Mobile phones and tablets have become incredibly powerful over the past few years, but their batteries, sadly, haven't kept up. As a lead mobile phone analyst, I'm pretty used to phones not lasting a full day on a single charge. I've hunted down plenty of ways to keep phones alive long past their normal exhaustion points.
Backup batteries come in many different sizes, but they're all measured in milliamp hours, or mAh. Your phone or tablet's battery is measured in mAh, too - pull it out (if you can) and take a look. You can expect a backup battery to boost your device's life by as much as you'd expect from the mAh stored, minus about 20 per cent for inefficiencies. So if you have a 1500 mAh battery in your phone, a 4000 mAh backup battery should give you more than two full charges.
You have to balance size, convenience, and charging time. Small batteries are very portable and flexible, and also hold about one charge for most smartphones. Bigger batteries take longer to charge, and they usually require you to tote around a USB cable, but they're a lot more capacious than the little guys.
Battery technology is advancing slowly, so we're going to be struggling with phones running low for the foreseeable future. If you can't keep your hands off your phone, a secondary battery may be the right solution. We've reviewed eight backup batteries, so you're sure to find one that fits your needs below, from a tiny iPhone juice booster to beefy tablet refillers.
If you have an iPhone 4 or 4S, these battery cases might be your best bet.
Batteries Included in This Roundup
The Iogear GearPower Mobile Power Station is a relatively inexpensive, high-capacity add-on battery that can extend the life of even the most power-hungry mobile devices. It edges out the Sanyo Eneloop in terms of battery life and charging versatility with two USB ports, one of which features a high 2.1-amp output.
The Eneloop Mobile Booster offers a great balance between power, price and portability, fitting 5000 mAh of juice into a petite 4.6oz white body. You'll find it costs much less than £45.99 at retail, too.
XPal Power makes a wide range of backup batteries under the Energizer name. The XP4001 is the best bet for recharging most phones and tablets. While we prefer the Sanyo eneloop Mobile Booster overall, the XP4001 is less expensive and can charge some high-drain devices the Sanyo product has trouble with.
The tiniest battery in Mophie's backup line could easily be mistaken for a cigarette lighter. It's designed primarily to give an urgent burst of power to iPhones, with its tiny 700mAh capacity and built-in 30-pin Apple connector.
Aside from the unfortunate name (I'd like my battery to neither scorch or burst, thanks) the iPhone-exclusive FlipCharge Burst is a little expensive for the limited 720 mAh of juice it can provide. But it's unusually convenient to use, thanks to a built-in 30-pin Apple connector and a USB charging cable.
The Tylt Energi is a very convenient way to charge up any phone or tablet with a USB or MicroUSB port. We love its built-in AC power plug and MicroUSB tail. But although the size, price, and options are right, our ideal backup battery holds a bit more energy.
Made for iPhones and iPads - and nothing else - the Mophie Juice Pack Boost is a handy little extra battery which can give your iDevice a bit more charge. While it's slim, well-designed, and has more than double the capacity of the Juice Pack Reserve, there are much more flexible options out there.
The Mophie Juice Pack Powerstation gives you less battery life than competing products at the same price point, but it's a lot prettier. Whether that's worth it is up to you.
Need a lot of power to charge your phone or tablet? Zagg's ZaggSparq 2.0 is a powerful, elegant, and, at £79.99 pretty costly portable battery that can run an iPad all day long. There's one big caveat, though: you may need to go through a few units before you find one that works.