Many things can cause a battery to discharge at a variety of rates. Bad iOS, bad battery, bad other hardware, etc.
But one needs to understand the nature of wireless communication even in the climate of not consciously/actively using the device. You could charge it up to 100% and not actively use it and just by being mobile (drive around town, between towns, etc., the battery can and will discharge in a non-linear, non0predictive manner.
Why? Well, there are a variety of expected, by design and engineering, reasons.
There are power optimizations taking place constantly between your handset and MANY cell transceivers. If you think triangulation means you are communicating with only three towers at any given time then thing again.
Additionally your handset is constantly being told to lower its transmitting signal strength and then back up again by multiple radios on multiple towers. This is partly to minimize power on the carrier's network, but also to predict your movement and to prepare another tower in case that prediction is correct.
Then there is increased power on your handset's transmitter in increasing shielded environments, in the environment of others transmitting near you, near your optimized tower, etc. All of this dynamic power-related behavior is completely transparent to the end user--unless you are in RF design, of course. And yes, I took a couple of classes in RF design for one very large wireless carrier in the US.
Now, aside from all that mumbo-jumbo I just said please keep the following in mind ...
Before I bought my iPhone I had a My Touch 3G made by HTC and branded by T-Mobile. I had it for eighteen months before the charge on the battery changed dramatically over night. It use to have a standby for days, but then changed to 4 hours.
I could charge the battery and remove it and it would hold the charge. I could put the charged battery back into the phone but leave it powered off and it would continue to hold the charge. But as soon as I turned the phone on it would discharge in less than four hours.
It wasn't the battery, not how I let it sit on the desk doing nothing. It was the hardware in the phone. Something failed and that failure caused the battery, which was good, to discharge rapidly. My response was to "
F-HTC!" and never bought one of their products again.
Not advocating Apple, its hardware or software, but just keep in mind that the nature of battery energy going somewhere can have a large arena in the final root cause.
