My iPhone 4 boots when docked, else bootloop

I have a very strange one:

I bought the device to repair it (Why? I don't know ...). All markers have detected liquid contact, but the device starts when connecting to my LG Bluray Hometheater with iPod Dock. However all other USB Cables, no matter if connected to a MacBook, PC or 2A adapter will leave it in a Bootloop.

The Battery connector was torn out, I soldiered it back, but it seems like the battery is also bad (got just 2.2V and I guess it won't charge anymore) unfortunately when I tried to replace it, that crappy connector dropped of again (and I even glued it with industrial glue ...). So I'll have to soldier it back tomorrow.

Installed is iOS 6 if that is relevant.

So: why does it boot with the Bluray (then asks me to connect to iTunes) but not with USB-Cables? Both cables are not original ones, but work with my other iPhone 4.