sgs4Smartphones are now an indispensible part of our lives – you only need to watch how some people panic when their phone’s battery goes flat to see that in action!

But over the last three years, we’ve seen smartphone battery capacities at least double – HTC’s popular 2010 phone, the HTC Desire, featured a 1450mAh battery.

Today’s new Sony Xperia Z Ultra features a 3050mAh-capacity battery.

Yes, phone screens are now significantly larger as well – in the case of a comparison of the HTC and the Xperia Z Ultra, it’s getting well on the way towards a doubling in diagonal size – 6.44inches vs 3.7inches.

But the key thing that is often forgotten about smartphone battery life isn’t how long those batteries last, but how long they take to charge back up again.

So what is the fastest way to charge your mobile phone?

The average smartphone comes with a 5V/1A power adapter and combined with the phone’s battery capacity, tells you how long a battery will take to charge.

The HTC Desire’s 1450mAh battery for example being charged at a one-amp rate will take approximately 87minutes to charge. The Xperia Z Ultra with its 3050mAh capacity battery will require take 183minutes if charged at that same one-amp current rate.

Now some of these newer phones are now being supplied with 5V/2A chargers – so in the case of the Xperia Z Ultra, that 3050mAh battery can be charged in as little as 91minutes.

Charging by other means

However, there are plenty of devices offering to charge phones these days – AC power adapters, USB ports, USB battery packs, even solar panel based chargers.

As we’ve already showed, charge time is directly proportional to the charge current – the higher the current, the faster the battery chargers.

But not every device offering to charge your phone can deliver the same level of charge current.

Take the standard USB port from a PC or laptop – actually, there’s no such thing. USB ports come in different standards – USB3.0 on very new devices, USB2.0 on older models and USB1.1 if you go back far enough.

The problem with USB is that the charge current available varies depending on the device the USB port belongs to and the USB standard the port conforms to.

If you look at the basic USB standard, peripheral devices are allowed to draw current based on what’s called a ‘unit load’. For USB2.0, a unit load is 100mA (0.1 of an amp); for USB3.0, it’s 150mA. A device plugged into a USB2.0 port can extract up to five unit loads of current (500mA); on a USB3.0 port, it’s six (900mA).

But in 2007, the USB Implementers’ Forum created a specific charging port specification for devices like smartphones. A ‘charging downstream port’ type of USB port can provide up to 1500mA (1.5-amps). Apple (and other manufacturers) use a special voltage setting on the two data pins of a USB port to negotiate a charge current of up to two-amps. If the iPhone doesn’t see the specific voltage required from the charging source, it remains in the low-power charge mode. But if it does see the required voltages, it jacks up the charge circuitry to allow up to two-amps of charge current.

The majority of USB battery packs are capable of supplying at most one-amp of current. However, Optical disc maker Verbatim recently released a new range of USB battery packs into Australia with one that was the exception to the rule. The largest version – a 10,000mAh capacity type – has a maximum charge current of 2.1-amps to charge up an iPhone or iPad. Its priced at $100.

As for solar-powered battery chargers, most of the more compact models come with low-power solar cells that charge up a built-in Lithium-ion battery slowly over time and in practice, it’s the battery – not the solar cell – that charges the phone. So again, it comes down to the capacity and circuitry of the battery pack as to how high a charge rate it can provide.

Which is the fastest option?

So after all that, it shouldn’t come as much of a surprise that the fastest charge rate you’ll get is from your phone’s AC charger. If your phone only uses a one-amp AC charger, most USB2.0 and all USB3.0 ports should be able to match this level. However, if your phone uses a 2.0/2.1-amp AC charger, it’s unlikely many USB ports will be able to match that. You might be lucky with some USB2.0/3.0 ports but only if they conform to the ‘Charging Downstream Port’ specification.

The occasional USB battery pack may meet that 2.1-amp charge current level but it will only be the more expensive models of USB charger with higher-capacity batteries that will do it.

Make sure the phone is off

And one other thing – when you’re charging, just make sure your phone is powered off. If your AC charger has a limited charge current, having the phone powered up will make the charging take longer. If you have an iPad, you may have noticed it either won’t charge or only charges very slowly when connected to a USB port and powered up.¬†Next time, turn it off and let it charge – it should charge faster. How fast will depend on the charge current the USB port can deliver to your device.

But certainly, having your device turned off leaves whatever charge current is available to go all to the battery rather than having to charge the battery and run the device at the same time.

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