Mophie Juice Pack Air vs. Mili PowerSpring 4 reviewSee all photos
Mili apparently jumps from the gate with big wins in terms of price and capacity. Amazon lists the PowerSpring 4 for $68.97 with a 1600mAh lithium polymer battery. The Juice Pack Air for iPhone 4, on the other hand, lists for $79.95 on Mophie's own store with a 1500mAh lithium polymer battery listed in the specs. Be careful about jumping to any conclusions based on this information alone -- the Mophie price will likely drop a few bucks when it become more widely available (it's not listed on Amazon yet) and rechargeable battery capacities can be reported using either minimum or typical values. Regardless, what really matters is performance, not bullet points. We'll get to that soon enough.
The PowerSpring 4 measures 124 x 65 x 18 mm making it a bit shorter but slightly wider and thicker than the 128.8 x 63.8 x 17.2 mm Juice Pack Air. In practice, however, the size difference isn't noticeable. Both the Mili and Mophie offer pass-thru charging and syncing for the iPhone without removing it from the case.
Out of the box
Both Mophie and Mili gave reasonable consideration to the packaging of their products. That's a must for Apple fans notorious for their sense of style and is a critical step in securing a positive first emotional response. But we'd have to give the nod to Mili here. The packaging is more compact, economical, and consistent with Apple's own products.
More importantly, both batteries shipped about 75 percent full (showing 3 out of 4 LEDs) making them ready to use out of the box. A nice, but expected touch. The cases are also much lighter than expected. The Mophie weighs just 2.5 ounces. Mili doesn't list the weight of the PowerSpring 4 but it's roughly the same as the Mophie, maybe a tad heavier.
The Mophie Juice Pack Air is the hands-down winner when it comes to design. While both are inflicted with a painted metal-look band encircling the case to mimic the iPhone 4's antenna, the Mophie was clearly loved and cuddled as it progressed from art to part. For all we know, the Mili was raised by an inattentive wire-frame monkey in Harry Harlow's lab.
Unfortunately, the cutouts on the Mophie's cap for the power, mute, and volume buttons inhibit casual, one-handed access to the iPhone 4's few buttons. The volume and mute, in particular, were only accessible when deliberately holding the case for two-handed fingering. The Mili, with its more aggressive cutouts didn't have this problem. While the Mophie's cap can be removed without affecting charging, that flexibility comes at the expense of protection.
As a baseline, we usually get between eighteen and twenty hours of battery life from our iPhone 4. We're that annoying heavy user at the dinner table constantly checking updates be it browsing the web, checking email, streaming video, listening to music / podcasts, Twitter, Facebook... you know, the usual, over WiFi and 3G.
So, let's get to it, the fact that these can be used as oversized cases is of secondary consideration. Can the Mili and Mophie really double the life of the iPhone 4's internal battery? To put it simply: almost. Was there a winner? No, not in terms of raw battery performance.
Both battery packs performed roughly the same under real world use, though the Mophie did so with more grace and elegance thanks to a red/green standby/charging switch and brilliant string of 4 white LEDs used to indicate the juice left in the battery. The Mili also had a standby mode which essentially meant not hitting the power button after inserting the iPhone. Once charging, the only way to disable the Mili's battery was to remove the iPhone then resinsert it. The Mili's quartet of blue battery-level LEDs come tucked inside a clear plastic tube worthy of decorating an Appalachian shack at Christmas.
There are essentially two ways to use these battery packs. The first involves using the pack as an external battery (inserting a fully charged iPhone into a fully charged battery pack), the second as a charger for the iPhone (putting a depleted iPhone into a fully charged battery pack). Mophie recommends using the Juice Pack Air as a charger, not an external battery. That means putting the Juice Pack Air into standby mode (turning it into a passive case) and then flipping it into charge mode when the iPhone battery hits 20 percent. In practice, however, we didn't notice any significant advantage to this approach.
When used as an external battery (fully charged iPhone 4 inserted into a fully charged battery pack), the Mili gave us 16 hours and 30 minutes before giving up the ghost. Of course, the iPhone's battery was still fully charged. The Mophie lasted 16 hours before its battery was completely drained, leaving the iPhone to fend for itself. Keep in mind that this was real world use but still less than the 18 to 20 hours we typically get.
We then tested the packs as chargers with the iPhone 4 battery completely drained (not the 20 percent Mophie recommends). Using this approach, the iPhone battery charged 10 percent for every 16 to 17 minutes connected. After 2 hours, both the Mili and Mophie batteries were spent with the iPhone 4's battery registering a 69 percent and 73 percent charge, respectively. Again, not double the life.
Finally, we tested using Mophie's recommended threshold of 20 percent. Again, we saw the iPhone 4 charge at a rate of about 10 percent every 16 to 17 minutes on average finishing at exactly 90 percent for both cases. So yeah, no matter how we tested, we're still not seeing twice the battery life.
It's also worth pointing out that although the Mili is rated at 1600mAh, the 1500mAh Mophie exhibited equivalent performance characteristics. Remember what we said about minimum and typical battery capacity values at the top of this review? Mophie says that 1500mAh is its minimum capacity while Mili tells us that 1600mAh is the typical value. In other words, the batteries are likely twins.
Neither the Mili PowerSpring 4 nor Mophie Juice Pack Air actually doubled our iPhone 4's battery life. However, they did extend life by more than 16 hours when used as external batteries. Otherwise, they were capable of charging the iPhone 4's discharged battery to 70 percent full when used as chargers. And with the batteries being equal, choosing the winner comes down to price and design. To us, the Mophie Juice Pack Air's superior build and flexible, two-piece design more than offsets the $10 premium it currently demands.