Within every electric vehicle is a battery that outputs a certain voltage.
Given that this voltage might not be appropriate for the various devices being driven, we need to install another device along the signal path.
This device is known as a DC/DC converter, and without it, the modern electric car wouldn’t be possible.
Why is a DC/DC converter necessary?
While DC/DC converters can be based on a number of different designs, the underlying function remains the same. We might be talking about a step-up converter, which turns a low-voltage input into a high-voltage output, or a step-down converter, which does the opposite.
The batteries of an electric vehicle typically output several hundred volts of DC. The electric components inside the vehicle, however, vary in their voltage requirements, with most running on a much lower voltage. This includes the radio, dashboard readouts, air conditioning, and in-built computers and displays.
In electric vehicles which use a DC motor, actually running the motor can use up to three times the voltage provided by the battery. With the help of the right converter, we can bridge this gap without having to resort to a larger, prohibitively-heavy battery.
DC/DC converters constitute an entire subcategory of electrical engineering. There are many different ways to get the job done, all of which are based on the principles of Ohm’s Law.
A simplified step-down converter might work by generating a series of on-off pulses, and then using a combination of inductors and capacitors to smoothen this into a consistent DC signal, whose current is constant and whose voltage is determined by the duty cycle (that’s the duration of ‘on’ states relative to “off” ones).
What’s the difference between AC/DC and DC/DC?
The plugs in your home put out an alternating current, which oscillates between positive and negative potentials relative to ground. In the UK, the wall voltage is around 230V, and the frequency (the rate at which the current alternates) is 50Hz.
In the US, the voltage is half that, and the frequency slightly higher at 60Hz.
Most of the appliances in your home run, not on alternating current, but direct current. And thus, they’ll ship with a special box called a rectifier.
This device functions in much the same way as the step-down DC-DC converter, except that it runs the incoming signal through a series of diodes which clips the negative portion of the signal.
The resulting positive wave can then be smoothed into the desired voltage using a combination of inductors, capacitors and resistors.
Electric vs traditional car converters
The motor used in an electric car is considerably different from the starter motor found on a traditional internal-combustion engine. They require a mere 12 volts – which is comfortably provided by the car’s battery.
However, we can install an inverter into a car and thereby run electrical appliances with a mains power connection. This device mimics the sine-wave of an AC outlet and allows passengers to run televisions, Blu-ray players and game systems from within the car, just as if they were plugging into a wall outlet.
There are a few reasons to be cautious of running an inverter. If used when the engine isn’t running, they may deplete the battery very quickly. Moreover, they may place excess stress on the vehicle’s alternator.
Many modern electrical devices can be charged over a 5v USB connection, and the remainder can be charged over the cigarette lighter with an appropriate adaptor. As such, the investment in an inverter is difficult to justify for most motorists.