Power supply vs battery

[albafrati]

Found out that the resistor value greatly affects stability. With 1.5KOhm phone works almost flawlessly. I'd like to know why but I think it's beyond my skills and knowledge.
Anyway I found the cable... I'm gonna build it up to check serial console messages than I'll post results for anyone interested.
Thanks 

[albafrati]

Last update:
serial console doesn't report too much. When the phone hangs gets stuck with "Starting kernel ..." but it never starts though there's current flowing (0.4A).
Anyway I reached an acceptable condition with 1.5K resistor.
Thanks everybody...