Is it proper to connect my laptop that runs on 18.5 V directly to a 12 V battery

[Sheeran]

I have days of electricity outages here 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?

 

[iampoor1]

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?

You dont want a linear regulator, you want a switching regulator. Just buy a switching regulator module on Ebay. Plenty of them come with 7 segment displays where you can set the output voltage. Julian Ilett on youtube has quite a few videos and playarounds with these types of modules. I dont have any links, but they should be easy enough to find.

Good luck.

 

[Khron]

Whether it works on 12V or not, depends entirely where the DC input (ie. charger input) undervoltage threshold for the main voltage controller is set, inside the laptop. The ones nominally supplied with 19-20V usually have that set for 16V or so; some older IBM Thinkpads came with 16V chargers, and those might've worked well with a 12V DC input. That chip also supervises the current drawn from the charger / battery, so it shouldn't fry anything - these things aren't quite THAT dumb Quite the contrary, sometimes they're a bit too "smart" for their own good...

Whether 12V works or not, is a crapshoot, without a look at the schematics. Assuming you make sure you get the +/- polarity right, feel free to try. Worst case, it simply won't work / start, and that's all, but nothing should blow / fry because of too-low a voltage.

24V is, to say the least, risky as hell. The capacitors on the main power rail (coming from the charger / battery) inside the laptop are rated for 25V, and you usually want at least a 20% "headroom" between their rating, and the voltage they're subjected to.

I don't suppose finding a used UPS might be an option, would it? With a swap of fresh batteries, that oughtta help, with a bit less headache than MacGyver-ing some half-a*ssed contraption, imho.

 

[RuudNL]

There are 'car adapters', that will convert 12 Volts DC to 19 volts.

 

[Khron]

Oh yeah, forgot about those They can be kinda-sorta janky in quality, but for such (relatively) low power, one of those should be fine.

There are 'car adapters', that will convert 12 Volts DC to 19 volts.

 

[abbey road d enfer]

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?

I guess 10.8v is nominal (probably 3 Li batteries with nominal 3.6), which means a fully-charged battery outputs 12.6V (3x4.2). There is nothing in a laptop that uses directly the battery voltage, everything is powered via regulators, so replacing the battery with a 12V source should work, but you must have good overvoltage protection in it. In particular, battery chargers deliver way more than the batteries they are made to charge. You need a well regulated PSU. You may want to install a large electrolytic cap in order to help with instantaneous current demands (hard disk start-up).

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!

Misconception! Moderate undervoltage results in lower current.

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!

Now you are being dangerous!

Can I connect my laptop that runs on "18.5 V" directly to a 12 V battery

I wouldn't do that; the regulators may simly refuse to work and stall. In principle they would go in shut mode, bust some of them may start. Not having all the right voltages is to be avoided.

OR 24 V (2 batteries)? If I have 2 batteries outputting 24 V how do I reduce the voltage without wasting energy?

A switching regulator would do that, but I think it's not necessary at all.

 

[Khron]

10.8v nominal does indeed sound like a 3-series cell arrangement. That being said, even if you connected a random 12v battery or DC supply to the (appropriate) battery terminals of the laptop, there's a good chance it still wouldn't work, due to the "smart" controller inside the battery needing to "handshake" with the circuitry inside the laptop, and "open the taps" for the current from the battery to flow. No comms => no power.

I've already explained what would happen (or rather, not happen) if one connected 12V to the laptop's DC input - namely, not much, if anything at all. 
As an example, the MSI GX720 (an older gaming laptop) has the undervoltage-lockout set at 17.4V.

I guess 10.8v is nominal (probably 3 Li batteries with nominal 3.6), which means a fully-charged battery outputs 12.6V (3x4.2). There is nothing in a laptop that uses directly the battery voltage, everything is powered via regulators, so replacing the battery with a 12V source should work, but you must have good overvoltage protection in it. In particular, battery chargers deliver way more than the batteries they are made to charge. You need a well regulated PSU. You may want to install a large electrolytic cap in order to help with instantaneous current demands (hard disk start-up).

With a linear load, perhaps. Switchmode power supplies, though, of which plenty are present in a laptop (mostly synchronous buck converters) are actually a constant power load (rather than constant current, which a linear load would be), from the battery / charger's perspective. They'll draw as much input current as they need, to maintain the output voltage, regardless of the load current (up to, of course, the overcurrent protection tripping point built into said buck controller chips).

Misconception! Moderate undervoltage results in lower current.

Now THIS i'll agree with - supplying 24V, as i've pointed out earlier in this thread, would be at the very least risking the integrity of the filter / bypass capacitors on the DC input. You really don't wanna "help" let the magic smoke out of those...

Now you are being dangerous!

See first paragraph, as well as my first post in this thread.

I wouldn't do that; the regulators may simly refuse to work and stall. In principle they would go in shut mode, bust some of them may start. Not having all the right voltages is to be avoided.

 

[abbey road d enfer]

As an example, the MSI GX720 (an older gaming laptop) has the undervoltage-lockout set at 17.4V.

I don't get this. A laptop doesn't need to be connected to a working power adapter/charger to operate...?

 

[Khron]

Quite the contrary. The chip that handles the battery charging also supervises the voltage coming from the power brick. In most laptops, that's nominally 19V (20V in Lenovo ones), but "accepts" anything down to, in this case, 17.4V as valid / ok. Say, the brick-to-laptop cable's flimsy and there's some voltage drop across it, at high load.

Tolerances and all that, y'know? We don't live in a perfect world, yadda-yadda-yadda...

Anything less than that, and it decouples the DC input from powering the laptop, and switches to the battery.

And then we have these even-"smarter" laptops, which need an ID signal from the charger, to identify it, and auto-set internal power limits so it won't demand more "juice" than the charger can provide (or they b*tch & moan if you hook up a smaller-than-designed charger, even if it's legit). In Lenovos from a few years ago, that was just a resistor from the ID pin to ground; Dells have a one-wire serial PROM chip in the charger that the laptop reads; not sure how HP's do it.

 

[abbey road d enfer]

Quite the contrary. The chip that handles the battery charging also supervises the voltage coming from the power brick. In most laptops, that's nominally 19V (20V in Lenovo ones), but "accepts" anything down to, in this case, 17.4V as valid / ok. Say, the brick-to-laptop cable's flimsy and there's some voltage drop across it, at high load.

So you actually mean a laptop needs to be connected to its PSU to operate? Come on...

 

[Khron]

When its own battery is disconnected, yes

All i'm saying is, if you provide a lower voltage at the DC input, than whatever that undervoltage threshold is set at, and the laptop has no battery, it won't power on. As in, a laptop that nominally has a 19V charger, and with the battery removed (since that's the premise in the first post in this thread) VERY likely won't (even) turn on.

The laptop's internal battery is dead and I have actually removed it

 

[abbey road d enfer]

All i'm saying is, if you provide a lower voltage at the DC input,

The idea was to connect the 12V supply at the battery connection, not at teh charging input.

 

[Khron]

For reasons i've already explained, that's pretty much a no-go as well, unfortunately.

 

[iampoor1]

For reasons i've already explained, that's pretty much a no-go as well, unfortunately.

All he needs is an adjustable switching supply. Hundreds of these are available on Ebay. Seems simple enough?  ;D

 

[Gene Pink]

... which is why I'm not using DC-AC power inverter in the first place,...

Why not? They are very efficient, cheap, and ideal for this, IMO. Certainly more efficient than dumping 10 / 28 of the energy through a linear regulator.

A 400W non sine wave inverter should cover a laptop and a router/modem just fine, with less than 10 percent loss. It also covers a desktop with a power-hungry 17" CRT monitor and a printer, I can attest to this.

My own setup, a 100 foot of 10/3 SO cord scrap from the pile, quality automotive jumper cable clamps on one end, and a pair of large bolts in plywood on the other. This gets serious 12VDC into the house from my 24' boxtruck from the three large truck batteries in parallel, meant to start a Caterpillar diesel. Multiple inverters clomped on for AC, to whatever I need to power up in the house, just keep an eye on the DC volts, and go start the truck before it gets too low, and idle it for a little while to recharge, with the 220 amp alternator. With an 85 gallon tank, I'll have power for months.

Gene

 

[Khron]

All he needs is an adjustable switching supply. Hundreds of these are available on Ebay. Seems simple enough?  ;D

Oh yeah, forgot about those They can be kinda-sorta janky in quality, but for such (relatively) low power, one of those should be fine.

There are 'car adapters', that will convert 12 Volts DC to 19 volts.