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XPS 15 9550 - charging at 60W with 3rd party 87W USB C PD charger.
I bought an 87W USB-C charger to use while resolving a mechanical failure of the standard charging port.
BIOS reports 60W being delivered and a check of the input current to the charger measured 0.45A @ 120V, so the BIOS is probably telling the truth.
Is there a limit to USB-C charging power on this relatively early example of PD when not using a Dell dock?
Or is this likely another 3rd party charger that doesn't meet its spec?
jphughan
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July 31st, 2019 08:00
@dave_n_s that sounds reasonable. Just be aware that using a 60W power source while the system is actually in use (as opposed to asleep or shut down) can cause the system to throttle its performance compared to how it would behave when connected to a 130W source it's designed for, or for that matter even compared to how it would behave when running off the battery. In very heavy load cases, the system will continue draining its internal battery even while connected to that power source. The reason is that a 60W source is less than half the wattage the system is designed for, and when an undersized power source is present, the system first tries to reduce its power consumption to operate within the lower power budget rather than maintaining normal performance levels by drawing from the battery to make up the deficit. The underlying assumption is that you'd rather have the system perform more slowly for an indefinite period of time than perform optimally by using a method that will eventually cause the system to die even while it's connected to power. But if you're only using that 60W source for charging while the system isn't in use and/or don't care about the performance hit, then you're good to go.
Yes, learning more about the negotiation would require something in-line between the power source and system. There are USB PD analyzers out there, in fact I believe Plugable makes one, but they're pretty expensive and their setup is rather complicated.
jphughan
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July 30th, 2019 23:00
One quick note in addition to the above. If you want to look at actual power consumption, you'd be much better off using a USB-C multi-meter to look at the output from the power source rather than checking the input. The latter is subject to inaccuracies from AC/DC conversion efficiency losses, for example, and a multi-meter on the output side will show you what voltage was negotiated. I just got this one recently and I'm having some fun with it. For example, I've found that my Latitude 7480, which is designed for a 65W source and will display 65W in the BIOS when connected to an appropriate USB-C charger with a 5A/100W cable, only actually pulls about 25-40W in typical usage. Plugable is a well-known brand known for producing high quality products and providing solid customer support if you've never heard of them, by the way. And at the same time I also decided to get a USB-A multi-meter for fun, specifically this one. I'd never heard of this brand, but I trusted the reviews and it's working fine. It's interesting to see how much power some devices draw, and what happens when multiple power hungry USB-A devices are connected to a power source that can't supply every device's maximum power level simultaneously.
jphughan
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July 30th, 2019 23:00
USB PD charging options are limited to profiles supported by both the power source and the device being charged, and possibly (especially in this case) the cable.
Since you mentioned 87W, which to my knowledge is a wattage only used by the 15" MacBook Pro, if you're using an Apple charger, their 87W charger offers some charging profiles that are not part of the USB PD spec. Even if you're using some other brand, if it's an 87W charger and therefore meant to be used as an alternative to Apple's option, it might be designed to offer the same non-standard profiles. Either way, I wouldn't be surprised if compliant USB PD devices didn't try to pull anything more than 60W in that situation, in fact I've seen that behavior reported with multiple devices when connected to Apple's 87W charger. I have no idea why Apple decided to go off the reservation there, but they did. They also did with their 29W charger, but that has since been discontinued in favor of a 30W charger that's compliant. Another fun fact about the 87W charger is that it doesn't support 15V output. That doesn't matter for a laptop, but it makes that charger less useful overall, ironically including with Apple’s own iPads that support USB PD. That limitation also existed with the 29W charger and was rectified for the 30W charger, but the 87W charger has not received an equivalent revision as of this writing.
Another possibility is the cable. Typical cables that support USB PD only support up to 3A (amps), and the 20V official max of the USB PD spec delivered at 3A results in 60W output. The next step up for USB-C cables for Power Delivery capability is 5A, since 20V at 5 amps is 100W, the current official max of the USB PD spec. Cables that support 5A advertise this capability with an "e-marker" chip, and consequently I can confirm from my own testing that if you use regular 3A cable, you will never get more than 60W even if both the power source and the device would otherwise support more power.
And a final possibility is that the XPS 15 itself might only support certain profiles, such as 60W, 90W, and 130W. That last option is proprietary to Dell because it exceeds the USB PD spec, but some Dell docks can provide 130W over USB-C, and the XPS 15 9550 can accept it -- but it's possible that it will only use those higher power levels with actual Dell power sources. For what it's worth, I don't ever recall seeing anyone getting more than 60W over USB-C with an XPS 15 system except from a Dell dock that can provide 130W. Part of the problem there is that there simply aren't many USB-C wall chargers that exceed 60W, except for Apple's 87W charger that isn't fully USB PD compliant. Nekteck makes a 90W USB-C charger that is compliant, so you could try that, but I'm not a fan of that option because it uses a captive (permanently attached) cable. But maybe the system will draw 90W from that, especially since 90W happens to be a standard Dell AC adapter wattage.
samos1111
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July 31st, 2019 01:00
XPS 15 is known to be picky regarding the USB PD voltages.This is normal.
It can accept full 130W from certain Dell USB-C docks, but this is in fact above the USB PD standard max 100W.
dave_n_s
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July 31st, 2019 08:00
jphughan,
Thank you.
This is effectively a generic USB-C charger, advertised as 87W, but marked with 90W in an oval on the body of the charger, and for what it is, with captive cable and integrated AC plug, is cheap enough for my purposes and certainly charges at 60W without getting hot and switches to almost nil current draw when a load is not attached.
My current measurement was simply to confirm that the BIOS was reporting accurately, so that measurement suffices. I do like Plugable stuff though, their USB microscope is very good.
It sounds like the key to this puzzle is how the charging level is negotiated and I'm not going to get any further until I can see what happens when this charger and this laptop are performing this process.
However it is doing what it is supposed to - it's charging in a reasonable time - and it's arguably unreasonable to expect charger manufacturers to confirm charge level negotiation with laptops that are 4 years old.
Does that make sense?