I used my phone while charging on the go with my DC-18 portable charger. DC-18 was fully charged when I connected it to my phone. Why didn't my phone get fully charged?

Your phone consumes power when it is used as well as when it is on standby. Your phone’s battery may not end up fully charged in such occasions. DC-18 is a top-up charger that gives your phone an extra boost wherever you wish.