off-board DC charger.

Off-board charger

The off board charger has three main functional parts:

• the rectifier to convert AC to DC
• the DC to DC converter
• the variable DC supply

As you are familiar with the first two parts, we could talk a bit about variable DC supply. This is because the battery pack Voltage will not be same for all vehicles and to assure the ability to charge any vehicle, there needs to be voltage adjustment. Also Voltage needs to change to fit with the charging mode- Constant Current or Constant Voltage. We will talk about more about that later.

But we need to talk now about how the off-board charger and the battery will talk to each other. The battery needs to tell the charger what Voltage and Current it needs, and charger needs to tell the battery what it can provide. They need distinct languages for communication- CAN and PLC. CAN or Controller Area Network is very popular language for vehicles. The other one Power Line Communication is equally popular for talking through power lines. The simplest different between the two would be that in the case of PLC there is a pulse width modulated waveform, where has CAN has two waveforms corresponding to high and low levels.

So you have two languages , and the most used communication protocols for DC charging follows these languages. The CHAdeMO or CHArge de MOve uses CAN, where as CCS or combined charged system uses PLC. The third most common protocol the Chinese GB/T also uses CAN protocol. Other than the language the most distinguishing factor about these protocols is their connectors.

Whats in the connectors ?

The connectors have big fat pins which help in power transfer and small pins which help in communication. As we have to transfer DC we need only two pins: one for positive and one for negative to do the power transfer. All the connector types have two big fat pins as you can see.

Just remember that CCS, as the moniker goes, is designed such that the same pin can be used for both AC and DC. There are two types of pins one for US and other for Europe, which differs on the design of the upper part where the AC and communication pins are. The lower part has the DC pins, which is same for both.

Proximity and control pin

Apart from the power pins the all the connectors have something called proximity pin. The purpose of the proximity pin is to ensure that the vehicle does not move while it is charging. This pin monitors the connection between car and charger.

The CCS pins also have control pin, which actually controls the charging rate. The control pilot sets the actual charging current and thus controls the speed of charging. In the case of CHAdeMO and GB/T pins there are CAN-H and CAN-L pins to perform this function. CHAdeMO also has additional pins to start and stop charging.

The Tesla Connector

The Tesla connectors are unique in design. The same connector is used for AC and DC charging. I am not entirely sure of the communication protocol used by Tesla. However, its appears to me that it could be similar to PLC communication. Tesla enthusiasts among my readers, please do share if you know more.

Final thoughts

It 's also possible to charge batteries from a DC source such as Solar or another battery. This battery could be inside another electric vehicle also. In fact I read in news of a Hyundai Kona being charged by another Kona. If you find it interesting I will write another article on the DC/DC charging. Or else as planned we will proceed to dissect CCS and CHAdeMO in the next one. Let me know what you think.

Link to Part 1: EV Charging 101

Link to Part 2: EV Charging 102

Link to Part 3: EV charging 103

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 an 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.