iPhone reboots after apple logo

I have some very strange problems with my iPhone 4S (16GB), I got it from my friend and he doesn't know why the iPhone can't start anymore.

If I turn on the phone, I see the Apple Logo and after some seconds, the phones crashes and is restarting in a loop, always crashing on the Apple Logo. It looks like something is termenating the phone - is there a problem with the bootloader?

I tried several things:

• Turn phone in DFU-Mode -> iTunes Recovery -> iPhone is restarting during the Recovery and loops
• Turn phone in Recovery-Mode -> iTunes Recovery -> iPhone is restarting during the Recovery and loops again
• Turn phone on and hold the upper volume button -> reboots after some more seconds

*

I tried to restore the IPSW file from Apple Server for iOS 7.1.2.

Here is a video:

https://vimeo.com/107279805

I also can't restore any files with programs like "Wondershare Dr. Fone" - the program needs a phone which can boot up.

So, does anyone know any more tricks or something how I can "repair" the iPhone?

OR: Does it look like a "dead battery"?

Thanks!