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.
The chargers will be recalculated automatically. Note that the Complete button becomes active against the collection that you can complete (which you already collected).