Cell phone always running out of juice? Consider a portable battery packSo, it’s the end of another long day, you’re heading home, and you need to make one last phone call. But when you pull out your smartphone, bad news: the needle on the digital battery meter is buried in the red. What do to?

Well, you could always try ducking into a nearby Starbucks to plug your phone into a wall outlet—provided you can find one, of course, and assuming you have the charging cable handy.

There’s also another alternative: a portable (and rechargable) battery pack, small enough to fit in a pocket or a purse and ready to give your iPhone, Android phone, BlackBerry, or iPod a much-needed shot in the arm, even when there isn’t a wall outlet in sight.

A quick search on Amazon reveals dozens of makes and models, some of which are small enough to fit on a keychain while others are closer to wallet size.

Cell phone always running out of juice? Consider a portable battery pack

Some portable battery packs (like Mophie's new Juice Pack Reserve) are small enough to fit on a keyring.

Your typical portable battery will cost anywhere from $20 to $100, depending on the size, and they all work in more or less the same way: you charge them at home by plugging them into a wall outlet, then bring them with you for a quick boost whenever your phone’s battery is running low.

Of course, the bigger the battery, the more power it’ll store.

For example, a smaller, keychain-sized battery probably couldn’t completely recharge a dead iPhone 4; in a pinch, though, it could give you enough power for a few extra calls or to check your email one last time, and it would also be compact enough to bring in your purse every day.

If you want more portable power—say, enough to keep your smartphone going for a cross-country flight—you’ll need something a little bigger, perhaps the size of your phone itself.

Another clever option is a smartphone case with its own, built-in battery pack (such as those made by Mophie, Kensington, and Incase).

When shopping for a portable battery pack, make sure it’ll charge your particular make and model of phone.

Some portable chargers come with Apple’s 30-pin iPhone connector, which plugs directly into the bottom of your iPhone; others may have USB, Micro USB, or Mini USB ports. Check your phone’s charger to see what kind of connector it uses, then make sure the battery pack you’re considering will fit before you plunk down your cash.

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(Image: Mophie Juice Pack Reserve)