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.
Best portable battery: 6-10
6 Devotec Fuel Micro Charger
It's only good for a tiny charge; this novelty charger for Android phones and Kindles is all about the look. Weighing just 19g and measuring 38x27x13.5 mm, this red or green device is shaped like a jerry-can and features a micro USB tip that needs to be 'poured' into a smartphone.
At a mere 220 mAh the battery is only going to send 10% – tops – into a smartphone, but that could be enough to get you out of a jam (or give you an extra couple of days' reading on an E-Ink Kindle). The upside is that it only takes a few minutes to fully recharge itself from any USB slot.
Including a lanyard for attaching to a keyring, the Devotec Fuel Micro Charger is by far the easiest battery here to travel with – it's so tiny – but there are better value approaches to portable power. A version with an Apple Lightning adaptor will follow later this year.
7 Patriot PCPB90002 FUEL+
If you're constantly on your smartphone and are heavily using a tablet every day, too, it's probably time to … get some exercise, but start by hunting down the Patriot PCPB90002 FUEL+.
Though the brand makes all kinds of different shaped and specc'ed portable batteries, this flagship product houses a giant 9,000 mAh battery.
With a couple of USB outputs for simultaneously charging a smartphone (through its 5V-capable slot) and a tablet (via the 2.1-amp slot), the Patriot PCPB90002 FUEL+ is a little weighty at 190g and 114x93x25mm.
However, since it's capable of charging-up a smartphone three or four times (it can fill-up an iPad Mini 2 from scratch), the Patriot PCPB90002 FUEL+ is the one to hit on for long haul flights.
It's also one of very few portable batteries that isn't black; the Patriot PCPB90002 FUEL+ comes only in white with a red flash.
8 Energizer XP4006 Portable Power Tab Charger
£39.99 ($66 /AU$71)
For a battery to be portable, it needs to be lightweight and slim, but it's normally a trade-off with capacity. Not so on the Energizer XP4006, which weighs just 110g despite offering a 4,000 mAh battery.
As adept with a tablet as a smartphone, dual USB outputs make it handy when travelling with two touchscreens; the Energizer XP4006 can re-charge an iPhone 5S from scratch and lends an iPad Mini 2 a 40% blast.
Best of all, the Energizer XP4006 is shaped like a big-screen smartphone, and at 21mm slim it's primed for a jacket pocket.
9 Proporta TurboCharger 7000
This 7,000 mAh battery is all about value. Pound for pound one of the most attractively priced portable batteries of all, the Proporta TurboCharger 7000 has a smooth silicon shell – most are shiny and fingerprint-hungry – and a charging button on its top side.
It weighs just 180g and measures 119x73x12mm, but it's easy to travel with in other ways, too; Sony Ericsson, Nokia, Apple 30-pin and mini USB adaptors are supplied in a small travel pouch, though any device using USB is compatible.
Better still, the Proporta TurboCharger 7000 ships with two retractable USB cables – one for charging-up the battery, and one for connecting to those adaptors – thereby promising tangle-free trips with your gadgets.
10 Monster PowerCard
How about filling a credit card slot in your wallet with portable power? At just 19mm slim, Monster's effort is rather too chunky for most wallets, but this is still the ultimate in convenience.
Better thought of as pocket-sized (it's ideal for a shirt pocket), the PowerCard has a 1,650mAh battery so is good for around a 50% charge of an iPhone 5S/5C.
Though it doesn't play nicely with tablets, the PowerCard claims to supply a quicker than usual charge by dint of its 1.5-amp USB output. In our tests it wasn't faster, taking about 50 minutes to completely discharge.
Unfortunately the PowerCard makes a bit of a fuss as it recharges flashing blue LEDs on and off, so it's best not used overnight in a bedroom.
Still, the PowerCard in silver, black or red is a good valuer on-the-go portable battery to get you through the commute home.