I’ve come across lots of tweets, blog posts, and in-person comments regarding battery life on older devices after having upgraded to iOS 7. While I know many people simply enjoy complaining, perhaps a dose of knowledge will help ease misguided anger.
There are two primary factors in the degradation of lithium-ion batteries. The first is charge cycles. Apple rates iPhone batteries at 400 charge cycles.1 An iPhone battery that has been recharged more than 400 times will experience decreased performance. Second, these batteries have an expiration date. The useful life of a lithium-ion battery is two to three years, even if it goes through zero charge cycles.2 The battery in an iPhone 4s purchased at launch in October 2011 has likely reached the end of its usable life.
iOS 7 was launched alongside new iPhone models with improved batteries. New, more power-intensive features were likely developed with the new battery technology in mind. Older devices not only have previous generation battery technology, but those batteries are approaching, if not already past, their usable life cycle.
If you’re not ready to upgrade your device, but your battery has exceeded its usable life in charge cycles, age, or both, You can purchase a new battery at an Apple Store or Apple Authorized Service Provider.
1: http://www.apple.com/batteries/iphone.html
2: http://electronics.howstuffworks.com/everyday-tech/lithium-ion-battery2.htm
The Mac Admin 
great info, however, you did not define what a “charge cycle” is –
from http://www.apple.com/batteries/
“A charge cycle means using all of the battery’s power, but that doesn’t necessarily mean a single charge. For instance, you could listen to your iPod for a few hours one day, using half its power, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two, so you may take several days to complete a cycle. Each time you complete a charge cycle, it diminishes battery capacity slightly, but you can put notebook, iPod, and iPhone batteries through many charge cycles before they will only hold 80% of original battery capacity.”
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Thanks, this is great info to keep in mind as well.
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