installed any new apps recently? added a new widget? something that is always running in the background or updating every hour or so. these wouldnt show in ur task manager. check out settings/applications/running services.. and see if there's any unnecessary process running.

 

Take a look at your battery usage stats, Menu>Settings>About Phone>battery Usage. This will tell you whats using your battery the most.

 

widgets not being used on the homescreen should not drain battery, nor should they show up in ur running processes list.. however, just note any new app/widgets that u installed.. and check if theyre using a lot of battery, using the battery usage as steslatt said.
upgrading firmware is very easy, u just need to get started once and u'll realize its not rooting, u dont have to root to flash new firmware.. go here:
[ROM + Guide]Official i9100 Firmwares KF1/2/3/4/, KG1/2/3/5/6/7, KH1/3 Download - xda-developers

flash XXKG5 if u wanna stick to official firmware, or XXKH3 if u dont mind trying out a leaked firmware.. both are great on battery, and nothing will be changed on ur phone..

rooting can also help save battery life, by freezing unused system apps that drain battery.. u can also control which apps auto start, etc etc.. these methods too have really helped me extend my battery life.. the best and easiest rooting method is the CF root method.. here's the guide

[17.08.2011][CF-Root v4.1] KE2/7/8, KF1/2/3/4, KG1/2/3/5/6/7, KH1/3 - su+bb+CWM4 - xda-developers

hope this helps

 

You dont lose any data when updating firmware.
It can be undone by flashing your original firmware (download a copy first)
Yes sometimes some functions are lost like text message skins are no longer present in the newer firmwares.

 

You are likely using more power because of an application that is polling the server for data. This could be a weather or social networking software. If the battery is all of a sudden being consumed quicker, there has to be a reason.

 

ur welcome steslatt has already answerd those questions. i hope take the plunge n flash an update (even without rooting) the only thing i can think of thats missing is the messaging app themes.. apart from that, its pretty much all benefits (the biggest one being battry life almost doubling, in my case)

 

Where can i find original firmware for download?

 

For carrier branded firmware see here

For unbranded firmware see my sig.

 

I found when I had similar issues.. I rebooted the phone... and that fixed the weird fast draining of the battery (when nothing seems to be running).

 

Not sure if its gonna make any difference but...
I followed one of the latest posts in tips & tricks sticky on this forum, started the phone in safe mode and it sorted my phone out as regards to memory usage etc, my phone seemed to be alot more responsive after rebooting, maybe nothing but try it, it maybe because there are less things running in the background and using more power.

 

thanks on battery usage

 

how do you start the phone in safe mode?