[[Image:Iphonebattery.jpg|630px|thumb|left|Want some fresh-squeezed Apple juice? Photo by Ged Carroll/[http://www.flickr.com/photos/renaissancechambara/5422937479/ flickr]/CC]]
Pop quiz: Your hiking guide is suddenly mistaken for an afternoon snack by a bloodthirsty grizzly bear, leaving you stuck in the middle of the woods somewhere far from civilization. Let's assume, for the sake of argument, that you somehow manage to escape the bear. You have 38% battery left on your iPhone, and luckily you use Verizon, so you actually have a signal and can use the Maps app.
How will you make sure your battery lasts long enough that you don't get lost and die of exposure or dehydration before you make it to the nearest town?
In all likelihood, your typical battery crisis is nowhere near as serious (or potentially life-threatening) as the one above. Still, few things are worse on a busy night out than running out of cell phone juice. Here's the rundown on what will — and won't — make your iPhone or iPad battery last the longest, and some tips to make sure it lasts as long as your device.
''This how-to was written by [http://90wpm.com Ben Jackson], a writer and app developer living in Brooklyn. He likes clean typography, dirty language, strong coffee, apple pie and comfortable chairs.
First, let's dispel a few myths about battery life on iOS devices. The Internet is littered with conflicting advice about how best to charge your device, but the most commonly purported methods of extending battery life are effectively useless.
Killing background apps [http://speirs.org/blog/2012/1/2/misconceptions-about-ios-multitasking.html will not extend your battery life]. Those apps are effectively "frozen" in the background and, [http://support.apple.com/kb/HT4211 according to Apple's own support documents], multitasking does not put unnecessary strain on the battery. The exceptions to this rule are apps which use ambient location services ([http://www.readwriteweb.com/archives/geofencing_whats_next_for_location-based_services.php aka "Geofencing"]), background audio playback (the play symbol will appear in the status bar when audio is playing even if the volume is off), and background downloads in Newsstand. As we'll see, however, you can turn these background features off selectively without quitting the app.
===Myth 2: Calibrating Improves Battery Life===
It's impossible to physically measure how much power remains in a battery. Because of this, mobile device manufacturers have developed sophisticated algorithms to estimate the remaining charge. Still, over time the reading will begin to drift away from the actual value.
Calibrating (using the phone until it dies then charging to 100%) helps snap these readings back into place. It does not, however, extend battery life; it mainly makes the indicator more accurately reflect the battery's state. Apple advises users to calibrate monthly, but any more often is overkill. While there are some ancillary electrochemical benefits to running full charge cycles, in practice you shouldn't have to worry about this too much if you're using your phone regularly.
===Myth 3: Overcharging and Overdischaring Damages Batteries===
While overcharging or undercharging is a concern for gadgets that use lithium ion batteries, Apple has built-in protections to keep these things from affecting iOS devices. As [http://allthingsd.com/20120327/apple-ipad-battery-nothing-to-get-charged-up-about/ All Things Digital recently reported], when the battery fills up, the phone will let it drain to 99%, then fill back up. This cycle will repeat as long as the phone is charging, although the battery indicator will report "full" rather than reflecting the actual fluctuating charge to avoid distracting users.
Likewise with letting the battery "die" too often. (I use quotes around the word "die" because all lithium ion batteries will actually shut off with a bit of charge remaining as a safeguard against damage from overdischarging.)
What ''will'' damage batteries, however, is letting the phone die and then tossing it into a drawer for two months. Batteries lose charge over time even when not in use, and letting the battery sit with little to no charge will cause it to lose capacity.
===Myth 4: iPad Chargers Charge iPhones Much More Quickly===
iPad chargers deliver twice the current as iPhone chargers (2 amps instead of 1 amp). Both are five volts, but the iPad charger has double the wattage as well. (Remember Physics? Me neither. Current, measured in amps, is equal to Wattage divided by Voltage.)
But wait! Current isn't the only factor in charging time. Batteries charge in three stages: The first stage, constant current, is the only one where the battery might use the maximum amount of current that the charger can deliver. But even then, most devices with lithium ion batteries will limit the current to only what they need via software controls.
What this means in practice is that while you will get a quicker initial charge from an iPad charger due to the higher current, the extra current won't be drawn throughout much of the charging cycle. And since batteries charge much more quickly at first, slowing down as they reach the final 20% of capacity, you probably won't notice much difference unless you're running really low.
In addition, it means that if you're in a hurry you should avoid charging your device through your laptop. Newer Macbooks [http://support.apple.com/kb/HT4049?viewlocale=en_US will detect iPhones and iPads and deliver half the current] as the device's equivalent wall adapter. Gizmodo did a [http://gizmodo.com/5535631/the-fastest-and-slowest-way-to-charge-an-ipad side-by side comparison of different power sources] that illustrates the difference nicely.
==Extending Battery Life in an Emergency==
There are many factors that affect battery life. As the iPhone grows its feature set, that list grows as well. Apple acknowledges this, and maintains [http://www.apple.com/batteries/iphone.html its own short list of battery tips]. Here are the most common battery drains, and how to minimize them.
It should go without saying, but if you are in fact lost in the woods, a few rounds of Angry Birds is not the best way to spend your hiking break. Apps which make extensive use of the CPU (e.g. a music app like [http://www.intua.net/products/beatmaker2 BeatMaker 2]) will drain the most power. Graphics-heavy apps such as [http://venturebeat.com/2011/03/13/ipad-2-graphics-performance-processor/ Infinity Blade] will still use considerable battery even though the GPU processes the graphics more efficiently.
By far the most common drain on battery life is the iPhone's LED backlit screen. Apple recognizes this, and iOS devices come with settings that automatically adjust the screen to ambient light. If you're really strapped, you can adjust the brightness manually under Settings > Brightness. Lowering it will conserve battery life, but not by much. Most battery drain occurs when you increase the brightness above the default setting.
While [http://www.displaymate.com/AutoBrightness_Controls_2.htm there has been some controversy] over the effectiveness of auto-brightness controls in Apple products (as well as Android phones), for day-to-day use it's a decent option unless you relish manually changing the brightness every time you step inside or outside.
Remember, darker pixels use up less energy than lighter ones, so if you're reading then consider using a "dark mode", or if your app doesn't support it, you can invert the screen in Settings > General > Accessibility > White on Black. If you scroll to the bottom of the Accessibility Settings screen, you'll see an option to assign this to the Triple-click Home action.
#'''GPS and Geofencing:''' Many apps, like Foursquare and the recently released Highlight, keep tabs on your location constantly. This is a huge battery drain, and you can turn off access to location selectively in Settings > Location Services. You'll see a purple arrow next to any apps that are currently using your location, and a gray one next to those which used it in the last day. Turn off all Location Services to save the most power.
#'''iCloud:''' iCloud is extremely convenient, but unfortunately constantly backing up your data to the cloud will demolish your battery. Make sure to only turn on iCloud for services you need, and turn everything off (or just cut the network connection) if necessary. You can adjust this in Settings > iCloud.
#'''Bluetooth and WiFi:''' Bluetooth and Wifi are generally good about going to sleep when they're not being used, but they still check periodically for new connections. Turn them off in Settings > General > Bluetooth and Settings > Wifi.
#'''Cellular Network:''' Your phone is constantly scanning for cellular towers. When you add 3G data services on top of that, you're firing up the cellular radio regularly, which will suck the life straight out of it. In addition, iMessage will poll the network constantly even if you have push turned off elsewhere. Go to Settings > General > Network and turn off 3G if you still need some data service, or turn off all network services if you want the most battery life. Turning on "Airplane Mode" will disconnect cellular service, GPS, Wifi and Bluetooth all in one shot.
#'''Push Notifications:''' I know, "Email can't wait". Except it will have to wait, or you'll be unable to return it in a few minutes when your phone dies. You can turn off push notifications selectively in Settings > Notification Center. Previously, there was an option to turn push off for all apps, but in iOS 5 you must turn them off individually. This is a pain in the ass, but necessary. Make sure to remove the app from Notification Center, set "Alert Style" to "None", and turn off "Badge App Icon".
#'''Mail and Calendar Fetching:''' Even if you turn off push in Notification Center, iOS will still fetch email, address book, and calendar data via push unless you turn it off in Settings > Mail, Contacts, Calendars.
#'''Personal Hotspot:''' Sharing your device's 3G or LTE connection is convenient, but taxes the battery. Turn it off when you're not using it.
#'''Spotlight:''' Spotlight is used by iOS to generate the search results that pop up when you swipe right from the first page of the home screen. The price you pay for this convenience, however, is that Spotlight is constantly indexing the contents of your device, taking up battery life with it. You can turn off Spotlight indexing selectively in Settings > General > Spotlight Search.
==Caring for Lithium Ion Batteries==
All batteries will eventually wear down and store less charge over time. There are, however, some measures you can take to keep your battery in top shape.
Batteries lose capacity over time due to two factors: The first is the constant electrochemical interactions that occur inside the battery, even when at rest; the second, more significant one is the act of charging itself. Lithium ions expand when charging and contract when discharging, and high volumetric expansion causes degradation of the battery. Scott Mullin, a former Apple intern currently working in R&D at [http://www.seeo.com/ Seeo, a next-generation battery startup], compares the chemistry to breathing, saying "The deeper the breaths that you make it take, the more damaging that can be."
The main thing you can do to extend your battery's life is to follow one simple rule of thumb: ABC, or "Always Be Charging". The battery's typical Depth of Discharge, or DOD, is [http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries directly related to how many cycles it will take] before its capacity drops to 80%. The more you have to charge your battery before it's full, the less full charge cycles you'll get out of it.
You'll also want to calibrate once a month by running the device until the battery causes it to shut off, then charging to full. This will help ensure that the indicator always reports an accurate reading.
The temperature and charge level at which you store your phone also matter: Lithium ion batteries run best when stored at colder temperatures at around half charge. This is because all chemical reactions speed up at higher temperatures, including the ones which naturally degrade batteries over time. Unless you live in Ecuador, however, temperature shouldn't make much difference over the typical life of a mobile device. Just remember to avoid leaving your phone in your car on hot summer days.