I purchased two of these Samsung OEM 5V 1A chargers from Amazon after having purchased 2 .7 ma chargers elsewhere.
I don't know how much power draw there is from background apps running while a phone is charging but I'm thinking that it can be close to the limited 500mha a P/C is capable of delivering. Most PCs will automatically dispense 100mha immediately upon communication with a USB device until an electronic "hand shake" is established at which time the PC will know what kind of draw the device is demanding....up to the rather limited 500mha hour the USB circuit is capable of delivering. The 1A chargers did not double the charging times of the 500ma, they quadrupled the charge time as the 500 is having to overcome whatever background apps are running then use whatever is leftover to charge the phone. The 1A chargers deal with the S3s resting state demands and all the extra, up until the battery is taking at it's limit rate which is everything between its resting rate and your 890ma and/or the chargers 1A capability. Pick up a couple of these chargers guys. They are very good. And they are stainless steel instead of coated tin. Very sturdy.