[userbinator]

20V is not a 'stock' DC/DC output voltage, so we're going to look for something roughly 24V and find something that has an adjustable sense line for reducing the output to 20V.

Note that the absolute voltage limit is likely due to 25V electrolytic capacitors in the input circuitry (find a schematic for a model that uses the "20V nominal" PSU and inspect it to be sure), so 24V should be OK as long as you're aware of how far the tolerance of the output of the PSU you're using can be. I suspect the fact that laptop PSUs seem to come in a wide variety of very-close voltages around 20V (I've seen 18.5, 19, 19.5, 20, 21, even 18.75 or 19.25) is more for vendor-lock-in reasons than anything else --- 19V is only 5% less than 20V, and a PSU rated at 20V may even end up at 19V or less under full load from resistive losses. Due to using DC-DC converters, a slightly higher voltage may even increase efficiency as there's less I^2R losses with the lower current that results.

I did check the stock laptop power supply and it came in at 20.4V, which was somewhat curious.

That's only 2% high, and if it was measured unloaded, I wouldn't be surprised.

I explained the reasons for the ~20V standard for laptops in this comment from almost a decade ago, and not that much has changed since: https://news.ycombinator.com/item?id=7594383

[myself248]

Funny, I just found myself posting this comment: https://www.udoo.org/forum/threads/why-the-narrow-12v-5-volt...

It's only been a few days but nobody's responded yet telling me not to, so I'll get on with the experimentation shortly.

And yeah, as I just showed in another comment, I just take "18v" drill packs, which are 5-series strings of lithium-ion and thus range from about 15 to 20 depending on their charge range, and pipe 'em directly into my thinkpad's input jack. It charges quite happily until they get down to about 17.5v at which point the battery doesn't have a ton of charge left anyway, and the thinkpad neatly stops drawing current and avoids flattening the pack.

https://i.imgur.com/hYRje5h.jpg

I run the exact same arrangement, with a different plug tip, into my Evolve III Maestro. It comes packaged with a 12v brick but it's experimentally happy between 10.4 and 25.1 volts, so 15-20 is right on the money.

https://www.reddit.com/r/linuxhardware/comments/tk6hdp/evolv...

[consp]

Some laptops with changable batteries have 4/6/9 cell variants. I know Lenovo has them in the 11-14v nominal (from my head) range, 4s/3s2p/3s3p, so the lower voltage spec for the DC/DC converter might even be lower than 12v.

[transistor-man]

Woah that comment is a call back, thanks for digging it up.

I think one of the major boundary conditions, aside from 25V caps are just thermal. Laptops can't really radiate more than 100W continuously, and 20V at 5A is a fairly happy balance of wire thickness (defined by current rating) and system battery voltage defined by battery chemistry. A 4-cell lithium string tops out at 16v, and a 20v to 16v dc/dc is marginally smaller than a 60v to 16v dc/dc

[LeifCarrotson]

> 25V electrolytic capacitors in the input circuitry

Note that the 25V maximum rating on the side of the cap is not a design goal: Good designs will de-rate capacitors for longer life. A typical guideline is to apply no more than 70% of an aluminum electrolytic capacitor's maximum rated voltage, 60% of a ceramic chip capacitor's max rating, or less at elevated temperatures and ripple current levels. See [1] for more guidance, or look up the manufacturer datasheet.

It's just not that expensive to install 35V, 50V, or 63V rated capacitors, and it does so much to extend the lifetime and reliability of the products. I always hate taking apart a $2000 servo drive, PLC, or TV, and seeing a blown $0.40 capacitor where a $0.45 capacitor would have been fine, hopefully it didn't take out any more expensive components when it died.

Of course, my experience is primarily in one-off lab PCBs and <100 unit industrial hardware, so perhaps consumer price sensitivity for mass-market electronics makes this guidance different...

[1] https://www.navsea.navy.mil/Portals/103/Documents/NSWC_Crane...

[nextaccountic]

What about laptops that charge from USB-C?

[ajb]

They generally use PD which they negotiate up to 20V. Theoretically they should also charge off lower voltages as well, not sure if they bother.

[pletnes]

My macbook charges on 9V phone chargers. Slooowly.

[gorbypark]

I love the “new” MagSafe charger on my MacBook Air but also love that I can grab a charge from any random USB-C charger and cable if I need to. I regularly charge from a little 18w charger that I use for my phone.

It’s enough to keep the thing going without draining the battery and will charge it overnight fully. It’s pretty great having the option.

[paraselene_]

USB-C has been great, I now daily carry a 30w pd charger that's basically the same size as those old 5w iphone ones. Works great as a fallback option in a pinch.

[jon-wood]

You can even charge a MacBook from another MacBook if you happen to find yourself in a situation where you forgot the power adapter but not a USB-C cable.

[iakov]

How do the devices negotiate who gives the charge and who gets charged? I’ve never tried it - I imagined it wont work at all!

[jon-wood]

I have no idea, but the device that was plugged into mains power was very happy to pass some of it on to my laptop.