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The default USB voltage hould have been specified to 6 instead of 5 volts.

Six (6) volts would allow for longer cables than five (5) volts do, since the spare voltage compensates for the resistance of cables. This is even more crucial for USB hubs. USB hubs are highly dependable upon these days due to laptop vendors dropping the number of USB ports down to two or even one. I am looking at you, Medion.

If several devices are connected to a USB hub, the voltage can quickly drop below 4.5 volts due to the resistance between the USB hub ports and the computer's USB port, causing some devices to restart themselves even if the computer's USB port is not over capacity. If it were over capacity, it would just regulate down its output voltage to prevent overcurrent.

Lithium-ion batteries need at least 4.3 volts arriving at the battery terminals to fully charge, and mobile devices are typically not equipped with a boost converter. Even if they were, they are rather inefficient, meaning they would produce significant heat and waste a power bank's energy. Other USB devices such as flash drives and peripherals might power off below 4.5 volts. However, 6 volts have solid 1.7 volts of margin to 4.3 volts, more than twice the margin of 0.7 volts that 5 volts have. On the way from the power supply to the end device, the voltage has to pass several barriers which weaken it, including the cable, connector endings, and the end device's internals such as the charging controller.

Sure, there are quick charging standards such as by Qualcomm and MediaTek which support elevating voltages to nine (9), twelve (12), and even twenty (20) volts. However, they require support by both the charger and mobile device. If six (6) volts were the default USB voltage, all devices would have been designed to accept this voltage, and longer cables could have been used anywhere. Obviously, all USB devices should be able to run on five volts as well.

Six volts would have been more stable, flexible, and reliable.