I upgraded my original Iphone(2G) to 3.0 yesterday using pwnage from 2.2.1 and have been experiencing battery issues ever since.
After fully charging my battery and not using the phone, the battery lasts for about 3-4 hours.
Also did a full restore from ITunes to 3.0 and used redsnow to JB/Activate but no luck(I have no apps installed from CYDIA or anything).
But I do have a few apps from AppStore, as I restored from my earlier backup.
Any suggestions?

It's because push is broken on 2g hacktivated phones. Turn off push notifications in settings. Read more here: Push Notifications on hacktivated 2G iPhone Bram Bourgonjon

Yes indeed it was push notifications which was killing my battery. Once I turned it off my phone is functioning normally.

wish i could turn off notifications. my battery has been dismal since upgrading to 3.0

when i go into settings, notifications isnt even listed there!

Install a app that has push like taptap2 2.6, ap news, textfree light. Then go into settings and disable it!

i already have: AP and AIM installed, but notifications arent showing up. i'll try unistalling them, then reinstalling to see if that helps.

*EDIT*

thanks, installed tap tap and now notifications showed up. i dont know why it wasnt there before from the other apps ??

AIM does not currently support push. Try textfree lite, I know for sure that supports push. The setting in the settings app should appear right under wifi and its just called "notifications" with a red dot as an icon.

Edit: Cool glad you were able to get it fixed! I made a new thread explaining the problem in more detail. Later

Moreover i noticed that when battery is low it first show me 20% message...then, after 10 mins or so...it switches off. No more 10% message. Has anyone noticed that?

I just upgraded my 3G to 3.0 OS and I noticed the same thing. After a quick burn through the battery in about 6 hours I received the 20% message then it shut off shortly after.

Yeh same here, I restored but the battery still only lasts around 6 hours before dying and I don't get the 10% warning either.
iPhone 2G by the way.