How To Prolong Laptop Power When Running Off A Car Battery

Ryan Dube 03-09-2013

Have you ever found yourself hovering on the edge of an online catastrophe, trying to resolve some issue at work via your VPN link, while your laptop’s battery level continues to sink down to 0%? This is a situation that a lot hard-working mobile users often find themselves in, and the options available to keep that laptop battery charged are not as well-known as you might think.


Yes, you need an outlet. If you’re camping, you might desperately attempt to locate an outlet available on the outside of a building somewhere. If you’re traveling, you might stop at a rest area and hope that the roadside facility is equipped to handle technology-laden travelers like you. However, if you really want to be prepared for high-powered computing when you’re on the go, there are certain things you can do to ensure not only that you have power for that laptop anytime and anywhere, but also that the power lasts as long as possible.

There are a number of different aspects to consider when you’re looking to prolong the battery life How To Benchmark Your Laptop's Battery Life With Battery Eater Read More of your laptop when you’re using it on the go. There are, of course, settings you can change on the computer itself to prolong its battery life How To Increase The Lifetime Of Your Laptop Battery A laptop with a short battery life is a nuisance, especially when you're on the road and nowhere close to a power socket. To make each individual charge of your battery last longer, learn about... Read More when the laptop is unplugged, but beyond that, what about the process of recharging that battery? What is the smartest and most efficient way to keep your laptop battery charged How To Benchmark Your Laptop's Battery Life With Battery Eater Read More without putting a tremendous strain on the charging system – like your car’s own 12 volt battery?

In this article, I’m going to explain how the charging process works when you’ve got your laptop plugged into your car’s battery (lighter socket), and how you can manipulate that setup to reduce the amount of fuel you use to recharge your car battery, and increase the amount of time that you can use your laptop while running off just the batteries that are available to you.

An Ideal Mobile Laptop Charging Setup

For my mobile purposes, I bought a 350 Watt inverter from Radio Shack that can plug into my car’s lighter/electrical system. Wattage is the power demand of your laptop, and may also be called Volt-Amps by some. It is basically the voltage rating of the laptop power supply, multiplied by its rated current. For example, a laptop power supply that needs 19 Volts and draws a maximum of 4.75 Amps requires approximately 90.25 Watts of power.

It’s nice to have a 350 Watt inverter, which would be powerful enough to charge two laptops and then some. Keep in mind that when you’re using the unit, you will draw much more current out of your car battery, reducing the time you have available to recharge your devices before the car battery itself starts to drop too low. Here’s my 350 Watt unit with the laptop plugged in and the unit turned on. This inverter has an LED on the front showing you that it is currently turned on.



Like I mentioned, I purchased this 350 Watt Inverter from Radio Shack. You can find them on Amazon for less than $40, or any other place that sells electronics. A good inverter unit will include both a switch as well as a fuse on the back, and a plug that goes directly from the back of the inverter into the car lighter/power socket. A unit like a 350 Watt Inverter also has its own cooling fan, so this will unfortunately draw some extra energy from your car battery as well – but it isn’t enough to be concerned about.


To understand how such a mobile charging system works and how to make it function more efficiently, it’s important to understand each component of the system and the part that it plays.


Components of a Car Charging System

The electrical supply system of a car actually isn’t all that complicated, so long as you’re not talking about the engine timing system or the computerized chip that runs it – that stuff can get a bit intense. However, the electrical power system that feeds the power outlets inside of your car actually consists of just a few major components that you need to be aware of in order to fully utilize the system with your mobile office.

Here is a very simple layout of the car’s power supply setup with an inverter hooked up like the one I have pictured above.


This is an extremely over-simplified layout, but the main point here is to recognize what the major components are and what they do. The alternator is the one key to maintaining the long-term charge in the system. Your alternator will be buried somewhere in your engine, usually recognizable by the copper coils visible in the holes along the side of the housing, and operated by the engine via a rubber belt.



The alternator is connected to your battery – a 12 Volt powerhouse of electrical current that feeds all of the systems inside the cabin of your vehicle, through a bank of fuses (not shown in the diagram above). So long as your engine is running and turning the alternator, this 12 Volt car battery will continuously charge, and stay charged.


With an alternator to work with, there are some steps you can take to cycle through charging and discharging the battery – taking advantage of the life of your own laptop’s battery as well – to reduce the time that your car has to stay running, and to increase the length of time you can use your laptop without the engine running.


Playing With Your Alternator

There are basically two modes of operation we’re going to play with when you’ve got your laptop plugged into the inverter, and the inverter plugged into your lighter jack. First, you’ll start your car, so the ignition key will be in the fully-on position.


Let the entire system charge while you’re using your laptop. If you previously drained the laptop battery at all, then you’ll see the laptop battery Should I Remove My Laptop Battery To Increase its Life? Does running your laptop on AC power damage the battery? Should you remove the battery to increase its lifetime? You've probably wondered about this. Find out the answer now! Read More charging up at this point.


Once your laptop batter is at 100%, go ahead and turn the ignition of your car to the half-way position, so that the electronics in your car – like your radio, for example – still work. This runs everything, including the power jack (lighter port), directly off your car battery. While you were driving your car around, or running it to charge up your laptop battery, your car’s battery was also fully charged by the alternator.


In this key position, the alternator is no longer turning, so while you’re saving gas, you are now running completely off your car battery. You’ll see that when the key is in this halfway position and your inverter is plugged into the car’s power port, the power light on the inverter will still be on, indicating that it’s still receiving input power from the car.

If you have a smartphone that you’re using as part of your work in your mobile office, don’t forget that some cars have a second port inside of the center console between the two front seats. Some people can own a car for years before realizing that they have this second source of power. This is a convenient way to keep your smartphone charged even though you’re using the other power jack to power your laptop.


So, you might have run the car for a good 20-30 minutes while the laptop battery was charging. That’s half an hour of good, solid power-fed computer time. Then, once you switch the car into this battery-powered mode, nothing will change. You can use your laptop in this state, drawing off your car battery, for an hour or more very safely.

Extending Your Battery Use Time

To extend the time that you can run in battery-only mode, make sure you’ve enabled the “battery saver” power option on your laptop.


If you want to increase the time that you can run off your car battery, manually switch your laptop to power saver mode even though it’s plugged in.

During this time, you’ll maintain 100% power on your laptop battery, but in the process you’re slowly draining power from your car battery. How long this can go on depends upon how old your battery is, and what other systems you have running. If you’ve got the radio and fan turned on, it might draw down the juice a little faster.

A quick test to see how much power your battery has left is to quickly turn on the overhead light. Is it nice and bright, or do you notice that it’s fading a little bit and not quite as bright as normal?


When you notice a change in the strength of that overhead light, it’s time to completely turn off the car ignition and switch completely to laptop battery. Keep in mind that between running the car for about 30 minutes, running off the car battery for 2-3 hours, you’ve already worked for nearly 5 hours without even draining a single bit of your actual laptop battery life.

Once you’re on laptop-only battery power, you’re down to your last reserves. Here, it’s critical that you’re using power-saving features, like dimming the screen display for example. Depending how new your laptop battery is, and how much you’ve got running on the laptop, you can actually run in this state for anything from 1 to 5 hours.

The cool part is that after performing this slow stepped-drain routine that allowed you to work for 6 to 8 hours before draining all available power, all you have to do is turn on the car ignition again, let the alternator perform its magic and recharge both your car battery and your laptop battery in less than 30 minutes, and then you can start the process all over again!

Are you a mobile worker? Have you developed different tricks to prolong the amount of time that you can work off your batteries? Share your mobile worker tips in the comments section below!

Affiliate Disclosure: By buying the products we recommend, you help keep the site alive. Read more.

Whatsapp Pinterest

Leave a Reply

Your email address will not be published. Required fields are marked *

  1. mikia
    September 4, 2013 at 5:58 am

    photo from wp?

  2. Anay C
    September 4, 2013 at 5:12 am

    Ummm, using DC from the battery, converting it into AC and using the laptop's adapter to convert it into DC again for the laptop is the best way to prolong the car's battery? WTF?
    use a laptop charger which runs on the 12 volt outlet and step up dc12v to dc19v (assuming ur laptop requires 19v) or whatever voltage it requires. This is much more efficient!
    Any electrical engineer will tell you that an inverter is 65% efficient at best!

    • Ryan Dube
      September 4, 2013 at 6:07 am

      Anay - I'm not sure where you got the idea anyone was trying t prolong the car battery? The only goal is to discharge each stage in steps - rather than, as most people do, just turn off the car and switch entirely to laptop battery only. If the goal were to prolong the car battery, that would be the way to go, and avoid discharging it at all.

      At the same time, I do like your idea of using a 12 Vdc laptop charger and making the whole setup more efficient - that would definitely lengthen time available to be productive while mobile. Thanks for the feedback!

  3. Egzon
    September 4, 2013 at 3:25 am

    Hm, interesting idea but that is not enough energy for a laptop, it can just damage the batter IMHO.
    Why not just buy a solar panel?

  4. Ryan Dube
    September 4, 2013 at 2:50 am

    Awesome tips Jim - although you might have misunderstood the discharge rate I was referring to. Using the battery until the overhead light *starts* getting dimmer is hardly a deep cycle. However, your point is a good one - people shouldn't be draining the car battery way down, as they weren't designed for that and you could do more harm than good. In my own experience, for the short-term use of several hours, this shallow cycling technique works extremely well. If I were to head out into the wild and needed longer-term power needs, buying a marine battery would definitely be the ideal option.

  5. Jim Aspinwall
    September 4, 2013 at 2:39 am

    As someone who has been doing field communications and adding laptops to the mix for sustained camping and emergency (Code 3 vehicle and off-grid) operations, you do NOT want to do this with your car battery. A car battery is not "deep cycle' - it is designed for short-term high-current output to start the vehicle then run off the alternator for sustained DC. Long term discharge will destroy the lead components and deplete the chemistry. Dual car batteries only doubles your loss/expense. For the inverse reason you do not start/operate a car from a marine/deep-cycle battery which is designed for longer term sustained load, while the lead elements can be destroyed by high starting current draw. The best mix would be to leave the car alone, add a deep-cycle battery for laptop loading, separated and charging from the auto system with a charge isolator (a Schottky diode.) Just sayin' - if you're playing with technology, get it all, right.

    • Guy M
      September 4, 2013 at 5:53 pm

      Jim is right.

  6. Ashley Cardwell
    September 3, 2013 at 11:05 pm

    In the UK, i have an inverter and if the car battery drops below a certain level, the inverter cuts out to prevent from draining the car battery so the car can start on its own again.

    I have also seen cigarette lighter socket wiring catch fire because of the current passing through the cables with only a 240v 350w inverter.

  7. Marc
    September 3, 2013 at 10:37 pm

    you're much better off buying a second battery and keeping it charged. Cars are horribly inefficient at turning chemical energy (fuel) into electrical energy.

  8. Mike Merritt
    September 3, 2013 at 8:50 pm

    I use a 75 watt inverter that is a single unit. One end plugs into the car's power jack and the other end has a single 120 VAC receptacle on it. It's only 60 watts continuous usage - but more than enough for my netbook or my cell phone charger. It only costs $15.00 at the "dollar" store.
    It's much more compact, compared to a 350 watt unit. I keep it in the glove compartment of the car until needed.

    • Ryan Dube
      September 4, 2013 at 2:53 am

      You know - those smaller units could never handle my needs with a regular laptop. They would always end up cutting off on me unless I really cut back the laptop's power consumption by cutting the screen brightness down to the minimum level. I was forced to go to the 350 Watt unit because of this.

      The netbook is definitely the way to go for working mobile though, for this very reason!

  9. Ryan Dube
    September 3, 2013 at 8:45 pm

    I know, isn't it horrible? I have to make a habit of cleaning my battery terminals... One thing I do want to explore is setting up a portable solar panel either on top of the car or somewhere on the ground outside where I'm parked. Not sure how viable it would be to use it to recharge the battery during this kind of use, but it's worth a try. :-)

    • Guy M
      September 4, 2013 at 5:58 pm

      Yes it is possible to do it. I know a guy who mounted an 80w solar panel on the top of his toolbox in his truck bed. He had a two-battery deep-cycle AGM array on a charge regulator. Off of there he ran an inverter and wired a connection to outdoor 110v outlets on his tailgate. From there he could run most of his power tools. Not for hours on end, but for the tweaks he needed to make in the field. He also charged his cordless power tools this way.

      So, you could mount a panel in the back-window (securely of course) and run that to either your inverter or your charge controller-battery-inverter and power your laptop that way.

      You probably know, or can dig up, the formula for calculating charge rates and draw rates to size your system accordingly.

  10. Guy M
    September 3, 2013 at 6:16 pm

    You got some nasty terminal corrosion on your negative pole! ;-)

    I wish I had photos of some of the rigs we used in the oil patch. Some were quite elegant solutions and some were downright ugly, but it worked. I do recommend getting a RAM laptop stand and zip-tying your inverter and power bar to the upright. The upright can be removed from the car very easily when you have passengers.

    Many of the workers ran dual-batteries in their trucks. Some of them had at least one spare laptop battery that was kept charged as well. When you're 800 miles from the nearest village, redundancy is everything.

    • Skylark-Torch
      September 3, 2013 at 11:23 pm

      Note: Turning off Wifi capabilities on your laptop can drastically improve your battery life.