There's no more tragic first world problem than a flat battery on the commute or, worse, while halfway up a mountain.
However, portable batteries are now getting small enough to stow in a pocket or bag so easily – and cheaply – that it's worth carrying one, if only for emergencies. Choose carefully and know what you're buying.
Smartphone batteries, and those fit for using with a tablet, can be different. The former are usually small, and feature a USB output for fuelling the 5V battery of a phone, or almost any low-power device.
But not a tablet. Tablet batteries require a 2.1 amp-rated USB output if a recharge of their 12V batteries is to be successful. Many batteries have both types of USB, and so claim to be offer 'dual charging' or 'super-fast' recharging. Don't be fooled: without that second 2.1-amp USB they're useless for tablets.
To help you make your mind up, here's our pick of the best portable batteries around.
1 Powermonkey Discovery
This is easily the best-looking portable battery. With its a 3,500mAh lithium polymer battery, the Powermonkey Discovery from UK company Powertraveller seems a perfect fit for the latest smartphones, but its sole USB slot is a 2.1-amp version, opening it up to the world of tablets, too.
About the same size as an old Nokia handset from a decade ago, the Powermonkey Discovery's aluminium styling is thoroughly up to date.
It weighs just over 100g and uses a micro USB cable to recharge itself. It takes almost three hours to charge, and managed to refuel an iPhone 5S, and feed an iPad Mini 2 with a charge of just over 30%.
2 Mophie Juice Pack Air
In terms of the battery power it gives you, Mophie's stylistic triumph is actually pretty poor value. However, since the 1700 mAh battery inside the Mophie Juice Pack Air drip-feeds an iPhone 5/5S and essentially doubles the life of the handset without any need for cables and extra devices, it's understandably popular.
It's not always obvious how much power is left inside the case, and it's also slightly annoying that it needs a micro USB cable to recharge.
There's a fiddly headphones slot, too, which precludes the use of headphones that lack a super-slim 3.5mm jack. There really should be an adaptor in the box.
Despite these grumbles, the 1401x66x16mm Juice Pack Air – which adds only 75g to an iPhone 5S – is perfect for anyone who just can't leave their phone alone (i.e. everyone under 40 years old).
3 Innergie PocketCell Duo
Here's a nice middle way between carting around a huge battery and a pointlessly small one, but it comes at a high price. Eschewing the £10-per-1,000mAh of battery that most portable products stick to, Innergie is arguably being cheeky by asking for £99 for its 6,800mAh battery.
However, the design is particularly great; rounded edges to the 94x43x23mm Pocketcell Duo make it easier to stow than most, and it's got two USB slots. Weighing-in at 132g, the PocketCell Duo takes about three and a half hours to charge-up, then fully charges an iPhone 5S thrice over.
An unusual all-in-one cable combines mini USB, micro USB and a 30-pin Apple connector, but that won't impress owners of newer Apple devices.
4 Veho VPP-002-SSP Pebble Smartstick
This is the ideal handbag life-saver with a unique design and concept – and it far outperforms most other batteries. Housing a small-sounding 2,200mAh battery, it's super-efficient and gives up to a 50% charge to a smartphone, which considering its lipstick-shaped size (22x86mm, 70g) makes it remarkably good value. In our test it took a flat iPhone 5S to a stunning 79% without warming-up at all.
The Veho Pebble Smartstick charges-up via a micro USB slot that sits just above the USB slot it uses to refuel any non-tablet device.
As well as a black and grey 'sock', it comes with a tiny, clever cable that's terminated with USB at one end, and a 3.5mm jack at the other; in the box are adaptors to match almost any device, including pin tips for Sony Ericsson and Nokia as well as micro USB, mini USB and an Apple 30-pin connector.
Note the lack of Apple Lightning, though let's not get upset; just use your own Apple cable, as with all the other batteries featured here. Brilliant value.
5 Powermonkey Extreme
By far the most outdoorsy battery available, the Powermonkey Extreme is also the only solar-powered portable. The main unit, which houses a huge 9,000mAh capacity battery, weighs 242g and measures 152x59.5x28mm; it's here that devices can be attached to its two USB (one for smartphones, one for tablets) slots for recharging.
Meanwhile, the 214g, 18x171x90mm solar unit takes about 14 hours to charge-up, crucially even in overcast conditions; it can then recharge an iPhone 5S about three times, or an iPad Mini 2 to 90%.
There's a Powermonkey Aquastrap available for an extra £20 for anyone wishing to make the two units waterproof while in charging position. That's ideal for walkers. Campers, yachters and sunbathers.
There's a plethora of adaptors to pack, too, making this a bulky – if brilliant – choice. If you're going off-grid with your Garmin, tablet, GPS watch or Kindle – or you just want to make your smartphone sustainable – the Powermonkey Extreme is the way to go.