The $69.99 Energizer XP4001 power pack, made by XPal Power, is part of a long line of portable batteries of various sizes sold under the Energizer name. They range from the tiny, key-ring-sized AP650 to the bricklike XP18000, which is designed for laptops. The XP4001 is the best bet for recharging most phones and tablets, which is why we're reviewing it here.

The XP4001 is a flat, rectangular battery (5.2 by 3.2 by .6 inches HWD, 5.3 oz) that looks at first glance like it has rubbery grips. But those rubbery sides are just smooth, cosmetic plastic. The device has two USB ports and an AC adapter port on one side and two lights on top. The first light runs when you're charging the battery itself; it starts out red, but becomes green when the battery is full. The second light lets you check the battery's status when you're running it down, once more giving you either red or green depending on whether it's running out of juice.

The two USB ports are marked "phones" and "pads," and they put out different amounts of power: 500 mA from the "phone" port and 1.5A from the "pad" port. I found the "pad" port worked with an Apple iPad 1 and with high-drain mobile phones like the HTC EVO for Sprint and the HTC Thunderbolt for Verizon Wireless. Once you plug in your gadget, you hold down the button on top of the battery for a few minutes to start charging.

Performance and Conclusions
The XP4001 gave the Thunderbolt 5 hours, 52 minutes of LTE audio streaming time, more than double what I got on the Thunderbolt's built-in battery. It gave the iPad 4 hours, 49 minutes of video playback time. That's to be expected from its 4 amp-hour capacity, which is smaller than the competing Sanyo eneloop Mobile Booster ($79, 4 stars) and Zagg ZaggSparq 2.0 ($99, 3 stars) batteries, but much larger than the 1.4-1.6 amp-hour batteries you see in most cell phones. Of course, it also means the battery itself recharges faster, in four hours as compared to 7 for the Sanyo and 9 for the Zagg.

The XP4001 requires an external, AC power adapter to charge itself. Unlike the competing Sanyo and Zagg batteries we tested, the XP4001 comes with a velvety carrying case and two USB cables to connect it to your gear. The cables are short, curly, and don't terminate in standard microUSB ends. Instead, you get six plug-in "tips" that can terminate in microUSB, miniUSB, Apple dock connectors or Nokia, Samsung, or LG older proprietary phone connectors. This is a useful solution for people who don't already have their own USB cables, or for older devices that don't charge over USB.

We got seven XP4001 batteries for our Fastest Mobile Networks tests, and one of them was a bit defective. It was loose in its case, and rattled when I shook it. That didn't seem to affect its performance, though, unlike the sticky button on the ZaggSparq I tested.

The Energizer XP4001 is a good choice for a wide range of portable charging uses. While I prefer the Sanyo eneloop Mobile Booster overall, as it gives you more energy in a smaller, lighter package, the XP4001 is less expensive and can charge some high-drain devices the Sanyo product has trouble with.