Just to eliminate the OS as a factor, does the XPS 13 charge over USB-C while the system is powered off, as indicated by the white light on the front of the system that will illuminate (or flash if it's already at 90% or higher)?  Alternatively, go into the BIOS Setup and go to the Battery Information section.  Underneath the battery graphic, the system will identify the wattage of any attached AC adapter.  What does that show when you try your various USB-C chargers?

However, I think the machine was not charging when the 80W charger was connected because I observed a 3% drop in battery SOC when I left it there for a 10 min interval.

That's very strange.  However, I admit that since I haven't personally tested an XPS 13 with a third-party USB-C charger I don't know if this is normal for this model (even though it shouldn't be) or something specific to your unit.

For the system being unable to pull 45W from the 60W charger despite the latter being rated to provide it, the only things I can think of there are either a non-compliant USB Power Delivery implementation on the charger (relatively recently a Google engineer named Benson Leung analyzed a bunch of allegedly compliant USB-PD chargers and found most of them to be non-compliant and sometimes even dangerous), or else the 60W charger can supply 45W but not via the "20V at 2.25 amps" profile.  I believe Dell systems only accept 20V charging, so if your charger can only deliver 45W by running 12V at 3.75 amps, for example, that might not work -- although that seems unlikely because typical USB-C cables only carry 3 amps anyway and special cables are required to go up to 5 amps, so I would expect basically every 45W charger to support 20V at 2.25 amps.

In terms of the 80W charger delivering 45W but your battery continuing to fall even while just sitting on the BIOS Battery Information interface, I've never seen that happen on any Dell system, so I'm not sure what to make of that either.  Any chance you have access to another XPS 13 to compare against?

Not directly related to this issue, but I'm still waiting for my ideal USB-C portable charger.  I want one that has a USB-C port that can do 65W and then 2-3 USB-A ports that each do 10-12W, ideally with the charger being able to provide max power output from all ports simultaneously, so probably ~100W max total output.  I tend to use the XPS 15 that uses 130W normally, and that system needs at least a 65W power source to charge from; anything less and it won't even try.  But my go-to company for chargers and a few other items is Anker, and thus far their only combo USB-C and USB-A charger only provides 45W out of USB-C.  Sigh....

Sorry I can't be of any further help here!

@jphughan

Thanks for the info, I will try to look up what kind of profiles my chargers support. I guess it is possible that the 20V 2.25A is not supported, although I would think that PD chargers should be fully dynamic and supply any current that is requested (below their upper limit of course).

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@sola2wrote:

 

Thanks for the info, I will try to look up what kind of profiles my chargers support. I guess it is possible that the 20V 2.25A is not supported, although I would think that PD chargers should be fully dynamic and supply any current that is requested (below their upper limit of course).

 

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PD chargers are indeed fully dynamic up to their limit on current (amperage), but not on voltage.  USB PD specifies a few voltage options, and USB PD chargers are not required to support them all -- but among the voltages they do support, they can only supply those fixed voltages, not any arbitrary voltage up to their max.

I use Xiaomi 45W USB C charger and it charges my Dell XPS 13 9350 perfectly fine. You can find it on gearbest or aliexpress and they ship globally. Costs $20 - 25. 

https://www.gearbest.com/chargers-cables/pp_624045.html?wid=1433363

https://www.hardwarezone.com.sg/review-xiaomi-45w-usb-c-power-adapter-review-i-still-cant-believe-thing-only-costs-s20

USB C chargers are a bit flaky.

I would suggest that you charge the laptop when its OFF. That's what I do. If I've to charge it during use, then I do the following. 

1. Put laptop in sleep mode. 
2. Connect USB C charger. Turn on the laptop. 
3. To disconnect, put laptop in sleep mode.
4. Disconnect USB C charger. 

I NEVER connect / disconnect the USB C charger while the laptop is ON.

In the past, I removed the charger while in use, and suddenly my laptop shut down. Later I found that my battery life was gone (damaged) and I had to replace the battery. I'm not sure if the charger caused it, but to be safe, I follow the above steps. There have been times when I've removed / connected the charger, while laptop was in use and there were no issues, but I try to avoid it as much as I can. I always follow the above steps to connect and disconnect.

Dell XPS 13 has a restriction for 3rd party chargers, it's going to say that message no matter what you use that isn't Dell, but that doesn't technically mean they won't actually work. I heard about it but was skeptical so I tried a dozen different high output chargers that supply adequate amps/volts and they did in fact all complain. A couple seemed to work when the laptop was off but you're better off just getting one from dell if you want to charge while using it.

While all major brands (Apple, Google, Samsung, Dell, etc) tell you not to use third-party chargers for their phones or laptops, it seems Dell went a step further. This is a trick Samsung used to be known for, hopefully, they stopped but I don't have one anymore to confirm.