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I recently bought a laptop that is able to be powered via USB-C Power Delivery 3.0 on 100W (20V). From what I understand, PD (3.0) can be used to charge the battery, but also 'run' a device, i.e. the laptop recognises the power of the PD charger, and performance is reduced in order to cope with the lower wattage via USB-C instead of the default AC adapter. As I perceived it, it thus should not drain the battery on the way, and it should be even possible to run it off an empty battery (battery passthrough), given that it acts as a lower-power AC adapter.

Is that the case, or did I understand the concept of Power Delivery wrong and it only ever charges and discharges devices?

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  • What you are asking is entirely unrelated to USB Power Delivery though? It’s basically: “If I connect a weaker power supply to a laptop, what will happen?”
    – Daniel B
    Commented Sep 20, 2022 at 11:24
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    How a laptop (or any other device) uses a USB-C power adapter is probably entirely dependent on the device. Hence there is no answer to this question.
    – PierU
    Commented Sep 20, 2022 at 11:32
  • I thought I had asked a more general question about PD and what it is capable of. So the answer would be: USB-C PD is only a power standard giving X Watts of power to a device? And what the device does with the power depends on the device itself? I simply want to understand the concept, and how you put it, it's just a charger and has no 'features'.
    – physalis
    Commented Sep 20, 2022 at 11:45
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    To me it's an advanced adapter, as there is a negociation between the adpaterand the device about the voltage and the power the charger is able to deliver. Then it's up to the device to decide what to do with the delivered power: charge the battery, directly powering the components (battery passthrough), or whatever...
    – PierU
    Commented Sep 20, 2022 at 11:51
  • I see, then I really got it wrong - it's not the charger, but the receiving device deciding what happens with the power. I thought there was some built-in feature like a message or ID sending the info: allow for battery passthrough (or not). Thanks for explaining, that would actually be an answer to my question :).
    – physalis
    Commented Sep 20, 2022 at 11:55

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"USB charger" is a misnomer. They are just power supplies. All the charging circuitry is in the connected device1. USB is just a voltage source. What a device does with it is up to the device.

Before PD it was simply 5V with some current limit, which was usually slightly higher than the spec required and even higher with appropriate voltage divider between data lines (exact values expected varied between vendors).

With PD the voltage is negotiable and the power source can advertise its current capabilities, but it's still just a power supply. There is no battery charging circuitry inside.

1 With rare exceptions. For example OnePlus Warp Charge 30T wall wart contains actual charger. When a compatible mobile phone is connected, it can bypass phone's internal charger. This way some of the heat is generated away from the battery, allowing for faster charging.

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  • I had obviously misunderstood the concept of PD, and this cleared it up. I believed that PD had a way of 'signalling' itself how to be treated as a power source. I now understand it's up to the receiving device how to work with the power input.
    – physalis
    Commented Sep 20, 2022 at 12:03
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    I think it's a bit more than that. The device can inquire the maximum power that can be delivered and decide what to do then: if the max power is lower than the min required power for the device to operate, then it can decide to not use it at all. This was different with legacy USB-A chargers, where devices like external disks would start, and stop after a second because there was not enough power, then start again, stop again, etc...
    – PierU
    Commented Sep 20, 2022 at 12:05
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    @PierU Yes, that's correct. What I mean by "just a power supply" is that at the end of the day PD doesn't care what the device does with the power. In particular there's no circuitry in the adapter that would safely charge a Li-Ion/Li-Po battery by monitoring its voltage and adjusting the current.
    – gronostaj
    Commented Sep 20, 2022 at 12:14
  • Understood, it's still not telling the power-consuming device what to do though, and that was the crucial part for me to understand. It gives info about what it is capable of, but the device decides how to operate on the power given.
    – physalis
    Commented Sep 20, 2022 at 12:34

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