You need chargers to complete collections.
You can find the number of chargers you need in the collection. For each charger you have two numbers separated by a backslash. The first number shows how many chargers you have in total and the second shows how many chargers you need to complete the collection. If you have got enough chargers the area will be green if not enough the area will be red.
You can get chargers the following way:
You can look for the chargers in the buildings. Go to the building and tap the Level Info button. Level Info shows the information about the collection items and chargers you may find in this building.
Go to the collection and tap the charger you have not got enough. If you choose Find you will be prompted to the required spot in the city where you can find this charger. The chargers will be recalculated automatically. Note that the Complete button becomes active against the collection that you can complete (which you already collected).