I just checked and it says the battery on my iPhone 4s is 30% charged. If I leave my phone off, how much time will it have left? What if I turn it off?
I need to know if I'll be able to use this phone 6 hours later.
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Turning your phone off and back on will use up unnecessary power.
Take your phone, activate airplane mode, and then lock it. If you only need to wait 6 hours, this will probably give you the most battery remaining at the end of your wait.
If your wait time were longer, turning it off might use less power than having it on the entire time, but in this case I believe the duration is short enough to not warrant a restart (which is an electronically expensive operation for most machines).
While I do not think there is any official testing data that can accurately answer your question, turning off your iPhone should more than certainly leave you with battery 6 hours later.
In the past, I've tested out the standby capabilities of both my iPhone 4 and iPhone 4S. My test consisted of charging the iPhones to 100%, switching them to Airplane Mode, and locking the screen. I left them in that state over night (about 9 hours) and checked the battery percentage. They both reported 98% to 100% capacity, which surprised me greatly.
So I would like to think that you powering your iPhone 4s for 6 hours should keep the bulk of your charge in place. Maybe left with 25% of charge.
Of course, this is all assumption ;).
So yes you should be fine letting the phone sit for 6 hours, assuming it is not doing anything else and just idling in cellular Standby mode. If you really need the phone to be ready your safest bet is to power it down, then you know its not using any power for certain.
According to Apple's Spec for the iPhone 4S it gets about these times with 100% charge so roughly 30% of these number should give a rough estimate of how much time you can you run it for with a 30% charge remaining:
Talk time: Up to 8 hours on 3G, up to 14 hours on 2G (GSM)
2.4 hours 3G, 4.2 hours 2G
Standby time: Up to 200 hours
Internet use: Up to 6 hours on 3G, up to 9 hours on Wi-Fi
1.8 hours 3G, 2.7 hours Wi-Fi
Video playback: Up to 10 hours
Audio playback: Up to 40 hours
It depends! If you have app's running in the background using location based services and/or accessing network then it will be possibly out in 3-4 hours.
I would suggest you to turn off push notifications, turn off WLAN, turn off bluetooth and turn off automatic mail downloads. I would also consider shutting down apps like Facebook, Skype, Imo etc since they keep a network connection up in the background.
If I have such a low battery (and I can't recharge) I usually do shut down all running apps.