5Ahr phone bat is probably Li-ion single cell, therefore 3.7V. So your phone needs about 18Whrs (5Ahrx3.7V) to charge from 0% charge to 100% charge.
A 12V 7Ahr battery contains about 85Whrs of energy so assuming no losses you could completely charge your phone about 4.5 times from that battery. Now there will be some losses in both batteries and in the charger itself which will reduce the number of full charges.