Power A Laptop in a Car

Run a Portable Computer off the Automobile Battery on Long Trips

Aug 8, 2008

Laptop batteries only give a few hours of running time before needing a recharge from AC mains power. Here are options for when more running time is needed in a vehicle.

Most vehicles provide a cigarette lighter socket, supplying 12V (volts) at around 8 Amps. This is about 90 watts, more than enough to power a laptop and recharge the internal laptop battery. All you need to do is convert the 12V vehicle supply to the voltage your laptop uses. There are two ways you can run your laptop computer in a car from the vehicle power. Note that a few vehicles like some trucks use 24V vehicle power. The accessories are available in 24V versions but the accessory and vehicle voltages must match.

12V to AC Mains inverter

To use any other AC powered gadgets on a road trip, an inverter is needed that can turn the car’s 12 DC power into AC mains power. This plugs into the cigarette lighter socket. Some types are conveniently packaged in a tin the size of a soft-drinks can, to fit in the vehicle’s cup-holder and keep it from sliding about.

The amount of AC power available is usually limited to about 120 watts. With one of these fitted, it is possible to use the regular laptop power supply used at home to power it from the wall socket. What’s nice about this option is that as AC mains power is now in the car, other mains-powered gadgets an be used, for instance charging a phone and Ipod. There is no need to understand the voltage, current and power connectors of the laptop – all of this is taken care of by the original laptop power supply. What is less good about this is the tangle of wires and extra gadgets, which have to be stowed when leaving the car. Having this all out in the open would be like writing STEAL ME on you car. The second option offers a simpler approach.

12V Laptop power supply

Universal laptop power supplies are made that directly convert the 12V DC from the cigarette lighter socket to the higher DC voltages laptops run on, usually between 19 and 21 Volts. Using one of these is more complicated to set up, because it is necessary to set the voltage to match the power supply that came with the laptop. This can be determined by inspecting the rear panel of the original laptop power supply, and it should state what voltage it delivers. The voltage selector switch on the 12V supply is set to match this before plugging anything in. Note of the output current too, and check that the replacement can deliver this amount of current or more. For example, if the original voltage is 19.5V and current is 3.9A. The 12V supply is set to 19V and is good for 5.5A at this rating. That is more than the original 3.9A so the 12V supply will not be overloaded.

Finally, select the right power tip from the range supplied with the 12V laptop supply, and fit this to the cable the right way round. Ensure that the positive terminal is aligned correctly. It is essential to get this right – check and double-check it, because if it is wrong there is a risk of damaging the laptop. If a multimeter is to hand, check the two supplies match in terms of voltage and polarity.

Finally - Disconnect when the Engine is off.

Many vehicles power the cigarette lighter socket even when the engine is off and the key removed. While this can be handy for running your laptop without starting the engine, it is unwise to do this for more than about an hour. Vehicle batteries are designed to provide the heavy current to start the engine but do not have a particularly high capacity. Typically, one should not run a laptop for more than an hour from a typical fully charged car battery if the engine is not running*. Whichever type of inverter used, disconnect the inverter from the lighter socket when it is not in use as the inverter draws some residual current even if not powering a laptop.

* car batteries should not be discharged to less than 80% capacity for good service life. Assuming a 40Ah battery, and a laptop drawing about 60W (5A at 12V); 20% of 40Ah is 8 Ah. Running a laptop for an hour and a half would draw approximately 20% of the battery reserves. Realistically it is advisable to draw less than this, assuming the engine needs starting and the battery should still remain above 80% reserve capacity.

The copyright of the article Power A Laptop in a Car in Computer Hardware/Accessories is owned by Richard Mudhar. Permission to republish Power A Laptop in a Car in print or online must be granted by the author in writing.