Can't charge a cell phone with a power bank I built

I built a power bank using a 6 V, 4.5 Ah battery.

When I connect a cellphone for charging, my phone shows that it is charging, but the phone's battery level is not increasing. Instead, if I run some online games while charging with it, the phone's battery level is decreasing.

I used the 7805 IC and two capacitors to establish a 5 volt, 1 A current.

When I measure the output voltage and current, it is fine - but when I measure it while charging a mobile phone it shows the output voltage as 4.1 V.

What should I do to fix it now?