Because the iPhone 3G 
doesn't have a user-replaceable battery -- and because it's so easy to burn through a full charge before the end of a day -- some users may need a variety of power charging solutions. They need to keep the included universal serial bus (USB) cord or dock handy at home, and perhaps have one in the office. One also might need a car charger and might seriously consider an external portable battery.
There are several external portable batteries on the market, and I chose two different types of portable battery power from reputable dealers to evaluate. After all, who wants to risk an electrical problem or a freakish fire on a gray-market battery pack? I chose the Kensington Mini Battery Pack and Charger for iPhone and iPod and the APC Mobile Power Pack (UPB10).
Direct Attach
The US$49.99 Kensington Mini Battery Pack charges via an included USB cable through any powered USB port on a PC, Mac or plug-in charger. Its attachment to the device, however, is through the 30-pin dock connector at the base of the iPhone or iPod. Although this unit was on the market prior to the launch of the iPhone 3G, it does indeed work with the iPhone 3G.
To charge the high-capacity rechargeable lithium-ion polymer battery pack, simply plug it in and watch the four lights on the front. One LED (light-emitting diode) indicator lights up when it's at less than 20 percent capacity; another is lit when the charge is 20 to 59 percent, a third light indicates 60 to 99 percent, and the fourth indicates a full charge. As you might guess, the LED indicators show approximate charge levels.
Once charged, the lightweight battery plugs directly into the bottom of the iPhone and is mobile 
-- you can use the iPhone while charging. Sliding it into a pocket might disconnect the battery, for example, but it can be carried around without much trouble.
Making and taking calls while the Kensington Mini Battery Pack is attached is a little awkward next to your face, but hey, it works. As for the speakers located on the bottom of the iPhone, there's enough of a gap that audio still works fine, too. In addition, it does connect well enough with some thin iPhone cases to seat and maintain a charge -- a handy ability for those iPhone customers who use cases.
Kensington says the battery pack will give an iPhone 3G a three-hour boost of talk time, but I didn't bother burning up 180 of my AT&T (NYSE: T) minutes to test talk time. What I can tell you is that it'll recharge at least 50 percent of your iPhone, and with little-to-no usage while charging, you can sqeeze out even more of the green goodness shown by the battery level indicator on the iPhone's screen. The point of this mini battery pack is mobility , so it's kind of hard not to plug it in and use your iPhone 3G normally while it's charging.
Oh, one more thing -- you can charge your iPhone 3G and the battery pack at the same time while they are connected -- it's quite handy, and it doesn't attempt to connect your iPhone to iTunes if you're using a USB port on your Mac.
APC's Cable Attach Battery
There are two major differences between the Kensington Mini Battery Pack and the APC Mobile Power Pack. The first is that the APC unit only plugs into an iPhone via the iPhone's USB charging and syncing cable. There's no direct attach, and while you can use your iPhone while charging it, the cable will be dangling from the end of it. The second major difference is that the APC battery pack will work with virtually any mobile device capable of being charged via a USB cable -- this is cool if you have other phones or MP3 players.
As you might expect, it can also do a dual charge in a pass-through connection -- the charging USB cable is plugged into a wall socket or Mac/PC and the iPhone is plugged into the APC unit. Like the Kensington, this is quite handy.
The APC battery is more than than twice as large as the Kensington Mini, and it takes about 3.5 hours to fill; however, it's still small and light enough for easy packing. Besides, it offers up about twice the charge as the Mini -- I could fully charge a nearly dead iPhone 3G with the APC, with some juice left over.
Using the APC has one quirk that's either irritating or spectacular, depending on your situation: If the battery pack has not been used for a period of more than 15 minutes, it automatically enters a sleep mode to conserve the charge. To wake it up, you have to push a button on the top. As you might guess, you'll have to push the button several times while charging your iPhone or you're not going to get a full charge out of it.
Similarly, to see the status of the battery's tank, press the button and an LED status bar will show you the relative amount of juice remaining.
Pick One - You Can't Go Wrong
Overall, both battery packs are simple to use and effective. If portability and just a little jolt are all you need to get through the day, the Kensington Mini is a great choice. Equally effective is the larger and more flexible APC Mobile Power Pack -- it'll give you plenty of power with some extra to spare.
Either way, both will help keep your iPhone 3G running strong -- and that's really the point, isn't it?
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