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What is the best way to to turn 24 volts dc into 14.4 volts dc? Answered

I want to make a power supply for my laptop so I can use it for long time while camping.  I want to use car batteries.  I have tried using just one car battery but this does not work.  A car battery varies between 12.6 volts and 0 volts.  Most of the energy is gone by the time it gets to 10.5 volts.  An inverter is designed to be used when a car is running.  The alternator generates 14.4 volts so things which run off of a car alternators operate at 14.4 volts.  When I plug my laptop into an inverter which is connected to a car battery sans the car it will only work for a few minutes.  The voltage of the battery drops when current is being drawn from the battery.  This sets off a low voltage alarm on my inverter.  

So new plan is to hook up two car batteries in series running at 24 volts.  This brings me to my question.  What is the best way to convert the battery power to a regulated 14.4 volts.  The battery will be 25.2 volts fully charged and will drop all the way to zero when dead if you let it.  Most of the energy would be gone by the time the battery gets 20 volts.  

The set up will be
24 volt batteries---14.4 volt regulator---inverter---laptop


There likely exists a DC-to-DC converter capable of doing what you want.

Although, you might want to do the math for current (in amperes), power (voltage times current) and energy (time integral of power), for to discover how much, how many kilos, of batteries needed for your, uh, camping trip.

Well, let me start over. The most important consideration in buying a power converter, is knowing how much power it has to supply. Power is the product of two numbers: voltage multiplied by current.

You told us that your laptop wants 14.4 volts DC. But how much current does it want?

I left my crystal ball in my other pants, but I think typically laptops want around 30 to 40 watt, or approximately 2 to 3 amperes, at 14.4 VDC.

The price of power converters tends to scale with the amount of power they can supply, or convert, throughput.

Also the amount of energy stored in batteries is finite, some number of ampere*hours, in terms of charge, or watt*hours, in terms of energy.

For a truly complete overview of how this stuff works, you should probably read the Wikipedia articles for "Power supply" and "Switched-mode power supply",

In addition, I will mention a few, uh... exactly three, ideas.

There exist car powered laptop chargers, because, you know, peoples with laptops like to travel, so naturally there is market for that sort of thing. Answerer, WarenGonzaga, mentioned this too. Typically these car powered laptop chargers will want some DC input voltage in the range, of like 16 to 11 volts DC. The exact input range varies, but it is of course something that a nominal 12 VDC, car's electrical system could supply. For the output voltage, you have to find one matched to what your laptop wants.

I think 16 VDC, and 19 VDC out , are easy to find. But I am not sure about other voltages, like 14.4.

The second idea is to just use one of those car powered inverters that make pseudo-mains power; e.g. something approximately sinusoidal, with RMS (root mean square) voltage of about 120 VAC. Then you just plug your existing mains-powered laptop charger into this.

Using an inverter of this kind, might produce a lot of 60 Hz plus harmonics, (120 Hz, 180 Hz, etc), and that noise can find its way to the laptop's sound card. Not physically harmful, but annoying, if you want to music, or worse, record audio, voice for a video, etc. Or at least this is my experience, having used this kind of car-power-to-pseudo-mains-power inverter in the past.

The third idea is to try to use one of these modular, made in China, DC-to-DC switchmode converters, that can be found cheaply on eBay.

There are roughly two kinds available: boost and buck. Boost is for when the output voltage must be strictly higher than the input voltage. Buck is for when the output voltage must be strictly lower, or equal to, input voltage.

If you want to see a more complete explanation of buck and boost, well I think Wikipedia can help you there. Those are the first two entries in this table for "Non-isolated topologies" in the article for "Switched-mode power supply", here:


In practical terms, I guess what this means is that if you want output voltage at 14.4 volts out, from 12 volts in, that requires a boost converter, and if you want 14.4 volts out, from 24 volts in, that requires a buck converter.

Also regarding that phrase, "non-isolated", usually what this means is the negative terminals of both input and output are connected together, and usually that is no big deal for most purposes, but maybe worth remembering, or being aware of.

Final note: these cheap little buck (or boost) converter modules, are typically adjustable by way of a tiny little potentiometer, that you can adjust by turning it with a screwdriver. When it arrives in the mail, you can expect that pot to be set to the wrong output voltage.

The process of adjusting that little pot to the correct output voltage for your thing, is, well the way to do that is to connect some kind of cheap, test load, type thing to the output terminals, instead of your expensive laptop, which might be injured by having the wrong voltage fed to it.

As an example of an test load appropriate to your thing, try maybe one 10 ohm resistor, or two 10 ohm resistors, wired in parallel. What current would that give at 14.4 VDC? Answer: (14.4 V)/(10 ohm) = 1.44 A per resistor. I mean if you put both of them on there, that would give 1.44+1.44 = 2.88 amperes, and that was approximately what I was expecting for the current drain from a real laptop load to be.

By the way those test resistors, have to be rated for more than 20 W power dissipation, since,

(14.4 V)*(14.4 V)/(10 ohm) = 20.736 W

Anyway, once you have some confidence that the power converter gizmo's output voltage is set where it should be, and it can actually deliver as much current as your load requires, only then do you connect your expensive load, e.g. laptop, to it.

Come to think of it, some of these power converter modules come with a built-in display, that will tell you output voltage, and maybe output current too?

Upon request, I could point you to some example eBay pages for these modules, because I am pretty sure they are out there, at the time of this writing. Without that request, I will assume you have the eBay-foo skills, for to find these things by yourself.

I think I have seen Instructables, that included pictures of these modules, and maybe anecdotes about how to make them work, but I am trying to think what would be the words to search for 'ibles that have those things pictured.


A few of these are obviously using those cheap, eBay obtained, modules, like this one. I have not looked at this 'ible closely, but I think it is basically a DC supply, plus a buck type switching power supply module, plus a voltmeter+ammeter display module. The links to these various eBay pages are in the 'ible linked below, and amazingly, these links seem to be working, at the time of this writing.


Thank you for the detailed reply. A car powered laptop charger would not work because those run on 14.4 volts from the cigarette lighter adapter and a car battery is between 12.6 volts and 0 volts depending on state of charge.

I like the Boost idea. That might work. Stepping electricity up is a little inefficient compared to stepping it down though. I could get have two 12 volt batteries in series and then use a step down switch mode power supply.

I could see if there are inverters which run on DC between 10 and 12.6 volts, a little lower input voltage than regular inverters. If there are I could use one 12 volt battery. Or maybe there are inverters which run on DC between 20 and 25 volts, then I could run an inverter off of 2 12 volts in series.

My car laptop adapter works just fine off the battery. Not sure where your odd idea they only work off 14.4 volts comes from

You need to know how many total watt hours are in the battery and how many watt hours you can draw from the battery with the car laptop adapter before the car laptop adapter stops working. I bet that the car laptop adapter only works when the car battery is fully charged. I could be wrong though. I have never experimented with one.

Well, no, mine works from 11 to 14.8 volts, my laptop runs on 19V. At 11V the battery is flat, at a C/10 rate.

You don't want an inverter ! You're adding a DC-AC conversion, then converting it BACK to DC for the laptop, all you need is a suitable boost supply.

You are right. This is what I ended up doing. I found one which works between 10 volts and 16 volts.


PS at C/10, you can pull 10th of the terminal Ah of the battery - so a
150Ah battery will drain to flat in 10 hours at 15A - That's 1800 Wh

I ended up getting a car laptop adapter. This turned out to be the most simple solution. I found one which works between 10 and 16 volts which is perfect. It will work until the battery is completely drained. So thanks for all of your suggestions.

ultimately comma comma the question is what is the best way to produce the voltage needed by the computer from car batteries. an inverter may not be necessary at all. why have 120 volts volts if you do not need 120 volts?what voltage does the computer need?


theres a $2 solution, assuming your computer charges from usb .

How many amps?

If it is less than 4 amps like mine a adjustable DC to DC converter like this should do you on one 12 volt battery.


From 24 volts down to 14.4 a simple LM317 adjustable voltage regulator will do for 1 amp or less.

If you can get a UA78HGA adjustable voltage regulator it can handle what you want up to 5 amps.

have u tried 2 batteries in parallel?

The voltage would drop at a slower pace but the inverter would stop working while there is still a lot of power left in the batteries. The inverter stops working at 11 volts when there is still a lot of power left in the batteries.

Vicor makes some pretty efficient DC/DC converters with input and output ranges in the area of what you need, but they're not cheap..

You need a boost voltage regulator, that's all, with a 12 V input, 14.4 out, and a decent current rating, probably around 4A.

There's a pretty large amount of energy in a car battery - typically 150-250 Ah, that's about 83 hours of run time at 3A

I know I could buy a battery pack but I want to see if I could make one from car batteries. I meant to say two batteries connected in "series" not "parallel" in the question. Why would this not work?