A month ago I blogged about extended batteries for the iPhone (see “Longer battery life, continued“) , and mentioned the Power Slider by Incase. It is similar to the Mophie Juice Pack for iPhone, except that it is a full case for the iPhone 3G as well as an external battery.

I’ve been using the Incase Power Slider with my iPhone for several weeks now, and I am quite impressed. In the evening after having the iPhone on all day, making one or two calls, checking email and browsing the internet (as much as I can do during my lunch break), the battery of my iPhone is still at 100 percent. The Incase Power Slider is usually down to about 60 percent, or if it has been a particularly heavy usage day (for me anyway), down to about 20 percent, at which point it stops charging the iPhone’s battery.

The remaining battery charge of the Power slider is determined by a button at the lower back, which when pressed, lights up a line of 5 small LED lights which indicate the battery’s charge; 1 light means about 20 percent, 5 lights mean 100 percent, etc.

The iPhone can be synchronized with iTunes while still in the case, but the supplied USB cable (which is rather short) has to be used, and the button on the back of the case pressed for 3 seconds for “sync mode” prior to syncing.

Normally I remove the Power Slider before syncing, and leave it ready for recharging it overnight. It definitely works better when started with a full charge.

Physically, the slider consists of two parts. The iPhone slides into the bottom piece, which contains the battery and a connector. Then the top piece slides over the iPhone, so it looks like it is in a case with an extended lower end. SlashGear.com’s review of the Incase Power Slider has some pictures of what it looks like.

The Power Slider does add some extra weight to the iPhone, and my only gripe is that it leaves the screen unprotected, but then the majority of iPhone cases don’t protect the screen either.