Hi,
It seems to me you are mixing some information. A picture could clarify things.
In an USB system there is
* a host. Usually a PC. It uses "A" type connectors
* and a device. Maybe a printer or a camera. It uses "B" type connectors.
When you connect a device to a host, then first the communication speed is determind, then the device "asks" the host for a supply current. The PCcan refuse this, then the device must not draw that much current. If i remember right, then in ctive mode the PC should not refuse up to 100mA, but it may refuse more than 100mA.
Exeptions are 5V USB style power supplies with type "A" style connectors. There usually is no USB communication. If it can supply up to 1A then the supply connects both D+ and D- datalines to show the high current capability.
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I guess your charger chip is on the device side. So higher charge current reduces charging time. I'd prefer this.
So if you keep on USB standard you first have to check if there is a true host. If yes, then you have to check if 500mA is available.
If it is a 500mA supply, then 500mA is always possible...
Klaus