I recently purchase a Startech Thunderbolt 3 dock, model TB3DK2DPPD, for my XPS 15 9575. The specs say it can power a laptop with up to 85W using the Power Delivery specification which the laptop supports; however, the BIOS only reads the dock as a 65W adapter. After speaking with both Startech and Dell support I can't figure out why it won't use the full 85W, does anyone know if there are any settings I can change to get the extra 20W or if the BIOS has some predefined values for power adapters based on what dell sells, i.e. the BIOS will only display the values 45W, 65W, 90W, 130W etc.
First off, you shouldn't be trying to power your XPS 15 9575 that way anyway, because it requires a 130W power source for full performance. Anything less will cause it to throttle performance, sometimes significantly, and in especially high load cases or significant AC adapter wattage "shortfalls", it can even end up drawing from the battery to make up the difference. The only docks that can deliver 130W over USB-C/Thunderbolt are Dell's, because the official max of the USB Power Delivery spec is 100W, so Dell is doing something proprietary to offer 130W -- and that only works on Dell systems, because those docks say they only supply 60W to non-Dell systems. So you should look into one of two options:
#1: Buy the Dell TB16, and you'd need the one with the 240W adapter in order to have 130W available to the PC you attach to it.
#2: You may be able to save some cash by buying a TB3 dock that doesn't provide power at all and then connecting the system's own AC adapter in addition to the dock. Plugable makes one such dock, but it may or may not have the port connectivity you want.
#3: Keep that dock and attach the system's AC adapter in addition, because you won't find a third-party dock that can provide 130W.
But as for what you're seeing, under the USB Power Delivery spec, the power source provides a list of voltage and amperage configurations it supports, and the receiving device gets to choose from among those -- so there is no guarantee that a given device will select the maximum power profile offered by the source. Apple's MacBook Pro USB-C adapter, for example, supports up to 87W, but Dell systems I believe only draw 60W from it.
As a bit of extra background on USB Power Delivery in case you're interested, the wattage provided is based on the selected voltage multiplied by the selected amperage. On the former, USB PD currently maxes out at 20V. On the latter, every USB-C/TB3 cable is required to support at least 3A, but with the correct source, cable, and device, that can be raised to 5A. 20V * 5A is how you reach USB PD's maximum 100W, but that 3A "lowest common denominator" is also a big part of why you might see so many power sources, cables, and devices only providing/drawing 60W, which is 20V * 3A. So my guess is that the firmware in Dell's laptops is configured not to try to draw more than 60W from "unknown" sources, and Dell's docks seem to be configured not to provide more than 60W to any "unknown" devices. The reason amperage is a concern is because increasing amperage means more heat, and therefore carrying higher amperage safely requires thicker cabling. Higher voltage doesn't create that problem, which is why support for 20V is quite common, but support for drawing more than the baseline standard of 3A is much more rare. If you shop around for USB-C or TB3 cables, you will find that the vast majority of them only support up to 3A / 60W. The ones that support up to 5A / 100W are thicker, more expensive, and have much lower maximum lengths.
I understand that using a power adapter that doesn't supply the recommended wattage can cause the computer to throttle performance; however, as the 65w TDP of the 8705G also includes the discrete graphics card on die which I am not using in these scenarios in addition to not needing to charge the battery, I can't imagine the 45w difference in power draw would cause a significant decrease in performance or start to drain the battery if it were actually pulling the full wattage available through the PD spec. In addition, the cable that came with the dock is about a foot long and probably up to spec to deliver the power.
Your theory on the laptop being configured not draw more than 60w from an unknown source is a good one but then I have to ask why does Dell list this laptop as supporting PD if it doesn't actually support the spec correctly. Is there anyway to get Dell to confirm what is actually going on here? As for buying the TB16 dock; that would have been my first choice if it supported daisy chaining Thunderbolt devices which is a huge oversight for not including that.