This post is a continuation of our short series going over the basics of electricity. If you haven’t seen it yet, been sure to check out out first post on current, voltage, and resistance. This latest post will run through the basics of power and its relevance to EV charging.
What is power?
Power is the rate of transfer of energy. This could be any type of energy – for example, the power of an appliance such as your kettle is the rate at which it transfers heat to the water inside, or the power of your car is the rate at which it transfers energy to the wheels (this is the “horsepower” of your car).
Simply put, the bigger the power, the faster energy is transferred.
This concepts of power and energy are a fundamental part of physics. The first law of thermodynamics states that “energy can be transformed from one form to another, but cannot be created or destroyed”. In practice, transferring energy is never 100% efficient, and some amount of energy will always be ‘lost’ – generally by being turned into heat. This energy is not destroyed, however it is turned into a form that isn’t of very much use to us.
As electric car charger installers, the type of energy we are most interested in is electrical energy. This is transmitted to your house via the National Grid, and can be stored in batteries such as the lithium ion cells in your electric car.
The rate of transfer of electrical energy is called power and is determined by multiplying the current by the voltage.
P=IV
Power is given the mathematical symbol P and is measured in watts (W).
So, for example if you have a circuit delivering a current of 5A at the UK domestic supply voltage of 230V, then the power is 1150W. That’s roughly the amount of power that a normal kettle or toaster might use.
SI prefixes
You will often see units expressed differently from the way that they are presented in this series. For example, it is very common to see power expressed in kilowatts (kW) in the EV world – the ‘kilo’ part is called an SI prefix, however it really just means “multiply by 1000,” therefore, 1kW=1000W. A more complete list of these prefixes is expressed below.
- kilo (k) – multiply by 1000 (one thousand)
- mega (M) – multiply by 1,000,000 (one million)
- giga (G) – multiply by 1,000,000,000 (one billion)
- milli (m) – divide by 1000 (one thousand)
- micro (μ) – divide by 1,000,000 (one million)
- nano (n) – divide by 1,000,000,000 (one billion)
What is energy?
Scientfically, energy is the ability to do “work”… Although this isn’t a very useful definition to most people, so it’s usually better to look at an example.
For example, electric car batteries store energy which can then be used to drive the motor and wheels of the vehicle (this is called doing “work”).
To determine the amount of the energy transferred to the battery, you can simply multiply the power of the source (in kW) by the time (in hours) that the source is connected. Using this method gives the energy in kilowatt-hours (kWh).
E=Pt
Charging your car in the real world
In the real world, charging power is not constant over time. As your battery (or indeed any battery) charges up, it becomes increasingly more difficult to charge it up further. Therefore, the charging power decreases as time goes on. A graph of this is shown in Fig. 1 below.
In this case, the energy that is transferred to your battery is the area under this graph. As you can see, early on in the charging process, each additional minute of charging results in a big increase in the area under the graph (and therefore a big increase in the energy stored in your battery). However, later on in the charging process, the area under the graph increases only a little bit for each additional minute of charging, corresponding to a small increase in the energy stored in your battery.
For this reason, it will seem as though your car charges up very very quickly when you first plug it in; however, the last 20 or 30 percent will take a longer time to accrue. This is part of the reason why you often see charging times quoted as “20-80%” or something similar to this. There are diminishing returns when fully charging the battery, so when you are away from home it often make more sense just to top up to around 80% – provided this gives you enough energy to complete your journey.
Charging your battery to full capacity regularly is discouraged as it can degrade the battery and reduce its performace, so charging up to around 80% is often a good policy. Many EVs have charge limiters built in for this exact purpose.
Hopefully that gives you some more insight into the physics of electric car charging. If you have any other questions about electric vehicle charging, please feel free to contact us and we will do our best to help you out.
