The iPhone 4′s built-in battery should last you an entire day of moderate use, with room to spare. If, on the other hand, you’ve been holding epic conference calls on the iPhone 4′s spearkerphone — or worse yet, streaming YouTube all day in a spotty 3G coverage area — your precious handset will run out of juice in a jiffy.
One way to solve your iPhone 4 power woes is, of course, to pack Apple’s power cable in your briefcase and have a nose for electrical outlets. A popular — and more convenient — alternative, however, is an iPhone case with a built-in battery booster.
Several such devices are already on the market, including the Mophie Juice Pack ($80 to $100, depending on the model), and this a new kid on the block: the Third Rail System, which combines a slim plastic case (appropriately named the “Slim Case”) with a replaceable battery.
The hard, 0.9-ounce Slim Case adds minimal bulk to the iPhone 4′s shell and boasts openings for the camera lens, headset jack, sleep button, and volume controls. Meanwhile, the rectangular Smart Battery snaps onto the back of the Slim Case and delivers power through a trio of small contact strips.
The idea behind the Third Rail setup is that instead of keeping your iPhone in a bulky, battery-packing case at all times, you can snap on the slim, 1.4-ounce Smart Battery only when you need it. Another benefit over the competing battery boosters like the Mophie: you can toss multiple Smart Batteries into your travel bag, swapping them in and out as you go. Last but not least, Third Rail promises that its Smart Batteries are “future proof” because they could be snapped into a new Slim Case for, say, the iPhone 5 (or 6, for that matter). Anyone who bought an expensive case for their iPhone 3GS and then upgraded knows what a pain that can be.
It’s clever, but the Third Rail “system” isn’t cheap. Bundled together, the Slim Case and a single Smart Battery sell for $89, or $10 more than the cheapest Mophie Juice Pack. Mind you, that’s not a ridiculous price to pay; on the other hand, additional Smart Batteries cost a hefty $60 a pop.
How does the Smart Battery perform? First, we gave it a true torture test, streaming Gone with the Wind on Netflix over AT&T’s 3G network. The result: the Smart Battery (although not the iPhone 4 itself) was dead within three hours — not bad, considering the stress that streaming over 3G puts on the iPhone.
In a more reasonable test of average iPhone use, the Smart Battery powered our handset for a solid 12 hours before giving up the ghost, on a par with Mophie’s specifications for the $80 Juice Pack Air. Not bad, but it’s worth noting that the 1500 mAh battery in the Juice Pack Air has a slight edge on the Third Rail’s 1250 mAh battery in terms of raw capacity. (And Mophie also sells a larger, 2000 mAh-capacity Juice Pack for $100.)
In the end, we were impressed by the Third Rail System’s overall performance, and we like the idea of swapping fresh batteries into the included case — or removing the battery altogether when extra juice isn’t needed. Based on Third Rail’s capacity specs, however, you may have to settle for a little less power per battery than you would with a bulkier, battery-case combo, like Mophie’s.