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How to charge faster - Powering your tablet, phone & more

Richard Easton
6 May 2015
Smartphone USB charging
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Everything you ever wanted to know about charging but were afraid to ask

While the humble USB port, or Universal Serial Bus to use its official name, has been with us for decades, it’s only in recent years that it has become the default option for suppplying our mnay gadgets with much-needed power. Whether they’re smartphones, tablets, portable media players or numerous others, charging our devices is much simpler than it used to be. Now, with the new MacBook USB charging is coming to laptops, too.

USB power can be supplied from a number of different sources such as the USB ports on your PC or even your television. Then there are innumerable USB adaptors that most of us have hogging our sockets around our home, many of which came with old devices that are long forgotten. Not all these USB ports and adaptors are created equal, though, and not all devices require the same power draw.

What this means is that a high draw device, such as a tablet, might not charge from the USB port on your computer, or it might charge extremely slowly using the charger supplied with your smartphone. The reason behind this is largely a matter of amperage.

Getting amped

Amperage is the measure of the amount of electricity flowing in amperes (more commonly referred to as 'Amps'). Voltage, on the other hand, is the measure of the force of the electricity. A common analogy to wrap your head around is to think about a plumbing pipe and equate the amperage as the flow rate and voltage as the water pressure. The third variable is the level of resistance measured in ohms, but this isn’t completely relevant for discussions around charging. By multiplying the voltage by the amperage, you also get a wattage rating, which is a measure of the electrical power.

USB ports type A and type B

Power Sources and Adaptors

The main differences in USB charging sources, be they ports or wall adaptors, is the amount amperage and voltage supplied. The supplied iPhone USB wall adaptor, for example, is rated at outputting 5V and 1A. The charger for the iPad provides 5.1V and 2.1A, due in part to the iPad’s significantly larger battery. Then there are other fast-charging standards such as Qualcomm’s Quick Charge, which works with a number of smartphones using specific Snapdragon processors. Quick Charge is able to output up to 9V, 2A providing a charge up to 75% faster than a 5V, 2A charger, so you battery fills up far quicker.

A standard USB port on a computer typically outputs far less power. A USB2 port can be rated as low as 500mA (0.5A) whereas a USB3 port can output up to 900mA (0.9A), both providing 5V. These are the standard ratings, however, and some motherboard manufacturers might include special features designed for faster charging or charging high draw devices, and these can provide upwards of 1.5A. Asus’ Ai Charger for example can enable up to 1.2A over the USB ports whereas certain USB ports on a MSI motherboard might support its Super Charger function. Certain motherboards will even allow you to continue charging over USB even when the system is shutdown or in sleep mode.

USB hubs can be a bit of a mixed bag. Even hubs that have a separate power adaptor, meaning they need to be plugged into a wall socket, still might only provide the standard 500mA from each USB port or only one of the ports might provide a higher amperage. You’ll need to check the hub’s specifications and documentation to be sure. Then there are also portable battery chargers (also sometimes called ‘power banks’). Many will have two USB ports with one rated 1A and the other at 2A. As you would expect you’ll get faster charging from the 2A port, so prioritise using that and use the 1A port for when you need to charge two devices simultaneously.

What will also affect charging speeds is the device you’re looking to charge and how much power it’s designed to draw. Connecting a tablet that is designed to only handle 5V/1.5A to a 5V/2A USB adaptor won’t make it charge faster than it can safely handle.

Wireless Charging

It’s now possible to do away with the need to connect wires through wireless charging. By far the most popular wireless charging standard (also known as inductive charging) is the Qi standard (pronounced ‘chi’). Compatible smartphones and tablets are available from manufactures including LG, Samsung, Nokia, Motorola and Sony. There is also a rival standard known as Rezence from A4WP (Alliance for Wireless Power). A4WP will also soon be merging with PMA (Power Matters Alliance) to create a stronger competing standard to Qi.

Wireless chargers typically consist of a wired charging pad, which you then place the device you want to charge on top. It’s faster and more convenient than having to connect a cable to your device. The same power principles apply to wired charging devices with wireless chargers providing different wattages. The ‘low power’ Qi specification for example provides up to 5W of power. Chargers like the optional Google Nexus Wireless Charger are able actually able to provide 9W at 1.8A/5V. This should actually charge a Nexus 5 faster than its included USB wall adaptor that is only rated at 1.2A. There are also ‘medium power’ (120W) and soon to be ‘high power’ (1KW) specifications, although the latter is designed for high-power utilities such as kitchen equipment.

Rezence technology, on the other hand, is an exciting prospect as it uses magnetic resonance rather than inductive charging used with Qi, which requires some intricate lining up between your device and the pad to ensure charging happens. Rezence is far more forgiving. It will also be a technology worth keeping an eye on as its specification will allow for up to 50W, meaning it has the capacity to charge and power laptops.

Intel is a member of A4WP and has stated that it expects laptops supporting the wireless charging standard to appear within a year, which means your desks can finally be free of some of the clutter of cables. The Rezence charging pad can also work through reasonably thick surfaces as well, meaning it’s far easier to create a DIY wireless charging surface by simply mounting the charging pad to the underside of a table or desk.

Why does my iPad take so long to charge?

Calculating what different wattages mean largely comes down to maths. We’ve used an iPad as an illustration of how charging times can be affected.

 CurrentVoltageWattageTime 
USB Port500mA5V2.5W24:00
iPhone Charger1A5V5W12:00
iPad Charger2.1A5.1V12W5:00

As you can see, using the supplied higher wattage iPad charger is going to be the optimal way of charging your iPad. Using a PC USB port will take a very long time indeed while an iPhone charger will get you there but it will take more than twice as long.

However, while the iPhone charger is rated at providing 5W, in testing using a wattmeter, we were only actually getting a 3.7W power draw while charging the iPad with the tablet turned off. This will also contribute to a slow charge speed. With the iPad actually turned on it will begin drawing 6.7W but this doesn’t mean it will charge faster as some of the additional power will be used to actually power the iPad without also charging the battery.

In contrast, while charging using the iPad’s 12W charger, it managed to draw 12.7W both with the iPad turned on and turned off. The other interesting thing we found was that you’ll want to specifically use the iPad’s 12W charger to get faster charging for your iPad. A similarly-rated adaptor might not get the same charging speeds. For example, using a 5.2V/2.1A charger from an Nvidia Shield Tablet, which should have a slightly higher wattage than the iPad’s adaptor, the iPad behaved identically to if it were connected to a an iPhone USB adaptor, drawing 6.7W when powered on and 3.6W when powered off, meaning charging time is almost doubled.

Connecting an iPad to a PC on the other hand will result in the iPad reporting it’s not charging when the screen is on. With the screen off, it will actually charge, albeit incredibly slowly. You’ll also want to turn the iPad off entirely to help speed up the process but you’ll still need a lot of patience.

How much does it cost to charge an iPad?

The cost to charge your device doesn’t actually fluctuate based on the charger you use. Using the iPad as an example again, the average cost of charging an iPad across a year was found to be $1.36 (about £0.89) a year by the Electric Power Research Institute and this number won’t change based on the charger you use as everything scales. Higher wattage requires less time, and lower wattage requires more time but it averages to the same cost. 

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