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Last reply by 03-27-2022 Unsolved
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162782

Understanding Dell laptops and USB-C power sources

(UPDATE: Since writing the post below, it seems Dell may be changing its 65W max power draw limit from non-Dell USB-C power sources.  I've seen reports from owners of the XPS 15 9500 (released in 2020) indicating that it will draw at least 90W from non-Dell sources.  But what I wrote below would still be true for most of the Dell laptops in use as of this update in July 2020.  Original post below.)

I decided to write this post because there seems to be a fair amount of confusion around Dell laptops and USB-C power sources, particularly third-party USB-C power sources, including how to determine what wattage the system is drawing from them and why power sources are identified a certain way.  Hopefully this will help Dell laptop users understand how to learn more about how their setup is working and why it's working a certain way.

First off, most if not all recent Dell systems will display an AC adapter wattage readout in their BIOS Setup interface under the Battery Information section, specifically below the battery graphic.  You can access your BIOS Setup by pressing F2 during initial startup.  When using non-USB-C power sources, i.e. Dell-specific AC adapters that use a barrel-style connector, the wattage shown there will be the detected wattage of the AC adapter, even if it's more wattage than the system will actually use.  So for example if I plug a Dell 130W AC adapter into an XPS 13 9350 that's only designed for 45W, it will still identify the power source as 130W, even though it will never actually USE that much wattage.  Using a Dell adapter that can provide more wattage than the system needs will not damage the system in any way.  I've kept a Dell 130W AC adapter in my living room for years and have used it with Dell systems designed for anywhere from 45-130W and it's been fine.

With USB-C power sources, the wattage readout you see there is a bit more complicated.  The reported wattage will be the maximum wattage that particular system will actually draw from that particular power source.  That might sound simple enough, but it isn't quite so simple, partly because of a restriction around USB-C charging that I've found implemented on multiple Dell systems.  Basically, as of this writing, multiple Dell laptops I've tested will not draw more than 65W from a non-Dell USB-C power source, even if the power source can provide more wattage and the system is designed for more wattage and would therefore benefit from drawing it.  In addition, the USB-C cable you use can matter.  USB-C cables that support USB Power Delivery are available in two versions: up to 60W (sometimes written as 3A, or amps) and up to 100W (sometimes written as 5A).  The latter are typically thicker in order to carry the extra power safely.  The relationship between watts and amps here is based on the current 20V max of the USB PD spec.  Citing amps is technically more precise because the higher amperage requirement is what drives the need for thicker wiring, but most customers think in terms of wattage.  Anyhow, if you use an "up to 60W" cable, you will never get more than 60W, even if you have a USB-C power source that can provide more wattage and a system that would otherwise draw more.

So let's look at some examples.  All of these assume USB-C power sources (as opposed to the barrel-style Dell AC adapters) and that you're using an appropriate cable for the wattage involved.  They also assume that you're using a USB-C source that can supply 20 volts.  Some lower wattage sources only supply power up to 15V, and although some laptops will work with such sources, others will not.  I didn't test such chargers.

Case #1: An XPS 9350, which is designed for 45W.  If you connect a 30W power source, the system will show 30W since that's all that's being offered.  If you connect a 60W power source, it will show 45W because that's the most the system is designed to draw.

Case #2: A Latitude 7480, which is designed for 65W but will take advantage of 90W when available.  If you connect a 30W source, it will show 30W.  If you connect it to a 65W source, it will show 65W.  If you connect it to a Dell dock that can provide up to 90W, it will show 90W.  But if you connect it to a non-Dell 90W power source, it will show 65W, even though other non-Dell systems will correctly show that power source as 90W.

Case #3: An XPS 15 9570, which is designed for 130W.  That's actually more than the official 100W max of the USB Power Delivery spec, but Dell has done something proprietary on this and some other 130W systems and certain docks (like the WD19TB) to stretch that spec to allow carrying 130W over USB-C.  They also make a 130W USB-C power adapter.  If you connect this system to one of those devices, it will show 130W.  But if you connect it to a non-Dell 90W power source, it will still only show 65W.  This is frustrating because this particular system would actually perform better if it drew 90W rather than 65W and is still refusing to do so.  In addition, I believe (but have not confirmed independently) that the XPS 15 models will not work at all with any USB-C sources that supply less than 60W -- although even a 60W source would be inadvisable overall.  Battery charging will be slow to non-existent and CPU and GPU performance throttling would be significant as the system attempted to operate within the constraints of a power supply that offered less than half the wattage it the system was designed to use.  A 60W source would be fine to use purely for battery charging while the system was asleep or off, but would not be a great choice while actually using the system.

Hopefully this helps people understand how Dell systems identify and work with USB-C power sources!


Replies (61)
2 Bronze
2 Bronze
144068

I'm considering purchasing a large quantity of non-Dell USB-C monitors for my organization; however, the power delivery is only 45W while our standard issue Latitude 7390 2-in-1 laptops draw 65W. Your article is super helpful but it's still unclear to me if the 45W over USB-C will charge our laptops or if the monitors are incompatible or, worse, potentially damaging to the laptops. Would you clarify?

144082

@Broccoli Cobalt  Glad you found my post useful!

If your laptops are designed for a 65W power source, then you shouldn't be considering USB-C displays that only provide 45W unless you also plan to keep the laptop's AC adapter directly attached as well.  Otherwise, while you won't DAMAGE anything by giving your laptop an undersized power source, you will create a subpar experience for your users.  First, the laptop will display a warning about an undersized power source every time it boots while connected to one, which will be an annoyance.  But worse, if a laptop designed for a 65W power source is only fed 45W, the laptop will try to operate within that reduced power budget by doing things like reducing its battery charging speed and reducing its CPU and GPU performance.

There are several USB-C displays on the market now that provide 65W or even 90W (although as mentioned above, as of this writing you won't get more than 65W over USB-C to a Dell system unless it's from a Dell source).  I would recommend that you consider one of those options.  Also keep in mind the note above about the USB-C cable coming into play if you want to go above 60W.  If the displays you buy supply more than 60W and come with a USB-C cable, then it's a safe bet that the included cable will be rated for up to 100W.  But if you'll be buying USB-C cables separately, you'd want a USB-C cable that supported DisplayPort Alt Mode for video and USB PD up to 100W, like this one.


144076

@Broccoli Cobalt  CORRECTION!  I initially linked to the wrong type of example cable in my post above.  I've edited that post to link to the correct product.


2 Bronze
2 Bronze
142722

Hi, thanks for your insight. I've bought a Dell Latitude 3400, but I don't really want the expense of a few hundred £'s on a Dell dock too. I want to try a few cheaper alternatives (https://www.amazon.co.uk/gp/product/B07F2561H1https://www.amazon.co.uk/gp/product/B07MKC5Z1D/) but I need a USB-C charger, can I get anything that will supply 65, 70, 90 watts? Something like this? https://www.amazon.co.uk/Adapter-Charger-Ponkor-MacBook-Laptops/dp/B07Q9LRHH9/

142710

@Smeg Head  This thread is purely about how Dell laptops handle USB-C power sources, so I'm not sure why you're posting about docking stations and "mini-dock" products here.  If you want a "mini-dock" product recommendation for your needs, then I would suggest that you create a new thread of your own that describes what system model you have and what device(s) you want to be able to attach through this device.  You'll get more visibility for your question and you'll avoid taking other people's existing threads off-topic.


142707

@Smeg Head  Actually I just re-read your post and realized the only part you were asking about was the power source.  In that case, it seems the Latitude 3400 is designed for a 65W power source.  But if you'll be connecting it through a mini-dock, make sure that it supports providing at least that much to the attached system.  Some only pass through at most 60W because you need different cabling to exceed 60W, and some mini-docks might not incorporate that for their connection to the system.  Also be aware that mini-docks tend to "skim" some power from the attached power source when one is available in order to run their internal electronics and provide power to USB peripherals that are attached to them.  When the mini-dock does NOT have a power source attached, it would draw from the system, but the dock can't provide power to the attached system and draw from it at the same time.  The end result is that plugging a 65W power source into a mini-dock does not necessarily mean that the system will see a 65W power source.  It might see a 45-55W source instead.  So if you want to minimize the chances of warnings about a "Slow USB charger", you might want to look at a 90W power source to make sure you have enough capacity to pass through 65W even after whatever the mini-dock skims from that source.

And then you'll need a USB-C cable rated for up to 100W Power Delivery because most USB-C cables are only rated for up to 60W, and if you use a 60W cable, then that's all you'll ever get even if the power source could otherwise provide more and the device you're trying to power would accept more.


3 Silver
104107

Well, I tell you what...I bought my system...and I bought an additional power cord/charger/etc. Neither worked well from day one. I kept getting messages that said I was using a charger that didn't have enough power and that my laptop would drain battery power faster than it could charge! I was using Dell chargers! The one that came with it and the same exact one they sold me. I've replaced both...twice!!!

Seems to be a Dell laptop and charger problem.

142653

@jphughan Thank you. The one device I linked to seems to be 100 watts and it lists the Mac book Pro and some 15" laptops, and Dell Inspiron and Latitude laptops. The second quotes 60 watts output with the reduction due to the what the dock uses. If I buy a charger which delivers something over say 70 watts, that should then supply the dock and the laptop whichever I use (maybe better to utilise the higher wattage and buy a 90 + charger?), my query was more will a generic charger work, and Dell laptops don't require a Dell dock, it's a standard they should all adhere to?

142634

@Smeg Head  Dell laptops that support being charged over USB-C support that using the industry wide USB Power Delivery standard.  But it seems that they do place limits on third party USB-C power sources, for reasons that aren't clear.  But yes, they do work, at least up to 65W (or the max that the system uses if lower, such as the XPS 13 that only uses 45W anyway).  I personally have USB-C power sources from Anker, Nekteck, and Satechi with outputs ranging fro 60-90W, and my various Dell laptops work fine with all of them -- again, up to either 65W, the charger's max output, or the system's max draw, whichever is lowest.

As to the devices you linked, the one that only passes up to 60W might not be because it's account for skimming losses from a higher wattage source.  60W is a fairly common maximum limit to see with USB-C cables because it corresponds to 20V (current max of the USB PD spec) at 3 amps.  Currently, there are two "classes" of USB-C cables, namely those rated for up to 3 amps (60W) and those rated for up to 5 amps (100W).  Cables that support the latter need to include an "e-marker" chip to advertise that capability.  So I suspect that the 60W limit in the case of that adapter relates to its permanently attached cable.

As for a Dell dock, certain Dell systems like the XPS 15 are designed for 130W power sources, and Dell did something proprietary with systems like that as well as certain docks in order to send that over USB-C/TB3, stretching past the 100W max of the official USB PD spec.  So if you want that, you'd have to use a Dell dock.  And then Dell also has a WD19DC dock that plugs into two USB-C connectors on a system in order to provide up to 210W in order to properly support laptops that have even higher power requirements.


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