Created by the folks at Philips, the free app works on the iPhone, iPad, and iPod Touch. But I found it of most value on the iPhone 4S, where battery life can be at a premium. Following iPhone users' complaints of poor battery life, Apple said last week that an update to iOS 5 is coming and should fix the problem.

To start you off, the app displays a screen with various tasks, such as talk time, Internet use, game play, audio playback, and video playback. Tapping on any task tells you how many hours your current charge would provide were you to engage in that task.

For example, with my battery charged to 70 percent, I could enjoy five and a half hours chatting on the phone, six hours and 20 minutes using the Internet, more than four hours playing games, and seven hours watching videos.

An "All data" screen sums up all the tasks and breaks down items like Internet use depending on whether you're using Wi-Fi, 3G, or 2G. A "Charge time" button tells you how long it will take to fully charge your device. And tapping twice on any task displays the percentage of your current charge.

In my informal testing, the app seemed accurate. The battery charge time was often a single percentage point behind what my iPhone reported. So the app might tell me that my charge was 70 percent while the iPhone's own battery usage would report 69 percent. But the app would eventually catch up.

BatterySense also includes a "What if" scenario that tells you what you'd need to do to achieve a certain number of hours on a single charge. Moving your finger up and down a gauge triggers such suggestions as not watching online videos, turning off push notifications, and turning off Wi-Fi. I didn't find that feature very helpful, but the app provides its own suggestions on preserving your battery, which were more instructive.

Among other tips, the app recommends that you fully drain the battery from time to time and then do a full recharge. It also advises you to disable the "fetch new data" option whenever possible and to shut down background apps, such as GPS and instant-messaging software, which often keep getting push notifications even when suspended.

The Audio Equalizer apparently chews up power, so the app suggests keeping that off. And turning off 3G if you don't need it can double your talk time and Internet use since it forces your phone to run on the slower EDGE network.

Of course, many other sites and sources provide advice on conserving a phone's battery power . Apple even offers its own page of suggestions. So the tips in BatterySense can be found elsewhere, but they're helpful nonetheless.

The app can't fix the problem with the iPhone 4S battery drain (for that we'll have to wait for the next version of iOS). But it does provide some insight into your battery usage and what you can do to coax more power out of a single charge.