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I have a Thinkpad x270 and read that I can charge it using USB C Power Delivery and that it draws 45W. I also have a 36W USB C charger. What would happen if I combined those two?

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    Can you provide more detailed charger specs, specifically supported voltages? Or a model number.
    – Bob
    Mar 2, 2019 at 12:30
  • @Bob: It's this one: amazon.com/Cables-Portable-Dual-Port-Charger-Adapter/dp/…
    – d33tah
    Mar 2, 2019 at 12:33
  • Ok, somewhat aside from the USB-PD technical answer, I'd be wary of that charger in general - I suspect it either doesn't really support PD or only supports the easy 5V/3A mode of PD, and the higher voltages listed are only used under QC 2.0/3.0. Especially given the price.
    – Bob
    Mar 2, 2019 at 13:46

4 Answers 4

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tl;dr: Most likely, this will simply not work. Not worth trying.


Firstly, it's quite clear that it won't meet the 45 W the laptop expects. With traditional chargers (those that are a 'dumb' DC SMPS) what will likely happen is as the laptop requests more current, the voltage will start sagging until it reaches a cutoff and the laptop stops charging. Unlike phones, laptops tend to not 'test' supplies for the max current they support - they assume you're using a properly rated charger.

With USB-PD, the situation is a bit different. Since the laptop can communicate with the charger, technically it can discover the charger's capabilities. From there, it is possible for the laptop to e.g. limit charging rate by using a lower current. Whether a laptop does so or not would depend on the laptop; this is optional.

But we run into an issue: with USB-PD, it is also possible for the voltage to vary, and chargers can supply different voltages. Again, it is possible for the laptop to accept different voltages, but this is vanishingly unlikely: it requires significant additional circuitry that laptops tend to not include on the power input.

In this case, the biggest problem you'll likely run into is the USB-PD's power rules, as seen in section 10 of the revision 3.0 specification. Wikipedia has a simplified reproduction of the table, on the right.

In short: the max power you can get at 9V is 27W (3A). To get 45W, you need either 15V on 3A or 20V on 2.25A.

Your charger claims: "Type-C Output: 3.6V-6V 3A 6.1-9V 2.7A 9.1-12V 2.3A"

Most likely, the laptop only supports 15V input. It will try to negotiate with the charger, discover the charger does not support 15V, and abort. Nothing will happen.

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    Pretty easy for the poster to test if you are right. His worry was probably that something will blow up, but we all agree that this won't happen.
    – harrymc
    Mar 2, 2019 at 14:20
  • How does accepting different voltages "require significant additional circuitry"? Most of time the input voltage for laptops (and non-laptop PCs/equipment with DC-in) is just chosen to get the desired wattage over the desired cable gauge, not because of some constraint on the DC-DC converter, which usually accepts a decent (sometimes very wide) range of inputs. Mar 2, 2019 at 20:50
  • @R.. True, there's usually a buck converter involved. But AFAIK not often a boost (or buck-boost) converter. Which means that, yes, there is a range, but going too low very likely won't work. More specifically, that particular battery is rated 10.8 V nominal (6-cell, likely 3S2P). That means it'll need a charging voltage of approximately 12.6 V (Li-ion 4.2 V * 3). You'd need a boost converter to get that to charge at all from a 9 V supply. You might be able to run the laptop off a lower voltage, but probably not charge the battery.
    – Bob
    Sep 6, 2019 at 7:04
  • Now, if you fed a higher voltage like 20 V instead, it could work ... assuming it doesn't burn anything. We don't know the tolerances and it's dangerous to speculate. Try at your own risk, YMMV, etc.. Of course, PD won't even supply higher voltages unless requested.
    – Bob
    Sep 6, 2019 at 7:11
  • From what I understand, MacBooks are different and can accept a wide range of voltages, because you can even charge them with 5W input. I've noticed some Windows laptops don't allow this though—I tried a 30W input on my friend's HP laptop, but it didn't work May 25, 2020 at 18:40
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As long as the voltages agree, most likely this will work, although slowly. It would work better if the laptop was in standby mode.

Basically, all modern batteries are built with with a chip that regulates the input - they will only allow what they can handle. This is also correct for chargers which also support these smart features (except perhaps for the cheaper brands).

If the laptop has a proprietary charging port, use only the stock charger and don't try to force it.

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    This answer appears to ignore that the question is asking about a well-defined standard (USB-PD) and only discusses 'traditional' laptop charging that doesn't apply here.
    – Bob
    Mar 2, 2019 at 13:48
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Though details were provided in the comments that slightly changes the question, I'll add my personal experience addressing the title by itself.

I got a small 35W USB-PD charger as a "just in case" backup for my laptop. Then my original 85W charger burned out. I started using the new one hoping that it would be adequate for almost-idle usage, it was. Only after I started using it under a heavier load, and/or charging the battery, that I noticed it got extremely hot and started to smell. So I got a new full-wattage one. It worked ok, until it stopped delivering the full wattage. Back to Amazon it went.

I ordered another charger, even more powerful than before, upgrading the cable and supposedly the adapter itself. This one didn't deliver the full wattage right out of the box, maxing out near ~45W. Back it went.

Another try, again upgrading the cable to a certified one, and an even higher wattage adapter. This one finally took, delivering the full wattage, but only after I swapped out the certified 100W cable for a Monoprice cable.

The theme of the above story is that most adapters try to deliver the watts at the voltage required. Some even go above their own rated output, which is unlikely to help their longevity.

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    Other details: the small, generic adapter is still going strong, though it has other duties now. I was able to extend the time that it was charging the laptop by helping to dissipate some heat via an aluminum external hard drive enclosure taped to it.
    – joe
    Mar 2, 2019 at 19:50
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I have a Huawei USB C charger (from my Nexus 6P phone). It only says poutput 5v, 3A.

On my laptop (HP Elitebook 830 G5) it says input 20v 2.25A OR 19.5v 2.31A.

However, I can charge my laptop just fine with my phone charger.

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