A good friend of mine had been complaining that her iPhone 3GS battery was holding less and less of a charge. When we got together at 5 p.m. one recent day, it was at 5 percent full - and it had been fully charged that morning. She had barely used it all day. The phone was apparently running itself dry simply by being turned on.

The single biggest battery consumer is the screen brightness. But it wasn't especially bright on this phone.

So I suggested that she take the phone to an Apple store to get the $60 battery replacement service. In fact, there was an Apple store only two blocks away, so I accompanied her - and found out, upon arrival, that there is no $60 battery replacement service! There's one for iPods, but apparently not for the iPhone.

There are plenty of do-it-yourself and third-party battery-replacement services that advertise online, but the Apple store Genius, named Nicole, said none of that would be necessary. She tested the battery and found that it was perfectly fine!

Instead, Nicole pointed out a few things that were contributing to my friend's rapid battery depletion. I took notes and thought I'd pass them along.

* Push e-mail. This, I believe, was the big one. My friend has seven e-mail accounts, and her phone was checking each of them every 15 minutes. If you turn off the "Push" feature, and set it to Manually instead (in Settings->Mail, Contacts, Calendars->Fetch New Data), then your iPhone checks for e-mail only when you actually open the e-mail app. Your battery goes a lot farther.

(If you have a corporate Exchange account, your calendar and address-book data will similarly be updated only when you open those apps.)

* GPS checks. In Settings ->General->Location Services, you'll see a list of all the apps on your phone that are using your phone's location feature to know where you are. (It's a combination of GPS, cell-tower triangulation and, on some phones, Wi-Fi hotspot triangulation.) All of that checking uses battery power, too. My friend had dozens of apps with Location Services turned on, many of which didn't really need to be on. She turned most of them off.

* Notifications. Similarly, in Settings -> Notifications, you see a list of apps that are allowed to display pop-up notifications (those blue text bubbles that look like text messages). To do that, they have to monitor what's going on with your phone - and that takes juice. Turn off the ones you don't really need.

* Background apps. Nicole the Genius discovered that my friend had a huge number of apps open - maybe 40 of them. She maintained that they were using battery power, too, in the background.

Now, I kept my mouth shut. But I'd been led to believe that background apps are generally frozen into suspended animation precisely so that they don't use battery power. In fact, Apple was criticized when it introduced "multitasking" in the latest iPhone software, precisely because apps don't actually keep operating in the background. Only a few sanctioned features keep running in the background (Internet radio playback and GPS tracking, for example).

Even so, Nicole quit all 40 of the apps that were still open. (To do that, double-press the Home button to open the multitasking app switcher. Hold your finger down on any icon until they all start wiggling. Tap the little X close boxes to manually quit open apps.)

Did the Nicole treatment work? Very well indeed. The next day, my friend's battery, by the same time of day (5 p.m.), was still at 80 percent!

So there you go: How to make your iPhone battery last a lot longer. For free. You're welcome.

SoS