I often use mine with a my phone AC charger which has a USB slot. It sends more power which chargers the battery faster, it is harmless and just the same as when you either charge your phone in your computer or the wall. Lithium Ion batteries can handle the difference just fine
Usb is always 5 Volt. Wall charger send more Ampere, for example mine sends 2,1 Ampere. Standard usb port it's about 1/4 = 0,5 Ampere = 500 mA that is the usb standard current for computer port. Power is Volt x Ampere...
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More Ampere less time to charge, but it also depends on the internal electronics circuit, because if you send more Ampere to a device that for example accept only 1 Ampere, only one is used. ( it depends of the internal resistance of the charging circuits usually or regulations circuits...)
We're always working on new things
Any USB power source will push DC 5v to your PAX 2. So if you're using a DC 9V USB source, like a tablet charger, it will still only accept 5v. Using our PAX or PAX 2 AC charger, though, will allow it to accept DC 9v and drop the charge time by about an hour.
I think there are some things that are not very precise...
USB is always 5 Volts, as I said, change the current, that is express in milliAmpere or Ampere ( milli = 1/1000 ) DC is different than AC ( alternate current, like home, typically it's a sinusoidal wave)
DC is not alternate, has a positive side and negative side. + and -
Pax 2 can use both ( AC and DC), because there are circuits inside Pax unit that convert AC to DC and this is also the reason why you can put Pax 2 no matters the orientation using Usb ( that is DC current). You don't have to worry about positive and negative...
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