Problem in circuit that charges tablet/cell phone via Micro-USB port
USB chargers usually aren't chargers in the sense that they control the charging of the device.  They really just supply current at a specific voltage, and the device does all of the complicated parts of actually charging the battery.  So, all the so called "USB chargers" are really just 5 volt powersupplies.

USB does have one wrinkle, though.
USB is only allowed to provide a very low current until the device and the host have negotiated over the data bus for a higher current to be supplied.  Well behaved USB devices therefore will not draw a high current from USB port.

Since your powersupply can't negotatiate with the USB device, the devices draw only low current from the supply - it takes a long time to charge or doesn't charge at all.

The good news is that there are standards that make it possible for a USB device to recognize a "dumb" USB host that can provide high current.

The simplest is to just short D- and D+ together.  The USB device will then recognize that you have a powersupply that can deliver high current with out negotiation.

There are other standards (there's 200 ohm from D- to D+ and there's a slightly more complicated Apple standard) but many devices work with the shorted D- D+.



This site goes into some detail on implementing the various standards.