technically speaking yes you could make a backup of the stock rom from one phone and use it on another but you wouldn't want to backup the data and a bunch of other things. this would be way harder then just using odin. Follow these step word for word and it will be much easier and safer then what your thinking of doing:

Be Rooted

How to get back to stock from any rom or recovery

Step 1: Download and install these files to pc:
Odin- http://samsung-updates.com/Odin307.zip
Samsung Driver- http://tinyw.in/Fdoz

Step 2: Download one of these Factory STOCK Bloated 4.0.4 roms and the stock kernel to your pc:
VRALF2- http://bit.ly/Pp1Kdb (you will need to unzip these to extract the .tar file)
VRALG1(Latest)- Goo.im Downloads - Browsing C...w_ship.tar.zip(Don\'t forget to thank Invisiblek!!)
Stock VRALF2 Kernel- http://samsung-updates.com/kernel/Sa...LF2_Kernel.zip

Step 3: Power device off, then hold the volume down/home/power keys until you see a yellow triangle, then press the volume up key to access "download mode"

Step 4: Open Odin and connect the oem usb cable to pc & device, you should see a "comport #" in upper left hand corner of odin

Step 5: Uncheck auto reboot

Step 6: Click on the PDA function of odin and select the Stock ROM you downloaded in step 2. Should take roughly 8min, then odin will say pass/reset

Step 7: Click on the PDA function of odin and select the Stock Kernel you downloaded in step 2. Should take less than a minute, then odin will say pass/reset

Step 7: Disconnect usb cable, then pull and reinsert the battery

Step 8: Press and hold the volume up/home/power keys until you see a green android, you should boot into stock recovery

Step 9: Wipe data/factory reset

Step 10: Reboot! (You should now be booted into factory stock, locked, and bloated ICS 4.0.4)

 

Undertow,

Just to clarify, when I open Odin, there are two boxes checked on the right: "Auto Reboot" and "F. Reset Time". When flashing to my S3, I should uncheck "Auto Reboot" but keep "F. Reset Time" checked?

 

As described in that thread multiple times, that bug/fix only affects the battery stats display and has absolutely nothing to do with the battery's drain. If you're sucking juice in standby then something is wakelocking it; get the free BetterBatteryStats app from xda to see what's going on and check your settings. Also check your signal strength.

If your phone is only 3 days old, I doubt you have that issue; did it have an "updated" sticker on it and/or the box?

There are several stock ROMs available for download that you can flash without using a backup from another phone (which I have no idea what the effects of that would be); detailed procedures are stickied above. However, this isn't really necessary if you don't return a very good (the best) phone because of a user error.

 

yes, only uncheck auto reboot and leave reset time alone. But I do agree with rolo you should check for other problems before returning your device. before I rooted and switched to a custom rom i was getting a day and half to two days battery life, without using it a lot. I now get 3 days battery life without using it much. Check your apps and other programs you have going to see what is using up all that battery your talking about.