Is it safe to try charging my laptop with a USB-C PD charger that has less wattage than recommended?

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?

Solution 1:

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.

Solution 2:

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.

Solution 3:

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.