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Use 2x5V from USB ports to speed up device battery charging? Do we get enough current (1A) from this 2 USB ports as well as from AC charger? I assume that voltages need to be the same, but what'll happen if they are not?

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  • \$\begingroup\$ There is "faster" and "more current". If your device draws less than what your USB port is capable of, there is no point in trying to increase the current capability of the power supply, it won't make it any faster. Certain chargers do not have the features allowing the full 500mA of USB; then you would need more current but your first try should be to enable maximum current. Not sure how you can double the current capability, it's not as simple as 2 diodes in series with cathodes connected. \$\endgroup\$ – Mister Mystère Jun 11 '15 at 9:50
  • \$\begingroup\$ I saw a few devices which are using 2 USB ports as power supply, thats the reason why I'm asking this. \$\endgroup\$ – user76920 Jun 11 '15 at 9:52
  • \$\begingroup\$ It might be as simple as a parallel connection if the voltages are exactly the same. Each output has its own current limiter, so theoretically after enumeration you can have up to 500mA (100mA without successful enumeration) from each, giving 1A total. Incidentally, you can get up to 1.8A from a wall charger with the right resistance between D- and D+. \$\endgroup\$ – Mister Mystère Jun 11 '15 at 10:01
  • \$\begingroup\$ Can you provide a block diagram or part numbers of whatever parts you are considering using? There could be some difficulty involved in using higher charge current. How do you communicate to the charger that more current is available? \$\endgroup\$ – mkeith Jun 11 '15 at 22:19
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    \$\begingroup\$ It's also important to consider that unless your device negotiates with both USB ports for the full 500mA each, then you are only allowed to draw at most 100mA from each port (200mA total) \$\endgroup\$ – Tom Carpenter Jun 15 '15 at 22:21
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Supplying a device from two USB ports (but at one PC) is very common with a Y cable:

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You have some idealized view to the modern products, in our reality:

  • rare mainboard implements current limiting capabilities (beginning from the design); and

  • yet rare mainboard uses such capabilities even it implements them.

Typical mainboard has a simple PTC resettable fuse in series of the VBUS circuit. Frequently, one fuse is used to protect two ports, e.g. in a backpanel 2xUSB+something jack. Therefore, the greed degree of a device is a good will of that device. An intelligent charger may limit itself if it fails to negotiate with the host, but again, rare host can intelligently limit it.

Also, a non-charging USB 3.0 socket should solely provide 0.9 A and is backward compatible with same type USB 2.0 plug.

To charge faster it is preferred to use a shorer cable and/or a cable whose power pair has a larger square resulting in lesser resistance in series between the power source and the burden.

Relating to your scenario, of course, not in all possible cases, the different voltage problem exists only with supplying from two different sources (supplies) and the only right way to solve the problem is to avoid such connections for power feeding.

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