The iPhone has become the smartphone of choice for many business travellers. Dramatically extending your usage is now possible thanks for a range of innovative iPhone battery cases.

One of the main issues with Apple's iPhone is that you can't replace the battery yourself. This has lead to a massive ecosystem of battery packs and other exotic ways to keep your iPhone powered up and working. External battery-packs were first to market and can provide a compact solution such as the Primo range from Phonesuit [www.phonesuit.com] or the Gum Plus [www.xtand.net/gumplus.html] that can recharge your iPhone four times before its battery is exhausted. Or you can even use clip-on solar chargers, if your business trip is in a country blessed with plenty of sunshine. We still seem to be years away from using fuel cell technology to recharge our phones. A couple of years ago Horizon [www.horizonfuelcell.com/electronics.htm] showed one of the first commercial fuel cell technologies at CES but the technology has not moved into the mainstream. Horizon did, though, illustrate that the fuel cell could be used on a practical level to power and recharge consumer and business devices.

More recently PowerTrekk [www.powertrekk.com] have developed a portable fuel cell charger for use by business professionals in developing countries. PowerTrekk provides instant power anywhere to reliably charge mobile phones and other portable electronic equipment. PowerTrekk is a 2-in-1 solution that is both a portable battery pack and fuel cell. The portable battery pack can be operated on its own as a ready source of power or storage buffer for the fuel cell. The fuel cell enables instant charging from an exhausted battery without needing access to mains power.

These new technologies give us a glimpse of a power-rich future using alternative power generating sources; but for now, choosing the right battery case for your needs is essential to avoid being cut off during your next critical business call.

As battery technology has improved many iPhone accessory developers have used thin battery technology to create integrated battery and iPhone cases that offer your handset protection from knocks, and gives your phone a boost in battery power. You must also consider which iPhone you have. Many of the current battery case developers support both the 3G(S) and the iPhone 4. Ensure the case you go for is compatible.

iPhone power Taking even a cursory glance at the market as it stands for iPhone 3G/3GS battery cases you are spoilt for choice. Some of the market leaders you should place on your shortlist include the New Trent [www.newtrent.com] IMP190 ($69) that is available for both 3G models and gives an extra 11 hours of talk time via its 1900mAh battery. This model weighs just 6oz and includes a batter life indicator.

The Splash [www.splashproducts.com] O2 Lite ($54) offers a slightly less capacious battery at 1400mAh but beautifully matches the contours of the 3G and 3GS. Expect around 5 hours of additional talk time. You can also charge and sync your handset without removing it from the battery case. The Splash O2 is also available for the iPhone 4 offering a 2.4oz battery case rated at 1700mAh, which should give you up to 8 hours of additional talk time, and a whopping 450 hours on standby.

Mophie [www.mophie.com] claim that their Juice Pack Air ($79) for the iPhone 4 is the world's thinnest rechargeable case. The hard shell of the battery case itself is also reassuringly strong. You can be confident that your talk time will be extended and your phone protected from all but the most extreme conditions. The battery itself is, however, only rated at 1500mAh offering just 6 hours of additional talk time. But if you're looking for a very low profile battery case that only weighs 2.5oz this model is the perfect choice. There is also a version of this case for the iPhone 3G and 3GS with a slightly less battery capacity but that only costs $49.

Tekkeon have listened to the complaints that many iPhone users have made about their inability to change their phone's battery. The myPower [www.tekkeon.com] ($79) appears to be a standard battery case, but Tekkeon have what the company calls eSwap, which means the battery itself is housed in a separate module that can be swapped for a fresh battery. This system could, therefore, offer you infinite talk time. In addition, this battery pack can charge just itself and not your iPhone's battery, thus preserving its life. The battery is rated at 1200mAh, but only weighs 1.84oz making these batteries some of the most compact currently on the market. Additional battery modules will cost you $24.95.

One of the market leaders in this sector is MiLi [www.iphonemili.com] who also claim to use the world's thinnest batteries in their cases. The company's latest offering the PowerSpring 4 ($69) extends the battery capacity to 2000mAh, which should give up to 7.5 hours of talk time.

All of the battery cases so far have encased your phone usually in a hard shell to offer you an impact resistant case as well as more battery power. Dexim [www.dexim.net] have gone down a slightly different route that offers a leather flip case. The Supercharged Leather Power Case ($69) includes a 2000mAh battery offering up to 6 hours of talk time. The case flips easily to offer fast access to your handset and all the appropriate ports for recharging and syncing.

Which battery case you choose will largely depend on how much additional talk time you require, as this is directly proportional to the power rating of the battery embedded in the case. Battery technology is improving every year, which enables developers to pack even more performance into thinner batteries. In all cases the addition of a battery pack will only add a few ounces to the overall weight of your phone.