iPhone 3GS battery problems

1. See if this helps:

http://www.apple.com/batteries/iphone.html

2. Very doubtful. You could try a reset or restore to see if that helps.

3. There is no legit way to downgrade.

When restoring your phone many times, did you restore it from your back up or set up as new?

It is possible that you might have a corrupted software from your back up, so therefore setting up as new is the way to go.

If the battery still drains even if set up as new, then take it to your closest apple store. Make sure you set up an appointment with the genius bar.

There are other threads out here with things to try...Look for those. If you try all these other suggestions and are still having drain issues, restore as a new phone, and don't add anything back yet. Just browse web, youtube, maps, txt, etc for a while, as well as standby...Let it go for as long as possible, and see how your battery holds up. If it's not ok at this point, then do a DFU mode restore. If that still doesn't fix it, then I'd say it's a trip to the Apple store for you...If this fixes the issue, then add back ONLY 1 thing at a time. Mail first usually, and watch for a while to see if it drains again. If not, proceed to next item(apps 1 at a time). After each added item, watch for a while to see if it drains ok. Should be able to narrow it down to the culprit. I know this takes quite a while to accomplish, so patience is needed, but as has been previously stated, it's usually bad or corrupted data in a backup that causes this...I had posted in another thread of same issues here. I was fine after the 4.2.1 update for quite a while(enjoying almost 3.1.3 battery life), then for reasons yet to be determined, mine started draining. I went into Settings where the Tethering item is located, and it was constantly clocking on that. Usually it clocks for a few seconds, then says Setup Tethering. Well mine was stuck there, so i think it was constantly using something. A hard reset cleared it, but I still have to try and figure out why it did that. I just don't have the time right now. I will most likely try as restore as new phone at some point. Apple did something though, because like I said, I'm enjoying almost 3.1.3 battery life now. I think it may have to do with cell radio, as I've seen something about NCFD being supported now in 4.2.

I have had problems with my battery and I tried most things. I got a new phone a couple of weeks back and noticed the battery lasted really well for the first day. As I started to "use" the phone more the battery was getting much lower than I thought it should do and I noticed that the data use was increasing. I then closed all open background apps and over the next few days closed every app after use. My battery now lasts ages, I think some of the apps (BBC News for instance) have their own background settings that allow you to decrease the update interval but the longest is every 30 minutes, but you cannot tell it not to update.