How to get household current in your car

How to get household current in your car


Car inverter and laptop charger

Brian Cooley / Roadshow

If you want to power a laptop or other large electrical device in your car, you need an inverter that converts your car’s 12 volt direct current into 120 volts alternating current like you have in your home . Here’s how they work and what to look for when you buy one.


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Put a household plug in your car


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What a car inverter does

The name “inverter” doesn’t mean much to you. This should be called a “polarity pinball and a voltage amplifier”, but fortunately it is not. Simply put, a car inverter takes 12 volts DC electricity from your car and alternates its polarity. Imagine if you took the red and black cables from your car battery and quickly switched them back and forth: you would create a raw form of alternating current that looks like this:

Sine wave vs square wave

True AC sine wave power found in a household outlet looks like the smooth waveform at the tip. Quickly reversing the polarity of a DC power source, such as a car battery, results in the raw AC power of the square wave shown below.

Brian Cooley / Roadshow

An inverter takes this raw AC current and breaks it down into steps to more closely resemble the sinusoidal AC current coming out of a household outlet, while also increasing the voltage from 12 to 120. How elegantly does the inverter make great part of its cost.

Modified wave AC power

A DC inverter gradually increases the electric power of your car from top to bottom while regularly reversing its polarity. The result, shown here in purple, begins to closely resemble the alternating current you get at home.

Brian Cooley / Roadshow

How to buy an inverter for your car

When you buy an inverter to power the electronics of your car, add the power of all the devices that you will connect to it at the same time and buy an inverter 20% higher than this power to be sure. If you’re going to be running things that have motors or ballasts, like power tools or fluorescent lights, you have to buy a much bigger inverter to account for the high power these devices need at startup or during pulses during their operation.

Size table of DC inverters

This size chart shows that some devices require an inverter with a much higher power “cushion” than what you need for a laptop or other small electronic device.

Samlex

You will also need to decide on the quality of a sine wave you need. A simpler and cheaper inverter will provide “modified sine wave” power that is acceptable for simpler electrical devices, such as an electric drill or a light bulb.

Amazon

Modified square wave power at an affordable price.

For laptops and other sophisticated electronic devices, I recommend a “pure sine wave” inverter that emits alternating current that is almost indistinguishable from the power supply you have at home. Such an inverter can cost much more, but ensures that your delicate electronics work and last as expected.

Amazon

An inverter that faithfully reproduces the sine wave found in the household outlet and is designed to power devices up to a combined total of 300 watts.

Finally, decide how you are going to connect an inverter to your car. Small inverters that plug into a 12-volt “cigarette” outlet are limited to the rating of the fuse in that outlet. For example, a 12-volt outlet protected by a 15-amp fuse can only support 200 watts of power from an inverter, regardless of the size of an inverter you are connecting to. To determine this calculation for your car:

  • Look for the fuse size for taking 12 volts in your car by inspecting it or consulting the owner’s manual
  • Determine the voltage you will use: Your car will supply 12 volts when it is off, 13.8 volts when it is running
  • Note what power you want in an inverter, which is its nominal capacity

Plug this information into one of the different online watt / amp calculators and see if the resulting number of amps is equal to or less than the nominal value of the car’s 12-volt output fuse. If the calculation goes above the nominal value of the fuse, you can still use the inverter, but not at its maximum nominal power. To do this, you will need to find a more robust way to connect the inverter to your car, such as connecting it directly to the battery. Many inverters also include a connection cable with battery clips to temporarily establish a higher current connection when the car is stopped.

DC inverter cables

The battery clip cables on the left allow this 300 watt UPS to operate at full capacity. But feeding it via the practical 12-volt output cable on the right would limit its power, depending on the size of the 12-volt output fuse in the car.

Brian Cooley / Roadshow

With an inverter in your car, you get a decent preparation margin. You’ll know you can power a wide range of electrical devices, no matter where unexpected plans or emergencies take you.

First published April 7.



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