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As declared in the USB Battery Charging specification, a USB port can indicate that it is a Dedicated Charging Port (DCP), e.g. a dumb wall charger, by shorting D+ to D-. Version 1.1 of said spec declares that such a port must be able to supply up to 1.5 A. However, the latest (and final) version 1.2 increases this to 5 A.

This confuses me. Say a device following v1.2 is plugged into a DCP that follows v1.1, with no communication happening besides the shorted D pins, the device would identify the port as being able to supply 5 A and could happily start drawing much more than 1.5 A, which the charger may not be able to supply, or maybe it would.... and then catch fire.

How can this be in a system that is as backwards compatible as USB? Sure, nowadays with Power Delivery, we're back to having to check all parts of a USB charging chain (charger, cable and device) to get optimum performance, but surely not for safety? Am I missing something? Was version 1.1 only ever a draft, not meant for implementation?

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    \$\begingroup\$ However, the latest (and final) version 1.2 increases this to 5 A. but BC1.2 also states that a DCP is allowed to shut itself down if load current exceeds 1.5A (should be defined as \$I_{DEVCHG}\$ or something like that). Also check the load curves. A DCP can switch its operating mode to CC (constant current) at max 1.5A. \$\endgroup\$ May 5, 2020 at 20:34

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I did miss something. Upon further reading (also of this answer), I found that the spec requires a little more smarts of a DCP than I initially thought.

BC 1.2 Paragraph 4.1:

A Dedicated Charging Port shall go into current limit mode at some current within the range of I DCHG . A Charging Downstream Port is not required to go into current limit mode, but may do so at some current within the range of I CDP . In current limit mode, the output voltage of a Charging Port shall drop to a level that allows the Charging Port to continue outputting its maximum current.

The 1.5 / 5 A is the max current that a DCP is allowed to supply, not that it must supply. Chargers are allowed to be weaker than that, and are actually required to implement current limiting to prevent dangerous situations. The consuming device is expected to either suffer the voltage drop, or detect it and start drawing less.

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