[gnabgib]

The USB standard only allows 7.5W (5V @ 1.5A) of power. By negotiating over the data lines, the supplier and consumer can agree to higher amperage and voltage (up to 100W in USB3, 240W in USB3.1) - but you need data lines for this feature.

Some USB condoms include a chip to do this negotiation (with the other device) for you - but you still have to trust the chip.

You may very well have experienced this with a very basic USB cable (with just the power lines) - people call them cheap or bad quality, but because of the lack of data lines - only 7.5W can be delivered.

https://en.wikipedia.org/wiki/USB

[mrb]

This is incorrect. USB power delivery does not need the data lines. Negotiation happens over the CC line and Vbus line:

"A power-only receptacle Upstream-Facing-Port might only have VBus, GND, and CC pins populated, because they do not need the data transfer capabilities" source: https://acroname.com/blog/breakdown-all-power-delivery-types...

[dragonwriter]

> The USB standard only allows 7.5W (5V @ 1.5A) of power. By negotiating over the data lines, the supplier and consumer can agree to higher amperage and voltage (up to 100W in USB3, 240W in USB3.1) - but you need data lines for this feature.

Per the USB 3.1 power delivery spec [0] all communications related to power delivery occurs over the CC wire (with roles chosen according to Vconn), the data wires are not involved.

[0] https://www.usb.org/sites/default/files/USB%20PD%20R3.1%20V1...

[PrimaryAlibi]

So the charger won't work without the data wire and it could destroy the laptop. It's so crazy because I've seen in these tech communities people saying it's recommended to cut the data wires and everyone is upvoting it. I guess that's another popular misconception going around that it's generally fine to cut a data wire.

[gnabgib]

The charger will work, just at a low/slow power. No destruction, unless it's non-conformant (the source should only increase the volts/amps if it detects the correct signalling from the drain... and this should is defined in the USB certification specs).

You may want to charge without a data-wire, or use a cable with a correct power-negotiation chip if you don't know/trust the source (eg a charging nook in a library/school/bar/airport.. anywhere public). Some devices are very trusting of power sources, or have been (security is improving, modern phones require unlock before they even acknowledge they accept/send data).

[nucleardog]

> You may want to charge without a data-wire, or use a cable with a correct power-negotiation chip if you don't know/trust the source (eg a charging nook in a library/school/bar/airport.. anywhere public).

There's an alternate charging interface you can use that's pretty widely available and I'd highly recommend--the 120/240VAC outlets all over the place!

Yeah yeah, I'm only half kidding. If you're going out to the bar you're probably not gonna shove a USB charger in your pocket. But in most of the rest of those situations (library, school, airport) and more you _probably_ have a few things you're carrying with you. Just leave a small adapter and cable rolling around the bottom of your bag and you don't have to worry about this. (Or at least you're into the realm of _wildly_ theoretical attacks.)

This doesn't just avoid the potential security issues... A lot of those charging lockers and things are not exactly well designed or well engineered. If you use your own charger you also know some weird cheap out-of-spec setup isn't going to damage your phone and there won't be any incompatibilities with the charger/cable/device that leave you charging at 7.5W.

[Faaak]

USB PD without signaling won't work. It wont supply the needed voltage (e.g. 19V), and the laptop won't charge

[yread]

Is that true? I have a cable here that I use to charge my laptop with 65W PD but it doesn't make a data connection. Does it do some black magic?

[Kirby64]

There’s data, but then there’s also the “CC” pins. CC is mandatory for USB-C. It is what does the communication for PD. So, it’s data, but a very specific type of data.

[cesarb]

> I have a cable here that I use to charge my laptop with 65W PD but it doesn't make a data connection. Does it do some black magic?

The magic is that USB-C has not one, but _several_ mostly independent "data" connection wires. Chargers normally do not use or care about the USB 2.0 data channel (or the separate USB 3.0 data channel), they only care about the separate "configuration" channel used for USB-PD negotiation; IIRC, according to the standard pure chargers are even supposed to short together the USB 2.0 wires, to signal to older USB B or micro-B devices "I'm a dumb passive charger which can provide more than just 2.5W of power".

So, if you have a broken cable which does not have the USB 2.0 wires connected (which AFAIK is not allowed by the standard), but has the power and configuration wires correctly connected, it might (or might not) work as a charge-only cable.

[nixosbestos]

> I've seen in these tech communities people saying it's recommended to cut the data wires and everyone is upvoting it

Right, and what communities are those, exactly?

[dragonwriter]

No, the charger will work fine – and at full power. GP is incorrect, the data lines are not used in power negotiation.

[marcosdumay]

The data lines weren't used for charging until fairly recently.

Al of those people may not be up to date, or you may be seeing old discussions.

[cesarb]

> The data lines weren't used for charging until fairly recently.

Several proprietary protocols (like Quick Charge) used the data lines to negotiate the power and voltage, then USB Battery Charging standardized a way to indicate being a charger through the data lines, and that was all before USB-C. So unless you were satisfied with very slow charging, the data lines were always necessary.