Compared to some of its competitors, the iPhone 4’s battery life is surprisingly good. But few of us would complain about squeezing a few more hours between recharges. And, of course, many iPhone 4s have been in use for over a year now, so their battery life is gradually getting shorter. In my quest to extend my phone's daily life, I tested eight iPhone 4 cases that include built-in batteries for powering the phone.

As it's difficult to get a reliable measure of exactly how long each battery extends the iPhone's use time—I didn't have a spare phone I could dedicate to methodologically sound testing, plus different tasks consume more or less power—I've indicated whether each vendor's battery-life claims were in the general ballpark of my own experiences.

Each of these battery cases sports a dock-connector plug that couples with the iPhone's 30-pin dock-connector port; as a result, none let you dock your iPhone with a dock-cradle accessory—such as an iPhone speaker system—without removing the phone from the case.

Mophie Juice Pack Plus

Mophie's $100 Mophie Juice Pack Plus ( ) offers 2000mAh (milliamp hours) of power, which the company says should more than double your iPhone’s battery life; that claim meshed with my experience. I tested the black version of the Juice Pack Plus, but it’s also available in blue, pink, yellow, and purple.

Despite being one of the larger-capacity batteries I tested, the Juice Pack Plus weighs just 2.5 ounces and measures just 5.1 inches tall, 2.6 inches wide, and 0.7 inches thick. To my hand, the added weight and thickness were minimal. The pack actually consists of two parts: you slide your iPhone into the lower section of the case and then attach the upper third of the case. The case leaves both of the iPhone’s cameras, the Ring/Silent switch, and the headphone jack exposed. The case covers the Sleep/Wake and volume buttons with press-through buttons of its own, which work fine.

Though the Juice Pack Plus covers the iPhone's dock-connector port, you can still sync your iPhone using a Micro-USB port on the case's bottom edge; the case ships with a 30-inch USB-to-Micro-USB cable. You can plug the USB end into your Mac to charge the case and sync your phone with iTunes, or plug it into any Made for iPhone USB AC adapter to charge.

The Juice Pack Plus features a row of four LEDs on its bottom edge that indicate the battery's current charge level. The lights glow continually while charging, and while using the case, you can press a button to their left to see the remaining battery life. Near the bottom of the left edge of the case—in line with the volume buttons—sits a Standby (on/off) switch that lets you control exactly when the case sends juice to your phone.

Simply put, the Juice Pack Plus is excellent. I was impressed by its grippy surface texture, ease of use, and powerful battery. As it was the first model I tested, I had high expectations for the rest of the battery packs I looked at.

Mophie Juice Pack Air

I also tested Mophie’s $80 Juice Pack Air ( ), which is very similar to the Juice Pack Plus at 2.5 ounces and with measurements that are just a hair different: 5.1 inches tall, 2.5 inches wide, and slightly less than 0.7 inches thick. The Air includes the same 30-inch USB-to-micro-USB cable.

The Juice Pack Air's major difference is that it packs less power than the Plus—just 1500mAh versus the Plus’s 2000mAh. Mophie says you can get up to an additional 6 hours of talk time, 5 hours of 3G data, or 9 hours of Wi-Fi use out of the Juice Pack Air, and those estimates seem reasonable based on my testing. Like the Juice Pack Plus, the Juice Pack Air charges via a Micro-USB port near the bottom of its right-hand edge, and sports LEDs to indicate its charging status and a switch that controls when the pack charges your iPhone.

Unlike the Plus, however, the Air doesn’t use pass-through buttons for the phone's Sleep/Wake and volume buttons; rather, the case has cutouts that give you direct access. I’m not a fan of this approach, as it makes accessing the recessed volume controls and Sleep/Wake button needlessly frustrating. The cutouts also cheapen the case’s look for me. It’s just $20 more to spring for the Plus, and I think it’s worth it for both the more-powerful battery and the better design.

Incipio OffGrid Battery Case

The $70 Incipio’s OffGrid Battery Case ( ) holds slightly less juice than the Juice Pack Air: 1450mAh, which the company claims should “nearly double” your iPhone’s battery life—a fair estimate based on my testing.

The OffGrid is made of solid black plastic, and like the Mophie cases, consists of a larger, slide-in bottom section with a slide-on cap. The case weighs just over 2 ounces, and measures just over 5 inches tall, 2.5 inches wide, and 0.5 inch thick—notably thinner than the Mophie cases, though found the smooth plastic a bit slippery in my hand.

Like Mophie’s packs, the OffGrid charges via Micro-USB. The case includes a 2 foot charging cable, which plugs into a port located where you’d expect the OffGrid’s bottom-right speaker grille to appear. Along the bottom front edge of the case sit four blue LEDs that—again, just like the Mophie design—indicate the OffGrid’s current charge level.

The OffGrid, like the Juice Pack Air, employs cutouts in the case to make the iPhone’s buttons available. The Sleep/Wake and mute controls are easier to access than on the Juice Pack Air, but I still found the volume controls a bit pesky to reach.

On the front of the OffGrid, at its bottom left edge, is a small power button. Pressing the button once lights up the LEDs to show the current charge level; pressing and holding the button for two seconds turns on the pack so it can power your phone.

If you don’t mind the plasticky feel, and you'd like to save a few bucks, the OffGrid is a reasonable alternative to the Juice Pack Air.

Incase Snap Battery Case

Incase's $60 Snap Battery Case ( ) takes a different approach than the Mophie and Incipio packs. First, the case consists of just a single piece of rubber and plastic—there's no separate cap here. And the Snap Battery Case charges via a standard 30-pin dock-connector port of its own—the included cable is a black version of Apple’s familiar cable for charging iPods and iOS devices.

The case is impressively petite. It’s thin and very light, with just about the same dimensions as the OffGrid, though perhaps even a millimeter or two thinner.

The biggest benefit of the Snap Battery Case’s single-piece approach is that there’s no extra piece to risk losing. The downside, unfortunately, is that of the products I tested, the Snap Battery Case is the most difficult to use. Putting your iPhone into the case is easy enough, but to remove the phone, you must pry the top-center of the case off your iPhone, then pull the iPhone from the top a bit; only then can you remove the phone from the case’s dock connector.

Incase says you can expect up to three extra hours of talk time or 16 hours of audio playback from the Snap Battery Case’s 900mAh battery—an approximately 40 percent performance boost. That’s less than any other unit I tested, and a consequence of the Snap's thin profile. If maximal iPhone usage time is your goal, this case isn’t for you.

On the back of the case, along the bottom, sit five LEDs that show the Snap Battery Case’s charge status. Below them sits a power button. Hold the button down for four seconds to power on the battery and get your iPhone to draw juice from it.

I love the use of a traditional dock-connector port for charging and syncing. I just wish the Snap Battery Case packed a bit more battery oomph, and that it was a bit less painful to remove.

Third Rail Slim Case and Smart Battery System

Third Rail's $90 Slim Case and Smart Battery System ( ) offers a different take on the iPhone battery case. The company sells cases and batteries separately, but the batteries are interchangeable and attach to the outside of each case. This approach has a couple advantages. The most obvious is that if you don't need longer battery life on a given day, you can detach the battery for a slimmer package. The other is that if you buy the Slim Case for your iPhone 4, and you later buy a new phone—whether it's an iPhone 5 or something else—that requires a different case, you can buy just a new Third Rail case and keep using your current battery pack.

The Slim Case—which offers no juice on its own—measures approximately 5 inches tall, 2.4 inches wide, and 0.5 inches thick. The Smart Battery measures 3.3 inches tall, 1.8 inches wide, and 0.3 inches thick. On its own, the Slim Case weighs less than an ounce; the 1250mAh Smart Battery adds 1.4 ounces. On the back of the Smart Battery are grooves that let you piggy-back multiple batteries on to of each other to gain additional juice—Third Rail says you can use up to six batteries together, although doing so would make the case considerably bulky.

Like the Offgrid and Mophie cases, the Slim Case consists of two parts: the larger main section, which slide up over the bottom of the phone, and a small, slide-on cap. However, the Slim Case uses larger cutouts around the volume buttons, Ring/Silent switch, and Sleep/Wake button than the Air and Offgrid do, leaving the controls far easier to reach. However, the edges of the case sit flush with the screen in front, rather than extending past it to offer some added protection when setting the phone face-down.

Attach the battery pack, and the Slim Case system works fine for recharging your phone. Unlike the other battery cases I reviewed, however, the Slim Case with the Smart Battery attached just didn’t feel right in my hand. The rubbery Slim Case itself feels just fine, but the battery pack has a few sharp edges, particularly around the grooves where you can daisy-chain additional batteries.

That said, the Smart Battery does offer another perk: It sports two Micro-USB ports: In and Out. The In port is for charging the battery [and syncing your iPhone with your computer]; you can use the Out port to charge other USB devices. The case ships with a Micro-USB-to-female-USB adapter specifically for the latter purpose, as well as a Micro-USB-to-USB cable for charging and syncing.

Third Rail’s battery solutions are clever, but the Solo Slim Case can't match up with the better standalone iPhone cases, and the Smart Battery's clumsy and sharp-edged design renders the Third Rail setup less than satisfying.

IvySkin Quattro 4 Smart Case with Batterypak

IvySkin's $50 Quattro 4 SmartCase with the $45 Batterypak (  for case/battery combination) is another case/battery system. The battery offers 1700mAh of power, which nearly doubles your iPhone’s battery life. (Note that sometimes IvySkin’s marketing team refers to this as a “200 percent” increase; we won’t hold the bad math against the Batterypak.)

The Quattro 4 Smart Case itself consists of three parts. There are two rubber-and-plastic halves that slide onto your iPhone—one from the top, the other from the bottom—along with a thin, glass cover that sits over your iPhone’s own screen and is held in place by the case's halves. The glass makes me nervous—it seems more likely to shatter than the iPhone’s own glass would be, although maybe the idea is that IvySkin's glass will break instead of your phone's.

The case leaves the iPhone's Ring/Silent switch exposed and provides rubbery nubs over the volume and Sleep/Wake buttons. With the Batterypak attached, the case measures just over 5 inches tall and 2.5 inches wide. At its thickest point, it’s almost identical in thickness to the Mophie cases.

The Batterypak charges via a Mini-USB port—a 2-foot Mini-USB-to-USB cable is included—located in the center of the bottom edge. Five tiny LEDs sit on the bottom-rear of the case indicate the charge level. Immediately below those LEDs is the Quattro’s very small power button; you hold it down for several seconds to turn the battery on or off. IvySkin says that the battery won’t fully charge unless you turn it off, but if the battery is charging, it's difficult to figure out whether it's on or off: The LEDs light up in a different pattern when charging versus when the battery is simply powered on, so you must memorize those light patterns.

Overall, the SmartCase/Batterypak combo gets the job done, packs a lot of power, and feels nice enough in the hand. I’m not a fan of the glass screen cover, and you need to use it for the case to fit properly, but if you prefer an extra layer of glass to protect your iPhone’s screen, this is a battery case worth looking at.

Sol Solar-Powered Charging Case

Sol's $90 Solar-Powered Charging Case ( ) is another interesting spin on the battery case, though its name is a bit misleading, as the case's battery can charge via the sun’s rays, bright indoor lighting, or USB.

The Sol case measures just over 5 inches tall, 2.6 inches wide, and 0.8 inches thick, and weighs 2.4 ounces. Like the Snap Battery Case, the Sol case consists of just a single piece. The iPhone slides into the case quite easily—much easier than with the Snap Battery, since the Sol case doesn’t push against the top of your iPhone.

The case's surface is rubbery with textured bumps on the side, resulting in a very grippable feel. Dead center on the back of the case is a resin-coated Solar panel measuring 1.8 by 3.8 inches. Sol says that while you might scratch up the panel with rough use of the case, you needn’t fear that you’re going to shatter the thing.

A Mini-USB port on the bottom of the Sol case lets you charge and sync your phone (and, of course, charge the case's battery). Below the solar panel sit a power button, a series of four LEDs that show the battery’s charge level, and a separate LED that indicates the charging status. This last LED seems superfluous, but it's not a major issue. There’s also another LED on the bottom of the case, but this one is designed to be a “super bright” flashlight, and it is indeed quite bright; the flashlight lights up when you press the power button.

Unfortunately, unlike most of its competition, the Sol battery's can't itself be turned on and off, so there’s no way to tell the Sol case not to charge your iPhone. The company says it will soon launch a new version of the battery case that will include such a feature.

The Sol case’s 2400mAh provides the most power of the cases I tested, tripling your iPhone’s battery life on a full charge. It also charges more rapidly (via USB) than the other cases I tested, and it appears to charge the iPhone more quickly.

I like the Sol case. It feels nice and sturdy in my hand, packs a ton of power, and has the added awesome solar perk. I don’t love that it’s constantly feeding my iPhone power, even when the iPhone’s fully charged, so I’m even more excited about the promised sequel.

Exogear Exolife Recharageable Battery Case

Exogear's $90 Exolife Rechargeable Battery Case ( ) consists of two pieces: A dock-connector base with a battery backplate, and a plastic bumper that snaps over your iPhone and the case, holding them together.

The Exolife sports a 1500mAh battery, weights just over 2 ounces, and measures 5.1 inches tall, 2.4 inches wide, and 0.6 inches thick. The main case is black, although the outer bumper is available in several colors. (My review unit include a blue bumper.)

The Exolife charges (and syncs your iPhone) via Mini-USB; Exogear includes a three-foot USB-to-Mini-USB cable. The Exolife's Mini-USB port is a bit frustrating to access, as it’s protected by a tiny rubber flap that you must peel away to plug in the cable. I appreciate Exogear wanting to protect the port, but since so many cases don’t bother with such a flap, I bet it’s unnecessary.

Next to that flap is a small power button with a green LED that shines when the case is turned on. Exogear says the case detects when your iPhone is fully charged and shuts itself off automatically. In my testing, when the Exolife (finally) filled my iPhone to 100 percent, it still seemed to try to charge the phone—the power LED remained illuminated and the iPhone's status-bar battery icon remained in the "charging" state.

Exogear says the battery should give you seven additional hours of talk time, or six hours of Internet use, and those estimates seem reasonable based on my testing. But the device charged my iPhone noticeably more slowly than the other cases here.

I also didn’t love the case itself. The battery/bumper combo looks good and feels good in the hand, but the bumper is a bit flimsy and suffers from two flaws: The bumper makes the iPhone’s volume and Sleep/Wake buttons difficult to access, and the bumper doesn’t extend past the screen's edges in front, thus offering no screen protection. The Exolife shows a lot of potential, but I found it to be too annoying for daily use.

Macworld's buying advice

The Mophie Juice Pack Plus and the Sol Solar-Powered Charging Case were my favorite battery cases. Both felt comfortable in the hand, and both packed in tons of power. Of course, because of their high-capacity batteries, each is also on the thick side, although I didn’t consider either to be too bulky. If you crave thinness more than you power or protection, Incase’s Snap Battery Case is a fine slim alternative.