Power bank fixing

I built a power bank using 6V,4.5Ah battery.
When i Connect a cellphone for charging My phone shows that it is charging but phones battery level is not increasing instead if i run some online games while charging with it phones battery level is decreasing.
I used 7805 IC and Two capasitor to establish a 5Volt,1A current.
When I measure output voltage and current it is fine....but when i measure it while charging a mobile phone it shows output voltage 4.1V
what should i fix now
Sorry for grammatical mistake.