My Apple (NASDAQ:AAPL) iPhone 5s ran out of juice yesterday before the day was finished. Though this doesn't happen often, it's definitely not the first time. I guess I'm one of the "wall huggers" Samsung mocks in its newest Galaxy S5 smartphone ad campaign.

As smartphone use continues to surge, whether it's because of distracting tasks like compulsively checking email and Twitter, or productive tasks like managing to-do lists or writing everything down in Evernote (these are productive tasks, right?), battery life for mobile devices is becoming increasingly important. But Apple's quest to reach device thinness deserving of the "Air" designation (a title that has so far been reserved to products from its MacBook and iPad lines) has made the task difficult. And once you add in Apple's tortoise pace in launching a smartphone display larger than 4 inches, a move that would leave more room for the all-important battery, it's no wonder iPhone users are scrambling for the nearest outlet.

Fortunately, there may be light at the end of the tunnel (or more power at the end of that dying battery?) for those like myself who are wrapped up in Apple's ecosystem. The savior to iPhone users' battery problem may come in the form of the company's 5.5-inch iPhone 6.

4.7-inch and 5.5-inch iPhone 6 mockups pictured with the 4-inch iPhone 5s. Image source: 9to5Mac, used with permission.

An extra 400 milliampere hours?
Apple's current 1,560 mAh battery in the iPhone 5s pales in comparison to some of the competition, particularly to the HTC One (M8), the LG G3, and the Galaxy S5. These three smartphones boast 2,600, 3,000, and 2,800 mAh capacity, respectively. To be fair, these are much larger handsets. But such are the benefits of having more space for a battery.

A report issued earlier this year suggested that Apple's next-generation iPhones could see only modest improvements to battery life. The rumored 4.7-inch iPhone 6 would get battery life of about 1,800 to 1,900 mAh and Apple's phablet 5.5-inch iPhone would get a 2,500 mAh battery, according to G4Games. 

But two recent leaks of a 2,915 mAh battery for the 5.5-inch iPhone 6 suggest that G4Games could be off by more than 400 mAh for the larger iPhone. The first leak came from Chinese site Apple Daily, which shared a photo of a 2,915 mAh battery that appeared to be a battery prototype for the larger iPhone model. The second leak, from (via MacRumors) earlier this week, showed what appeared to be an identical battery, which the site asserted was for the 5.5-inch iPhone 6. While the leaks are not confirmed, MacRumors' Kelly Hodgkins said the markings on the battery are similar to those that appear on other Apple batteries.

Given the extra power consumption required by a larger screen, every bit of incremental battery capacity Apple can pack in its upcoming iPhone lineup is crucial, especially considering that the company is already lagging peers in this key area.

But what about the rumored 4.7-inch iPhone? Some reports suggest the battery will only sport 1,810 mAh, which could be a major disappointment. But one analyst from ESM-China electronics, citing China supply chains, shared an image of a 2,100 mAh battery that the analyst asserts will be used in the 4.7-inch iPhone. But even 2,100 mAh may not impress.

Alleged leaked 4.7-inch iPhone 6 rear casing. The part was first shared by Feld & Volk.

Of course, Apple can also improve battery life by semiconductor innovation and improved power management from the operating system. The tech giant has been particularly successful at power management on its desktop OS X line. But this efficiency leadership hasn't translated directly to iOS yet.

As both an Apple customer and an investor, I hope the company can deliver this rumored 2,915 mAh battery, as well a cutting-edge innovation in battery conservation, which could finally serve up some true all-day battery life.

Fortunately, speculation for this year's iPhone refresh will soon become history. Apple is supposedly readying the iPhone 6 line to be unveiled at its event next Tuesday.

Daniel Sparks owns shares of Apple. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.