Hi i have an S500i@ W580i and i was wondering if anyone else experiences a really short battery life with this phone? It barely lasts a day if i use it for a total talk time of about an hour or so and then a bit of texting here and there. How long does your phones seem to last?

There is always going to be a problem when you flash a different models firmware.
Try to go back to S500i firmware.

Are you using a Flash Wallpaper?
Flash menu?
Run JAVA app in the background?
Modified lighteffects?

I once put a flash menu and flash wallpaper on my w580 and it died in about one and a half day.

im using a flash theme, everchanging, i turned off the pedometer, using regular lighting effects, keep 50% brightness, no java apps in the background, no ringtones only vibrate

My phone lasts probably two days. But I use a LOT of the features. I use the pedometer, flash menus, games, texting, music (very often) and I call a lot.

Using any Elfs? Soundslide?

Mine typically lasts about 2 days, using DesktopFullScreen menu icons, static image wallpaper, SoundSlide elf, MiniGPS elf which sets my profile to Home when I reach Home(enables ringtones) and if I move away from home it goes to silent mode.

About 30+ messages and 1 phone call a day.