The Apple smartphone has perennially struggled on the battery-life front for many, but there is more than one way to make sure you are never interrupted midway through a mammoth NOVA 3 session again.
If you fume at the sight of seeing 'less than 20% battery remaining' flashing up on that beautiful Retina display, here's our pick of the best iPhone 4S battery-preserving cases (which of course will work for the iPhone 4 too) that will keep your Apple smartphone going much longer.
1. Incipio OffGRID Pro
This durable polycarbonate case features two interchangeable rechargeable batteries that combine to deliver a hefty 3200mAh of battery power for your iPhone.
Micro USB means you can sync as you charge with cut outs to give access to your headphone jack and on/off toggle.
The battery meter will alert you to your current power status, while a handy dual-flash diffuser will help make your photographs look more polished by reducing the interference when you are snapping.
2. Mophie Juice Pack plus Outdoor Edition
Ideal for hikers, bikers and generally lovers of the great outdoors, the Juice Pack Plus houses a 2,000 mAh battery which is protected by a hard-shell case, reinforced by a shock-proof rubberized band.
You can expect up to eight hours of talk time and up to seven hours of internet use over 3G, while an accompanying Mophie Outdoor Edition app means you can turn your iPhone into a GPS device to help discover unexplored off-road trails.
3. Pico Genie A100
Saving iPhone battery is just one of the A100's nifty tricks, as this lightweight case also works as a projector and a 2-watt speaker, all whilst keeping your iPhone in one piece.
The 1900mAh Li-Ion battery will add an extra three hours of iPhone life, while the 15 Lumen bright projector will expand the iOS greatness up to 60-inches on your nearest wall or ceiling so you can watch videos and browse pictures, or even take it into your next boardroom presentation.
4. MiSuny iPower 4
Harnessing the power of the sun and a 2,400 mAh battery, this charging sleeve promises two full charges before you have to juice it up again, and should give you around five hours' extra talk time, six hours' internet browsing and a massive 45 hours to listen to your iTunes playlists.
Thanks to the built-in solar charger, you can also top up on power during the day. Weather permitting of course…
5. PhoneSuit: The Elite Battery Case
Adding minimal bulk to an already slender smartphone, the slider-style case measures in at 17mm thick and packs a 2,100 mAh battery to completely revive your iPhone 4.
1-Amp charging technology promises to charge the iPhone in less than two hours, while the five-LED gauge located on the back of the case is discreet but will help to keep an eye on the current battery status.
Crucially, it's been given the Apple seal of approval to charge up iPhone 4 and 4S handsets.
6. Third Rail Slim Case
Claiming to be the thinnest battery case on the market, the robust hard-shell case uses Smart Battery technology that slides snug into the case and can also be used to charge other portable devices with the included adapter.
The integrated on/off switch ensures you are not wasting any emergency power, while additional smart batteries can be stacked inside the case, at the expense of its slender form of course…
7. Scosche Switchback Surge G4
With no ambitious claims to double the battery life of your iPhone 4 or 4S, the Surge G4 will get your dead Apple smartphone back to a maximum 80% charge from a 1,800 mAh battery.
Micro USB support means you can sync your iPhone as you charge, while a Scosche revive iPhone app will offer advanced battery management.
The built-in kick stand means you can watch videos hands-free, while the interchangeable polycarbonate backs let you add a little customisation to the proceedings.