Is it safe to use the cable used in Portable Battery Charger and connect to the phone via the PC USB?
It's very likely completely fine. You should be getting 5V on the non-PC end of the cable and your phone will limit current appropriately for charging. The only issue I can see is some companies will make cables proprietary by adding voltage dividers to the data pins, perhaps this is the resistance levels you're talking about? However, this will only mean your phone will not charge, it won't be damaged. This, of course, is all assuming the connector is correct.