Try to clean the metal contact at the iPhone USB cable. If it is not attached firmly, you may want to check at the connection, female end, of the iPhone. Clean it with proper tools, or if it is not available, try to use tip of paper.

If you been using it actively after 1.5 year (certain charge cycle) the battery is certainly, dead. Consider change the battery ur self. Get the iPhone battery that comes with a free prying tool. Require basic soldering skill. Check youtube.

You might want to consider buy extended battery pack at Low Yat, RM79.00. Do not have to open the iPhone at all. But it will look funny.