When USB 2.0 specifications were released back in 2000, the mobile market had yet to establish an industry standard for using USB as a universal charging solution. The original USB 2.0 allowed for a measly 500 mA of power, barely enough to charge even the smallest of devices. In response to market needs, manufacturers released charging USB ports in 2007 to supply between 500 mA and 1,500 mA. There are two types of charging ports: 1. charging downstream ports, supporting data transfers. 2. dedicated charging ports, without data support.
1,500 mA may have been enough for early generations of phones and smartphones but even that was not enough when tablets emerged. Most tablets and smartphones ship with dedicated chargers putting out at least 2,100 mA. These devices will typically not charge when plugged into a standard USB 2.0 port.
USB 3.0, release in November 2008, allowed for a maximum of 900 mA. Battery Charging Specification (Version 1.2 – December 2010), increased the power handling capability to 1.5 A but does not allow concurrent data transmission.
USB 3.1 SuperSPEED+
The USB 3.1 specification was released on July 31, 2013, allowed between 2 amp to 5 amp of charging power and introduced a faster transfer mode called "SuperSpeed USB 10 Gbps" putting it on par with Thunderbolt transfer speeds.
The USB Battery Charging Specification Revision 1.1 (released in 2007) defined new types of USB charging ports. As compared to standard downstream ports, where a mobile device can only draw more than 100 mA current after digital negotiation with the host or hub, charging ports can supply currents between 500 mA and 1.5 A without digital negotiation. A charging port supplies up to 500 mA at 5 V, up to the rated current at 3.6 V or more, and drop its output voltage if the portable device attempts to draw more than the rated current. The charger port may shut down if the load is too high.
Sleep-and-charge USB ports
Sleep-and-charge USB ports, typically colored yellow or red, can be used to charge mobile devices even when the computer is in sleep mode. Normally, when a computer is powered off, the USB ports are powered down. This prevents phones and other devices from being able to charge unless the computer is powered on. Sleep-and-charge USB ports remain powered even when the computer is asleep or powered off.
Mobile device charger standards
Away from computers, a number of worldwide organizations including the ITU embraced the Universal Charging Solution as its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger."
Currently, Apple is the only major company who does not use USB as a charging port opting to use their proprietary Lightning Connector. Despite the Lightning Connect being incompatible on the device end, the supply end is still compatible with USB.
Choosing the right power solution
Consumers looking for a charging solution should look for products offering at least 2.1 amps of output. 1 amp chargers may not charge tablets or charge very slowly.There is no harm is supplying more amps as chargers will only supply the amount of amps the device requires. To make things even more confusing, some vendors advertise their multiport products with the sum of all ports despite each port only supplying a maximum of 1 amp. Check the specifications carefully to make sure at least some of the ports supply 2.1 amps. Note that some cheap thin USB cables cannot supply 2.1 amps. If you’re having trouble charging your devices using a 2.1 amp charger, try changing the cable.
Tylt chargers and battery packs
Tylt offers several products which supply 2.1 amps of power. For autos, the Tylt Band comes in three versions with USB, Lightning, and 30 pin Apple connector. All the tangle-free Tylt Band versions include a second USB output port to connect another cable for charging two devices.
The Tylt Energi 10K is a 10,400 mAh battery pack offering 3 USB output ports with one supplying 2.1 amps for power hungry devices.