Looking into the circuits and logic behind USB PD (in particular USB-C PD), I wonder why it is not more often done to combine a wall-plug PSU and a power-bank (battery powered PSU) into one. Would that not save a lot of space and cost? I seems to me that a topology of:

90-250VAC -> DC -> Buck-converter controlled by battery charge controller -> Battery -> Buck converter controlled by USB PD logic + capacitors

would be cheaper and smaller than

90-250VAC -> DC + capacitors -> Buck-converter controlled by USB PD logic + capacitors + Buck-converter controlled by battery charger with USB PD logic unit -> Battery -> Buck converter controlled by USB PD logic + capacitors

and maybe even more efficient in a scenario where it is not mostly used stationary? (It could even skip the battery if it is wall-powered and fully charged)

Is there any major (electrical design) reason speaking against that? Why don't we see more of these combined devices?

(Side-thought: would it not also make integration of e.g. MPPTs for solar, car chargers, etc easier? I guess that's not a driving factor for most consumers though...)

Addition: As pointed out in the further discussion, I'm talking about rather small power-banks and PSUs here. Let's say a 30-65W PSU and 30~60Wh power-bank. Also the main advantage would of course be for multi-day traveling, not so much commuting and certainly not office/home use.

  • \$\begingroup\$ What problem would that solve? The reason those do not exist may not be technical, but they may not be economically viable. \$\endgroup\$
    – Justme
    Commented Jan 8, 2023 at 18:37
  • \$\begingroup\$ That's exactly what I wonder. As I said, I believe the benefits could be: smaller, lighter, cheaper (those are assumed and what I'm asking about), and easier to handle (I hate having to always handle all the cables and boxes. I would prefer one box with AC input and USB-C output). \$\endgroup\$ Commented Jan 8, 2023 at 18:51
  • \$\begingroup\$ No reason it can't be done but probably not much demand for it. \$\endgroup\$ Commented Jan 8, 2023 at 19:03
  • \$\begingroup\$ Why would you need an MPPT for a car charger? \$\endgroup\$
    – Solar Mike
    Commented Jan 8, 2023 at 20:06
  • \$\begingroup\$ @SolarMike, those were two examples of possible separate use-cases: 1. solar panel input (MPPT) 2. 10-36V DC charging (car/truck). \$\endgroup\$ Commented Jan 8, 2023 at 20:25

1 Answer 1


The three main reasons that come to mind (in no particular order) are:

  1. cost

  2. mass

  3. size

Note: for 1) most users have a suitable power adapter already so the money can be concentrated in the output side. 2) adding effectively useless mass to a powerbank which can re-power a smart phone 3 or 4 times is not effective, adding an extra cell for more capacity is a better use of the mass though. 3) the total size is also a limiting factor for many people (not all...) as convenience is part of the decision.

Others may decide that there is a priority order to these, but it always depends on the use each person has.

  • \$\begingroup\$ Thank you for the reply. I understand you arguments for stationary (home/office) use (where the mass and size do not actually matter that much). However, when traveling, many people now carry both a charger and a power-bank around. The two devices together should have more mass, volume, and cost than if you were to combine them in the same housing. \$\endgroup\$ Commented Jan 8, 2023 at 19:56
  • \$\begingroup\$ Ahh, but when you have a charger for the laptop that also works for the powerbank then your argument fails to fly... having to have a charger for the laptop with a second charger built-in to the powerbank is a waste as I outlined above. And I did not write my answer based on home/office. \$\endgroup\$
    – Solar Mike
    Commented Jan 8, 2023 at 20:05
  • \$\begingroup\$ Ah, I misread your reply slightly indeed. Sorry! But if I understand correctly this time, this only holds if you already have a sufficiently good power supply (which I think many people still do not, I just purchased a new one the other day) and/or for short trips (where you only carry either a PSU or power-bank). To the "charger for the laptop with a second charger built-in to the powerbank": yes, you need one AC to battery and one DC battery to USB-C PD converter. However, as I outlined in my original question, I think you need more components when you have a separate power-brick and -bank. \$\endgroup\$ Commented Jan 8, 2023 at 20:39
  • \$\begingroup\$ Less components as I only need one power brick for 2 things whereas you suggest 1 built-in to the powerbank and a second for the laptop. \$\endgroup\$
    – Solar Mike
    Commented Jan 8, 2023 at 20:41
  • \$\begingroup\$ Ah, is our misunderstanding stemming from that you assume the power-bank will be unusable for a laptop? My relatively small powerbank (5× 18650 cells) is also able to charge my laptop... Also as USB-PD allows constant re-negotiation of deliverable power, the same device could deliver 100W (20V@5A) or 60W (20V@3A) in AC mode and e.g. up to 45W (15V@3A) in battery-powered mode. 45W is still enough for most laptops. \$\endgroup\$ Commented Jan 8, 2023 at 20:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.