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Pesky Ngon

DIY laptop: making a 400W AC to DC UPS.

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Posted · Original PosterOP

bit of background: im trying to make a gaming laptop out of mostly desktop parts. plan to have a max power draw of 400W. only real challenge is making desktop parts run off a battery, and making that battery behave like it would in a laptop.

 

as far as i can tell, a laptop works kinda like a UPS; if you unplug the power, it switches over to the battery without crashing the system. if you plug it back in, the battery starts to charge while the system is running. Main thing that confuses me is why even low power desktop UPS are 10x larger than even a large laptop. why do electronics that perform such a similar task have vastly different sizes?
i understand they use large batteries, but from looking inside they also have monster PCBs the size of a laptop mobo.

 

im also aware there are different types of UPS (offline/standby, online/double conversion, line interactive) and that they are suitable for different tasks dew to differences in how fast they can change to and from the batteries, and how many times they can change back and forth. apparently the cheapest version, offline, can take 100ms to change. i have no idea if thats quick enough to avoid crashing a system. 
i assume id need line interactive because it can change faster and it can change a larger number of times?

 

another problem is (if ive understood correctly) a typical UPS takes AC, converts it to DC for the battery, then back to AC for the UPS output. the PSU i plan to use is DC to DC, so the conversion back to AC is unwanted in my case because id then need to add more parts to convert back to DC. is it possible to design a UPS that outputs DC?

 

final thing is keeping it low cost and compact. im hoping i can use an external AC to DC power brick to save space inside the laptop. as for cost, im aiming for £150 at most (not including the PSU). but im hoping for more like £50. i dont know if either of those figures is realistic?

 

TLDR: is it possible to make an AC to DC UPS for use with a 400W computer? could someone please point me in the right direction?

 

thanks for reading. any help or advice is appreciated.

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 trying to make a gaming laptop out of mostly desktop parts. plan to have a max power draw of 400W. only real challenge is making desktop parts run off a battery,

There's a reason why laptop manufacturers make everything very low power. Batteries are limited by both specific energy and specific power. Lithium Ion batteries, for example range (according to Wikipedia) from 100–265 W·h/kg specific energy and 250 – 340 W/kg specific power. What this means, in practical terms, is that a 400W machine would need somewhere around 1.5kg of battery just to turn on. Additionally, the more power you draw from the battery, the less energy you can pull from it (due to heat and other inefficiency losses). So, with that high of a power draw, we can probably assume that you'd be getting near the bottom range of energy; based on that, if we go with 100 W·h/kg, you would need a 4kg battery to run your PC for 1 hour. Even if we go with a more optimistic (but less realistic) 200 W·h/kg, that's still 2 kg minimum for 1 hour of runtime. Now, you haven't specified how long you'd want it to run for, but most laptops these days get 6-8 hours of runtime on a single charge. So now you're looking at a 12-16kg battery, minimum, and more likely a 24-32kg battery. Do you really want to lug that around in your laptop all the time? (Note that I'm not an electrical engineer, and this is all ballpark.)

Quote

another problem is (if ive understood correctly) a typical UPS takes AC, converts it to DC for the battery, then back to AC for the UPS output. the PSU i plan to use is DC to DC, so the conversion back to AC is unwanted in my case because id then need to add more parts to convert back to DC. is it possible to design a UPS that outputs DC?

If you do want to get your workout in, some UPS's have direct DC ports, often in the form of a "cigarette lighter" 12V port. I'd look for one like that, so you can pull it open and connect to the same connections as that port (don't use the port, you don't want the extra resistance it'll bring you). I have no idea about switching times, but that's probably your best option in this vein.

Quote

Main thing that confuses me is why even low power desktop UPS are 10x larger than even a large laptop. why do electronics that perform such a similar task have vastly different sizes?

The thing is, they're not doing the same job. A UPS typically contains all the circuitry needed to convert AC to DC and back again, as well as a battery large enough to power a desktop computer for the amount of time the UPS specifies. A laptop only needs circuitry for AC to DC, and even that gets offloaded into an external brick; additionally, the power requirements of laptops are typically much lower (usually less than 100W at full load, even for a high-end laptop {for example, my laptop is not high-end, but uses a 45W power brick, and that's enough to power the laptop and have enough overhead to charge the battery}). The cost of lower power requirements, however, is cost. There's a good reason why laptops are significantly more expensive than computationally similar desktops—the engineering of both power requirements and thermal requirements simply costs more.

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Posted · Original PosterOP
7 hours ago, fordy_rounds said:

There's a reason why laptop manufacturers make everything very low power. Batteries are limited by both specific energy and specific power. Lithium Ion batteries, for example range (according to Wikipedia) from 100–265 W·h/kg specific energy and 250 – 340 W/kg specific power. What this means, in practical terms, is that a 400W machine would need somewhere around 1.5kg of battery just to turn on. Additionally, the more power you draw from the battery, the less energy you can pull from it (due to heat and other inefficiency losses). So, with that high of a power draw, we can probably assume that you'd be getting near the bottom range of energy; based on that, if we go with 100 W·h/kg, you would need a 4kg battery to run your PC for 1 hour. Even if we go with a more optimistic (but less realistic) 200 W·h/kg, that's still 2 kg minimum for 1 hour of runtime. Now, you haven't specified how long you'd want it to run for, but most laptops these days get 6-8 hours of runtime on a single charge. So now you're looking at a 12-16kg battery, minimum, and more likely a 24-32kg battery. Do you really want to lug that around in your laptop all the time? (Note that I'm not an electrical engineer, and this is all ballpark.)

If you do want to get your workout in, some UPS's have direct DC ports, often in the form of a "cigarette lighter" 12V port. I'd look for one like that, so you can pull it open and connect to the same connections as that port (don't use the port, you don't want the extra resistance it'll bring you). I have no idea about switching times, but that's probably your best option in this vein.

The thing is, they're not doing the same job. A UPS typically contains all the circuitry needed to convert AC to DC and back again, as well as a battery large enough to power a desktop computer for the amount of time the UPS specifies. A laptop only needs circuitry for AC to DC, and even that gets offloaded into an external brick; additionally, the power requirements of laptops are typically much lower (usually less than 100W at full load, even for a high-end laptop {for example, my laptop is not high-end, but uses a 45W power brick, and that's enough to power the laptop and have enough overhead to charge the battery}). The cost of lower power requirements, however, is cost. There's a good reason why laptops are significantly more expensive than computationally similar desktops—the engineering of both power requirements and thermal requirements simply costs more.

thank you, that was very useful.
i have a lot of experience with desktops but little with laptops. i hadnt realised just how insane 400W is for a laptop. only just found out the 16 core XMG APEX 15 only has a 230W PSU. i might make a 400w laptop in future if i wanna go a bit nuts. but for now, id be aiming for 200W max. probably closer to 150w. 
1hr battery life seems pretty typical for mid/high end gaming laptops, so ill be aiming for that in terms of battery life. 
the info on UPS DC power was also really useful, ill have to look into that a lot more. im pretty confident i could cut out the conversion back to AC and make it small enough for a laptop.
thanks for all your help man. 🙂 

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