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Comment by CoastalCoder | Hacker News Reader

Is it possible for a (laptop + USB-C PD power charger) to support multiple charging rates?

My Lenovo Legion 5 has a 300W charger. Caveats: (a) I'm not sure if that's input power or output power; and (b) it's not a USB-C PD connector , but my question isn't specific to this laptop.

Airplane A/C power outlets aren't willing to deliver that much power. And even if I was willing to slow-charge the laptop on an airplane or with my car, the laptop isn't willing to cap the charge rate, so it's simply not an option.

So I'm wishing for some scheme that lets me slow-charge a laptop in those situations, even if it meant having two different A/C adapters.

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Great ask. There's no reason devices couldn't negotiate lower power. With USB-PD, power sources offer Power Delivery Objects: different charge modes they can do. It's up to devices to pick.

Right now most devices pick the highest power option (that they can handle). For a variety of reasons, it would be super sweet if we could pick lower power options! Maybe it's a multi-device charger & you want to leave more capacity for other devices. Maybe you want to slow charge your battery to extend it's life.

Almost every usb-pd device can accept lower power options, if plugged into a charger which doesn't offer the maximum rate. The only thing missing today is software, is user control over the power delivery negotiation process. Very very achievable, with minimal effort.

> Very very achievable, with minimal effort.

Not with how complicated the whole stack is...

We still can't say "don't charge if other device has no AC power" even tho that info can be propagated via USB-PD...

I genuinely don't think it's that complicated. There's a perception about that, but look at the wire protocol for a couple devices & it starts to feel extremely rote extremely quick.

There's a very old very extensive USB HID spec for a ton of battery & charger telemetry things, that for no good reason was never implemented. I'm not sure if USB-PD has overlapping way of sharing data like "is the other side also running on battery?" but we have had the technical possibility of answering that (and how much charge is left what the battery voltage is, but getting low power alarms, more) for literally decades & shame on us for never having implemented it.

> I genuinely don't think it's that complicated. There's a perception about that, but look at the wire protocol for a couple devices & it starts to feel extremely rote extremely quick.

USB-PD standard is 500+ pages. I remember some early adopters (before ready-made usb-pd controllers were available) complained about that, that the PD code was far bigger than rest of their gadget code. Hell, just look at amount of PD stuff that was subtly or not-so-subtly implemented wrong.

And it's like, they decided to invent their whole own PHY and wire protocol instead of just using CAN or one of the RSXXX ones, for no discernable gain.

Sure it is easier now but not because standard is, but because chip manufacturers made chips specifically to handle that mess...

PD interposer device that can mask PDOs?

Yes!

I hadn't considered such a widget until your post, but now that you mention it I would totally buy one.

Notably, depending on the device, it may not actually pick the highest option it could handle. It may automatically adjust the voltage over time (state of charge, usually, dictates this) to compensate.

There's also an optional usb-pd PPS (programmable power supply) where the device can ask for a specific voltage with 20mV granularity & adjust the ask on the fly. This let's devices just pass through power directly to battery while still letting them finely control charge curves. Neat stuff.

But even just adjusting between, I to make up an example, 15V while charging battery, down to 9V after charged, is quite possible. I forget specifics but I almost think I remember there being a minimum amperage draw, so devices canr just ask for 20V then just sip 0.01 amps... They are required to step down to lower voltages if they need so little power. I'm not sure about this though!

Yes, this is what USB-C PD is for. If you have a laptop capable of charging at 100+ W but you plug it in to a 45W charger, it will charge at 45W and maybe give you a notification about slow charging. (Sometimes you'll run into weird exceptions where either the charger or laptop doesn't support a particular voltage or has a lower current limit for some voltages so charging power can end up lower than you'd expect.)

My Lenovo Thinkpad X1 has a specific setting that allows it to charge off a lower power that you can access through a setting in windows.

Yes, there exist many devices that support multiple charging rates. My phone allows the selection of 3 different wired charging speeds, as well as two different wireless charging speeds, selected in software.

Unfortunately, due to cost or incompetent engineering, most devices only accept either a few charging configurations, often just one.

I didn't realize this as my first laptop with USB-C ports could be charged with any USB charger, even a dumb 5v 500mA phone charger would eventually get the job done over a couple days. I just assumed everything was like this, but unfortunately not. A lot of laptops and cameras will refuse to draw any power at all unless they can negotiate certain configurations.

It's too expensive for a feature that most people don't think to look for.

Lenovo connectors does have resistor on center pin that indicates charger rating. So you could jerry-rig adapter cable that makes laptop think charger is lower power than it is (or just buy lower power Lenovo charger).

the nice thing about USB-PD is that you can connect any PD-compliant charger to any PD-compliant device, and get some charge.

so even without any software or clever switches to slow-charge your laptop, if you took a USB-PD laptop that prefers a 240W charger and plugged it into your 50W tablet charger or 20W phone charger, it would charge. it would just do it a lot more slowly than witht he high-power charger.

I suspect your lenovo works the same way - if you plugged it into a 65W thinkpad X1 charger, it'd probably slow-charge your laptop.

This is not a given, though: it has guaranteed forward compatibility, not backward compatibility.

Charging a laptop from a 20W phone charger usually doesn't work because the laptop will have a minimum voltage/current requirement, but charging a phone from a 240W laptop charger will always work.

That has been my experience as well. I've not come across a laptop that accepts a charge from a low power USB-C PD charger. I'm aware the standards allow for falling back to lower voltages but it seems laptop manufacturers specifically go out of their way to stop this. I presume to avoid issues with their battery charging system.

I have this laptop and I've charged it with USB-C PD at 90W on multiple occasions. What chargers have you tried?

Wouldn't airplane outlets just limit how much watts come out? I'm not an electrical engineer, I don't know much about these things, but I'd assume they have protection built-in to just cap out instead of blow a fuse if something draws too much power.

The trick with my Lenovo 300W notebook in the airplanes is as following.

At first, with the adapter disconnected from the notebook, I charge the capacitors inside of the adapter by connecting and re-connecting repeatedly (1-2 seconds intervals). The first 5 times the breaker trips, but eventually the capacitors are charged, so no initial high current is drawn from the socket, and the breaker is happy.

Only then I connect the notebook to the adapter. I never tried to draw too much power from the airplane socket by playing GPU/CPU intensive games.

Wish you productive flights!

Oh so this is how airplane sockets get that way.

That's not really how electricity works. Wall power is a stable supply. 120v going up & down in a sine wave.

If someone is drawing 120w - 1 amp - your only control as a power source is to try to lower the voltage (voltage potential). Problem one: that will go out of spec with what devices expect, if you try to provide 80 vac instead. Problem two: switch mode power supplies that can handle sagged voltages will just try to draw more amps to get the same net power.

There's really not a good way to "cap out" power. The device just sees voltage potential (volts) and doesn't know to go lower. Blowing a fuse is the only real indicator we have.

There is, just not for "AC wall voltage". It could simply drop the voltage and expect the other device to say start limiting current when it goes 5% or more below nominal voltage. Akin to how MPPT works on solar chargers

It "just" requires both sides to support that. Which won't happen as we already have USB-C if you want to negotiate the power usage.

We could invent other signaling strategies too. Maybe we lower the voltage by 5% as a "low" signal. Now we can yell at the device in binary, and more explicitly spell out the conditions.

Maybe the source sends S-O-S in Morse code. Maybe it sends a requested max power.

We can invent signalling strategies to communicate yes. EV chargers use HomePlug Green for example to commitcate. This could be consumerized & put into all devices, used to perform negotiations. https://en.m.wikipedia.org/wiki/HomePlug

If we wanted to go entirely analog just simple CV/CC on the power source side (meaning "keep constant voltage up till max amps, then start dropping voltage) + MPPT-like controller on the sink is enough. That's easy enough to be done on off-the shelf chips.

>We can invent signalling strategies to communicate yes. EV chargers use HomePlug Green for example to commitcate. This could be consumerized & put into all devices, used to perform negotiations. https://en.m.wikipedia.org/wiki/HomePlug

I'm kinda surprised that's relatively rare approach, in-band communication like that saves 2 wires so would technically allow USB-C to get extra 20% power boost over same connector. I think I saw it used in some solar stuff to coordinated various devices of same manufacturer.

To be ultra pedantic, 120VAC refers to the root mean square (RMS) voltage of the sinusoidal waveform present in household electrical systems. The RMS value represents the effective voltage that produces the same amount of power as a DC voltage of the same value. In the case of a 120VAC circuit, the peak voltage, which represents the maximum value of the voltage waveform, is approximately 169.7 volts. This peak voltage is achieved when the voltage swings between +169.7V and -169.7V in a full sine wave pattern.

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