Here's a cheap way to double the battery life of the DS, if you don't mind random losses of power.
The Double Power Pack from Game Top is a simple, inexpensive device that connects an additional rechargeable battery to your Nintendo DS. It comes with one 850 mAh lithium-ion battery, and it has a compartment for the system's original 850 mAh battery as well. The two can be alternated with a small switch. With a total of 1700 mAh of charge, the system's battery life is clearly doubled, up to about 20 hours of gameplay under certain conditions. That time isn't continuous, since you must switch between the batteries at some point (they also have to be recharged separately).
Installing the Double Power Pack takes a few minutes, but it's a simple process. The unit ships with a tiny screwdriver to remove the DS battery cover. Then you pluck out the original DS battery and replace it with the new one, which is secured to a casing that fits snugly against the back of the system. Just snap the original battery into the "optional" compartment on the Double Power Pack, tighten the single screw, and you're done. It even has a specially cut corner to leave access to the stylus. With the device installed, the DS is slightly heavier and a quarter-inch thicker, but these changes are negligible after a few minutes of playing.
It sounds like a good package so far, especially for what it costs (less than $15 from Lik-Sang), but the Double Power Pack has one critical flaw: it doesn't fit. Oh, it looks fine when screwed on, and it feels solid enough, but electrical contacts are picky little things. They want the battery to be securely connected at all times, and this device just doesn't fit tightly enough to ensure a continuous power supply. While playing Super Princess Peach, the DS suddenly shut off on three occasions, once in the middle of a boss battle. It can happen even when the selected battery has a full charge and you're gently holding the DS in front of you…not even resting on a surface or getting banged around at all. This is an extremely frustrating problem which is just not acceptable.
Due to this major problem, the device isn't reliable enough to be useful. I can't recommend it unless you consider the current DS battery life so inadequate that you'd take the risk of suddenly losing power to the system, or maybe you're the handy type and want to mod the Double Power Pack to fit better. Most of us can live with eight to ten hours of gameplay, and the upcoming DS Lite promises even better performance.