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I have days of electricity outages here (due to war!) and I need to use my laptop. The thing is, it runs off 18.5 V 3.5 A (AC adapter output), so it is expecting 18.5 V, what will happen if I connect it to a 12 V? I know for sure some laptops will work! I tried some crappy old laptop and it ran even though the adapter its AC adapter outputs 18 V, but I did not want to try this on my laptop as I don't know what might happen.

The laptop's internal battery is dead and I have actually removed it, so should not the voltage be enough to run the laptop since now there is no battery to charge! And since the internal battery outputs 10.8V, should not the laptop be able to run on that low voltage from the power source as well?

But again I was afraid if I provide too low voltage the laptop will try to withdraw more current, and that might heat it up and fry it! But then I thought I can add a fuse to prevent the current from exceeding certain limit (say 7 A?)

And if we assume it does not work! Then I can add 2 batteries which will give me 24 V, but now I'm thinking 24 (actually almost 28V when the batteries are fully charged) might be too much for the laptop and now high voltage could really hurt!

A solution to that could be to add a linear voltage regulator, but that wastes too much energy (or does it not?), which is why I'm not using DC-AC power inverter in the first place, after all I'm trying to make this setup as efficient as possible (because as I said we have days of outages), so how do I reduce voltage without having to waste much energy?

Assuming the batteries are fully charged, thus outputting "almost" 28 V the regulator then will have to drop the volt down by 8 volts! Will this waste too much power? and by too much power here I mean like 50 watts! because the laptop runs with that much so if it will waste 50 watts or so, then it will be actually wasting the same as the actual load does! and that is a lot.

And an important question here, will I have problems with the regulator heating up? or that little difference isn't going to cause much heat?

To sum the questions up:

Can I connect my laptop that runs on "18.5 V" directly to a 12 V battery OR 24 V (2 batteries)? If I have 2 batteries outputting 24 V how do I reduce the voltage without wasting energy?

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  • \$\begingroup\$ Where are you located? Filling in your profile is not really for you but a courtesey for us. \$\endgroup\$ – Olin Lathrop Mar 28 '13 at 14:45
  • \$\begingroup\$ I live in Syria, I will fill in my profile :) Thanks \$\endgroup\$ – AL-Kateb Mar 28 '13 at 14:48
  • \$\begingroup\$ What exactly is the old internal battery made up as? 10.8V could be 9 NiCd batteries. That would charge at about 13V. Higher, if they are NiMh. If that is the case, you are probably safe connecting directly (with fuse, of course) to the 12V battery. Maybe, do not charge the 12V battery when the lap-top is connected. \$\endgroup\$ – Bobbi Bennett Mar 28 '13 at 15:20
  • \$\begingroup\$ I don't understand what's NiCd or NiMh, It's a Li-ion 6 cells battery with 4400 mAh, and 10.8V output! It's an HP battery. Nothing on the battery says NiCd or NiMh. \$\endgroup\$ – AL-Kateb Mar 28 '13 at 16:00
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    \$\begingroup\$ Li-ion. OK. They charge at 4.2 per cell, so that is 12.6V on the battery while charging. Down to 9V at low battery point. I take it you have removed the battery, and are running the laptop just plugged in to its power adaptor. I am doing the same thing, with an old Toshiba, with the same worry as Olin Lanthrop, that the battery might have provided some sort of regulation. But no problems. Only you can weigh the cost of a new battery and 12V charger against the (IMO minimal) risk of trying 12V instead of the power adaptor. \$\endgroup\$ – Bobbi Bennett Mar 28 '13 at 16:28
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There is no way to know from the outside how the laptop will react to 12 v, but probably not well. It may also not react well to having the battery completely removed. It could possibly be integral to the internal regulation.

The safe thing to do is to get a "car charger" for your laptop. These are intended to work from 12 V car power and supply whatever power your laptop needs.

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  • \$\begingroup\$ I've come across this DC adapter, but I thought if I bought another battery to give me 24V and reduce the voltage instead that would be more cost-efficient, as this DC adapter almost costs the same as a battery here! So what do you think about the 2 batteries thing? \$\endgroup\$ – AL-Kateb Mar 28 '13 at 14:57
  • \$\begingroup\$ @AL-Kateb it's probably worse, because while 12V is a standard voltage for adapters, you won't find a 24V one, and how are you going to feed 24V to the laptop directly? \$\endgroup\$ – clabacchio Mar 28 '13 at 15:05
  • \$\begingroup\$ @clabacchio I am not going to feed it directly to the laptop, but I will connect a voltage regulator as I mentioned in the question, and if I ever buy a 12V DC adapter then I have no problem with the 2 batteries, I will connect them in parallel \$\endgroup\$ – AL-Kateb Mar 28 '13 at 15:13
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    \$\begingroup\$ @AL-Kateb that's true, but consider using a switching regulator (boost for 12V, buck for 24V) because with the linear you'll have a lot of energy loss to dissipate as heat. Unless you need a personal heater. \$\endgroup\$ – clabacchio Mar 28 '13 at 15:20
  • \$\begingroup\$ @clabacchio I don't know where I read that linear ones are more efficient! But if you say it will even boost for 12V then that's perfect! \$\endgroup\$ – AL-Kateb Mar 28 '13 at 15:47
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Can I connect my laptop that runs on 18.5 V directly to a 12 V battery or 24 V (2 batteries)? That's 18.5 V on the laptop's DC plug. Laptops have voltage regulator circuits on the mainboard so they can accept slight differences in input voltage, but supplying 6.5 V less or 5.5 V more to a 18.5 V DC plug is very probably too much of a difference. For example, I have a ThinkPad X201 notebook (20 V DC nominal) that I can operate reliably with 18.8 V or 20.4 V DC, but supplying only 16.4 V DC will cause it to not even start. Other computers may react differently, and it might not be safe in all cases.

Since the internal battery outputs 10.8V, should not the laptop be able to run on that low voltage from the [DC plug] power source as well? No, not necessarily, since there are voltage converter circuits on the mainboard so that the supply voltages expected from battery and DC plug can indeed differ. Again for my ThinkPad X201: it has a 14.8 V battery, but would not even start up when supplying that voltage to the DC plug. Now you could try of course supplying the battery voltage (or something in the operating voltage range of the battery which is 9 V - 12.6 V for a 3s2p battery as you have) to the battery terminals. But again there's a problem: a typical battery connector has five or more pins. Plus and minus are usually at the very left and right, but there are data pins as well that let the laptop communicate with the battery. In my ThinkPad X201, just supplying battery voltage to plus and minus did not let the computer start up, since it did not recognize a "proper" battery via the data channel. It did not damage anything either, but it did not work. I even tried connecting the data channels from the dead battery in addition, but that did not work either …

Proposed solution. As proposed already, a DC-DC converter that supplies your 18.5 V DC (or 18 V or 19 V) from 12 V DC (one battery) or 24 V DC (two batteries) is the clean solution to go for. Their typical efficiencies are 85-90%, and they are more efficient with less voltage difference to cover. Since both 12 V → 18.5 V and 24 V → 18.5 V has similar differences, I recommend using a 12 V DC adapter since it requires only one battery for a start, and since DC adapters for notebooks with 12 V input (as used in cars) are more readily available than with 24 V input (only used in trucks). There are cheap adjustable ones available, no need to buy the expensive official "12 V car adapter" made by your laptop manufacturer. And before you wonder about the 10-15% power loss in the adapter, there are many ways to save more power by using the computer in an energy-saving way (display brightness, monitoring for runaway processes, frequent standby, auto-power-off for the monitor etc.).

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