A while back I took spliced a USB cable and was able to get +/- cables from it and use it as a way to feed power from a battery pack to an Arduino project.
Yesterday I spliced an apple iPhone charging cable and I can only get a voltage from the red cable and the threaded metal jacket surrounding the cables. Why is that?
How do I know which cables to tear apart and which ones not to?
Why is that?
Apparently, they are using the shield as ground. It's normal for the shield to be grounded (if there is a shield) but sometimes there is an additional ground.
How do I know which cables to tear apart and which ones not to?
You don't. They aren't sold to be torn apart... But, there has to be +5V and ground at the "far end".
You take them apart, and see if they're any good 
Luckily, they're cheap as dirt (I bought 9 pretty green braided flat micro USB cables for $7.50 this week - though I haven't chopped any) so it's not going to break the bank. You can also, for only a small premium, get cables that just end in the 4 wires, or in a barrel jack (ebay) - like 1.50-2.50 a pop.
I would prefer generic cables over fancy brand name ones for chopping up (and in general - I'm cheap). The branded ones are more likely to do something weird (like your apple ones did).
As it happens, I have a bunch of USB A male to USB A male cables. Like, over 100 of the damned things - I can't think of any use other than chopping them up to get USB power cables for projects (you get 2 for 1). I need to sell them or something...
The shield is the ground. Or use a proper usb lead instead of Apple Shite
Standard USB is red/black for power, green/white for data, shield for shielding.
Getting rid of the black wire allows a much thicker red wire for high current.
The original limit for USB is 0.5A maximum, but these days phone-chargers routinely used
2A which is 4 times the original design current and thus 16 times the dissipation in the cable
(I-squared-R losses) if you don't upgrade the wire thicknesses.
For just powering, get a USB lead without data wires, then the red wire can be a decent thickness.
Cheap long USB cables are not satisfactory for high current 5V power, rather large voltage drops
are seen.
Hey @Marciokoko, maybe you just want to get yourself a little usb/pcb adapter to attach it to your Arduino? Also available for other sockets.
Best
