is it possible to use a power bank after XPS13 is powered on?

@mf22  Typically this behavior is managed on the power bank itself.  I know that on power banks made by Anker, if you connect them to a running system, they default to drawing power to charge themselves from the system, but if you press a button on the power bank within a few seconds of making that connection, the power bank will switch to providing power to the attached system.  Did you check the documentation for your power bank?

thanks for the info. The Anker PowerCore+ 26800 PD explicitly mention it supports XPS 13, so I believe it should work. unfortunately,  the power banks I own do not work with the "Anker method", and I think they don't have a manual at all. (to my understanding, this kind of products are so simply that they don't need a manual. The Anker one doesn't have a manual on its website too.) previously, i thought any power bank that supports USB PD 45W (20W x 2.25A) would work, obviously, it is not the case. 

@mf22  The issue doesn't seem like it relates to the power output it supports, but rather the power bank not being clear which direction you want power to flow at any given time, since when a power bank is attached to a laptop, there are times you'd want it to go each way.  The fact that a power bank supports 45W doesn't mean it will always provide 45W whenever it's connected to a system that would use that -- because sometimes you might actually WANT to charge your power bank from your laptop.

The fact that the power flow switches direction when you power the system on sounds reasonable and normal.  If you connect the power bank to a system that's sleeping or powered off, in most cases the system's USB ports won't provide power to attached peripherals in that condition, so the only real power flow option would be for the system to draw power from the power bank.  But powering the system on causes a new USB PD negotiation to occur, and when the system is powered up, it of course WOULD be able to provide power to attached peripherals.  So it sounds like the power bank defaults to DRAWING charge when that option is available, in case you want to charge the power bank -- but there should be some way to switch that back.

With Anker power banks, there's a button you have to press to wake up the power bank regardless of which direction you want power to flow.  Then if you connect it to a running laptop, it defaults to DRAWING power from the laptop to charge itself, as yours seems to.  But if you press the Wake button again within a few seconds of making that connection, the battery pack reverses the flow.  Does your power bank not have any sort of button at all that you might be able to press to flip the power flow direction when you want to do that?  If not, then I'm not sure what to suggest other than the workaround you seem to have found.

@jphughanThank you for your thoughts. Your information is definitely useful.

I looked up a random USB PD negotiation document from TI to get some proper terms. When the laptop is powered on, both the laptop and power bank can be the sink or source. I didn't study in details the negotiation process. is it supposed the power bank's job to decide the flow of power? if it is the industrial standard/practice, then it is the fault of the designer of my two power banks. My power banks ain't those cheap cheap unnamed mainland China product. I've just sent email to ask for their technical support. I'll update this thread if they have positive feedback.

if there is no standard/industrial practice, as a user, it seems to me a power bank is a low cost dump device, and I don't expect it has any intelligent to decide the flow of power. On the other hand, a laptop, is a computer, I would expect it has the API and utility software to do the control, and maybe allow me to run a util and click a button or update a conf file to disable power output of a USB port when it is running. If it is not 2020 XPS 13 that have such feature, I wish the 2022 version would support it. 

edited:
1. Anker PowerCore+ 26800, at page 10, it mentioned about pressing for 2 seconds to "use as a hub"
https://d2211byn0pk9fi.cloudfront.net/spree/accessories/attachments/74048/B1362manual_.pdf?1539934376

2. with further test, one of my power banks, a Verbatim PD, actually does support reversing the flow of power by long pressing the button (of the power bank). My unit isn't 45W, and probably because of this, it fails to negotiate a wattage that works. But I guess a 45W Verbatim PD should work. (I use a USB power better to see the effect, the flow of power is reversed, but it is charging at 0A, i.e. not charging)

3. I actually have some other USB PD 45W/+ power bank to test. The one that doesn't work is MagicPro ProMini PM15. 

@mf22  Yes, it would be the power bank's job to influence the flow of power, because the power bank can control what capabilities it advertises.  When you first connect the power bank to the laptop, it could simply decide not to advertise the fact that it can act as a power source in order to cover scenarios where you actually do want to charge the power bank.  And if you activate whatever mechanism reverses the power flow, then the power bank WOULD advertise that it's a power source.  The fact that the laptop is more complex and expensive doesn't mean that it should be the one to control this.  First of all it would be more complicated to manage it from that side since there's no button for this on the laptop, and it would be a pain to have to jump around in Windows to manage this compared to just pushing a button on a power bank.  And second of all, it's just simpler to have the design I described where you can toggle whether the power bank advertises itself as a power sink or a power source.

@jphughan I'd guess so but given I'm not the OP I can't comment further.

Personally the thought of charging my power bank through the laptop has never crossed my mind. The USB ports only give out 15W of power at most, which will take hours to charge up a power bank fully. 

But yeah, I guess a possible issue with reversible charging is to decide who gets to charge who. I know that sometimes, it might be decided by battery level or wattage?

@describee  I wouldn't want to charge a high capacity power bank from a laptop either, but some people use regular USB-A wall chargers even for high capacity power banks, and USB-A wall chargers often provide only 5-12W unless they support a proprietary standard like Qualcomm Quick Charge, but that would require support from the power bank too.  So for those people, 15W from their laptop would be looking pretty good.

@describee  Ok, so maybe those have a different default behavior configured in their firmware?  But that doesn't really solve the issue overall, which is that when you connect a power bank to a laptop, there are some times when you'd want power to flow in one direction, and some times you might want it to flow in the other direction.  So for those power banks that default to charging the laptop even when the laptop is already powered on, what do you do if you actually want the laptop to charge the power bank?

I just tried charging on the following power banks:

- 30w Aukey PB-y7

- 45w Xiaomi power bank

- 65w zmi power bank

All three power banks seem to charge the laptop even when I plug in the cable after the laptop has been powered on.