Hello Fanil92,
Welcome to Apple Support Communities.
It sounds like there's an issue using iTunes to update iOS on your iPhone 3G, and there's also an issue with the battery draining quicker than expected on your iPhone. The article below should help you resolve any issues updating iOS on your device.
If you see an error when you update or restore your iPhone, iPad, or iPod - Apple Support
If the battery life issues persist after updating iOS, the information and troubleshooting tips in this article should help:
Batteries - Maximizing Performance - Apple
Cheers.