Ever wish you could squeeze a few more hours of life out of your iPhone? You can with a new power pack designed exclusively for Apple’s popular cell phone.

Mophie, the quirky company that brought the world such inventions as a shuffle case/bottle opener and product packaging that can be reused as piggy banks, is now shipping an external battery that delivers a surprisingly high ratio of power per ounce. It sells for $100.

While there are some power packs available for charging Apple gadgets that sport the 30-pin dock connector, the Juice Pack is there first that bears the ‘Works with iPhone’ certification logo.

Once you slip an iPhone into the battery pack, it is evident that the battery is, in fact, specifically designed for the gadget. The fit is tight, with attention given to such iPhone features as the speakers, camera and internal antenna.

While iPod touch owners might be tempted by Juice Packs’ sleek design, the touch is simply too wide for the pack.

Weighing in at 3 ounces and sporting a soft backing that is easy to grip, the battery pack is fashioned to be a near-permanent attachment to the iPhone. A handy pass-through connector allows the user to recharge both the battery pack, and then the phone, while they are coupled. Surprisingly, mophie – which has produced some exceptional iPod cases – does not attempt to provide full protection for the iPhone. The screen and the entire top of the phone are exposed.

Because of the added thickness and length of the iPhone/Juice Pack, none of the two dozen cases I have on hand fit over the combination. For that matter, the thickness of the unit around the bottom pass-through connector make it too big to fit into most dock speakers and chargers.

That’s the bad. Here’s the good: In the couple of tests I ran this week, the battery surpassed the company’s claims. Here’s what Mophie states about the pack’s performance: Up to 250 additional standby hours, up to 24 additional hours of audio playback; up to 8 hours of additional talk time; up to 7 hours of video playback and up to 6 hours of additional Internet use.

For my first test, I played a few feature movies at mid volume and the screen bright. The 6:45 hours of video playback was disappointing – then I realized I had failed to turn off the Wi-Fi connection. Once turned off, the pack delivered a whopping 9 hours of video playback. That’s a full 2 hours longer than the specs led me to anticipate. Likewise, audio playback under optimum battery savings conditions lasted about 26 hours (although I didn’t stay up the whole time to check that my songs continued to shuffle.)

As with all rechargeable batteries, it’s important to remember that they have a limited life, both in terms of working at peak performance and the number of recharges it will take.

Mophie says it will take up to 500 full recharges, or a larger combination of partial recharges. The partials do not damage the battery, according to the company, which encourages users to ‘top it off’ when near a charger.

In a feature not usual for batteries, the pack ships charged. The paperwork says it is fully charged, but my unit took about 15 minutes to top off.

Four blue LED lights on the back of the unit indicate the level of the charge. The iPhone drains the battery of the pack first, so users on the go can take off the battery pack once it is drained and carry around a normal-weight phone.

Without true competitors on the market, it is hard to judge the pack’s performance It is full of power and very convenient to use. Add to that, it’s the only made-for-iPhone option on the market. So maybe it’s unfair to wish mophie had included a little more protection for the iPhone.

Should future batteries come out at lower price points or offer better protection, then the rating of the Juice Pack might fall. For now, however, it pays to come up with a solid offering when beating competitors to the punch.