Making use of bike battery to charge mobiles without any other power resource?
It would be better to use a small DC-DC converter to charge your mobile from the other battery.  Most phones these days charge from 5VDC which they get from a USB connection. There are commercial adapters that convert 12VDC to 5VDC and provide a USB connection.  These are normally used in cars and plug into the cigarette lighter or 12VDC outlet in the car.  I would buy one of these adapters (12VDC to 5VDC USB) and attach it to the bike's battery using a 12VDC outlet/ cigarette lighter outlet.

The 12VDC  to 5VDC USB adapters are availabe pretty much any place that sells electronics.  The 12VDC outlet will probably have to be bought in a store that sells parts for cars or boats.

On a quick google search, I see that you can get both parts from Amazon.  I checked again, and there are finished units that do 12VDC to 5VDC USB - again from Amazon.

BTW:  Do you mean "Bike" as in "Motorbike" or do you mean "bicycle with battery assist?"