Ask the Experts: What’s the difference between Volts, Amps and Kilowatt-hours?

We turn to Steve Large, technical director of electric vehicle charging solutions company POD Point, for expert advice in our latest ‘Ask the Experts’ series.

By Will Dron on February 3, 2012 12:43 PM

@Ecocars1 sent us this via Twitter:

Q: “As a newbie to EVs I'm still confused on all volts / amps / kwhs etc and cables for EVs needed – they all seem to be different.”

Good question, and one that must be troubling many electric vehicle beginners. Steve answers...

“This is a very interesting question as it demonstrates a key problem with our EV industry,” says Steve. “When any new technology arrives it is often the case that important basic information is not reliably translated into a format consumers can follow.

“Let’s take a fuel station as an example. We are all pretty comfortable with fact you get the choice between diesel, unleaded and super unleaded but how many of us know the fundamental differences between these types of fuel? Most of us just know that we need to make sure we don’t mix the diesel with the unleaded and as long as we get the right one of these two our vehicle will carry on getting us from A to B.

“My point is that in reality most people don’t need to know about the volts, amps and kwh to buy an electric car. Many of the UK and European charging trials are finding that once people have a had an EV for a few weeks, they settle into a charging routine that matches their usage.

“So, in my opinion the honest answer to the prospective EV driver is to forget about the volts, amps and kWhs. You will take a few weeks to find your charging routine and then you’ll never look back.”

And now, for the boffins…

“However, with new technology it’s also understandable that people would like to understand some of the detail, so I will attempt to explain a few things for the more technically minded.”

Volts and Amps

“Volts and Amps (amperes) are engineering measurements used to define characteristics of an electricity supply. These measurements of a supply allow us to calculate the power a supply can deliver. The power is the key item of interest as this tells us how quickly we can charge an EV’s battery pack up.

“The difference between voltage and current (amps) can be understood if we look at them like water in a pipe (often referred to as the water flow analogy). Voltage is like the pressure of the water, while amps (the current) is the volume of water that flows past a fixed point in a fixed amount of time. The flow rate is determined by the width of the pipe and the pressure.”

Kilowatts and kilowatt-hours (kWh)

“Power is measured in kilowatts (kW) and the amount of energy that you use in a fixed period of time is measured in kilowatt hours (kWh). EV battery capacity Is rated in kWh – the 2011 Nissan LEAF has a 24kWh battery, for example, while the Mitsubishi i-MiEV’s is smaller and holds 16kWh.

“Power is calculated by multiplying the voltage with the current, so for example a 230v, 13A supply is capable of providing 2,990 watts of power (or 2.9kW). So the higher the power a charging point can provide (say a 7kW charger versus a 3kW charger), the faster it can charge your car.

“However, I should also point out that most cars have a maximum limit on the amount of power that they can draw, so the fastest rate your car can charge is controlled by how much power it’s on-board battery charger can take.”

Cables

“The cable between the charging point and car needs to be capable of transferring the power the car draws from the charging point. As an analogy, think of a pipe and water flowing down it; the larger the diameter of the pipe, the more water can flow down it. The electrical characteristics of wires are very similar. The more power that you want to pull through a wire, the larger diameter wire you need to make it.

“So a cable that can reliably (and safely) transfer 7kW of power versus a cable that transfers 3kW of power will need to be bigger to cope with the increased power transfer.”

“Today vehicles are typically being provided with the ‘Mode 2’ charging cables which have a standard UK three pin plug. These cables throttle the current provision to the car to 10A (2.3kW), this is a precautionary measure in case a driver plugs in to an unreliable/unknown power source. Some of the PiP regions are now providing ‘mode 3’ cables; these are cables with specific EV connectors fitted at each end capable of supplying 16A (3.6kW). Over the next few years I suspect we’ll see these ‘mode 3’ type cables become common place especially as charging infrastructure is introduced that supports these connectors and with capacity to supply increased power levels.”

Connectors

“This brings us onto connectors (the attachments at the ends of the cables). Connectors, like cables, are only rated for a certain power. So for example the standard UK three pin plug is rated for 2.9kW (13A). If we want to charge a car with a higher rate than 2.9kW, then we will need a different type of connector. This is why there are a variety of other EV connectors that you will have heard of. The benefits of these connectors are that they can handle higher amounts of power transfer and thus give us options to recharge a vehicle faster.”

Further information

I suggest that if you would like to know more on any of these points the follow web resources give good descriptions for further reading:

Connectors:

SAE J1772

IEC 62196

Charging (this article is actually battery specific but gives a good overview into the fundamentals of the charging process):

Battery charging



About Steve Large

Steve is the Technical Director at POD Point, where he is responsible for the hardware and software development for electric vehicle charging infrastructure. In addition to this Steve is actively involved with various industry organisations such as the DfT, IET and SMMT, where he advises on policy, industry developments and product specifications.



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