Need a lot of power to charge your phone or tablet? Zagg's ZaggSparq 2.0 is a powerful, elegant, and, at $99, pretty costly portable battery that can run an iPad all day long. There's one big caveat, though: you may need to go through a few units before you find one that works.

The ZaggSparq 2.0 is a 3.5 by 3.5 by 1 inch (HWD), 8.6 ounce black square that looks a lot like an Apple MacBook power adapter, except that it's black rather than white. On one side, there's a pop-up plug to plug the battery directly into the wall, a big plus over the separate power adapters that other batteries such as the Sanyo eneloop Mobile Booster ($79, 4 stars) and Energizer XP4001 ($69.99, 3.5 stars) use. On the other side, there are two USB charging ports, a four-LED capacity indicator, and an On button.

The two ports are marked "Optimized" and "General," and the battery can pump out a total of 2A at 5V, more energy than you can get from competing, smaller batteries. I didn't have any problems charging a power-hungry HTC Thunderbolt phone or an Apple iPad 1. As with the other backup batteries I tried, when you plug in your gadget, you must press the button on the battery to start charging.

Performance and Conclusions

As expected, the ZaggSparq 2.0's 6 amp-hour capacity let it outpace, outlast and outcharge competing smaller batteries. The ZaggSparq 2.0 powered an HTC Thunderbolt LTE phone for 11 hours and 30 minutes of Web streaming (that's more than quadruple the Thunderbolt's built-in streaming life), and allowed for 7 hours, 7 minutes of video playback on an iPad 1. The downside is long charging time—up to 9 hours to refill the battery from a power outlet.

All of this would be great if the ZaggSparq 2.0 worked as designed. But one of our two units ended up with a stuck on button after a week of use; the other one couldn't hold a charge very well, losing 20% of its charge over four days without use. Zagg had similar problems with its first version of the Sparq, as many customer complaints on sites like Amazon.com show.

Zagg immediately offered to send me a new battery. The company says it's absolutely committed to replacing defective units and that it has licked the original Sparq's production problems, but still, you should be prepared to have to send your Sparq back if you get a lemon (as I did.)

At $99.99, the ZaggSparq 2.0 could be a great companion for frequent flights or trips, as it delivers a full day's worth of charge for most mobile devices. I only recommend it with some reservations because you may need to try a few times before you receive a properly-functioning unit.