Why is the power output of a power only cable on a USB Y-Cable so poor? [closed]

Solution 1:

Normally devices are only allowed up to 500 mA (at 5 V) from a standard USB 2.0 type-A port. Phones and chargers implement a negotiation protocol, such as QuickCharge or USB Battery Charging, which allows the phone to ask for more power (a higher current limit and/or higher voltage).

But this negotiation is primarily done through the USB data lines, so with an Y-cable that only connects power, your phone is only able to talk to the head unit, not to the charger.

(With the Y-cable the phone could probably draw a little bit more, but I'd generally assume that it deliberately limits its current draw if it's unable to do the QC handshake with the charger, even if the charger itself does not. These Y-cables were meant to be used with devices that were designed to expect such a connection.)