You only need to charge your tarot cards when the cards are tired or fatigued. You know when the cards are in this state when the readings are poor or don't make sense.
Cards get tired for a couple reasons. One reason being that they reflect your emotions or feelings, and you may be tired for doing readings for too long. Since you are tired or worn-out, the cards are. Another reason the cards may be tired is that they make contact with the energies of the people you read for. This is also why it is very important to shuffle your cards thoroughly.
You would charge tarot cards like how you would change the batteries in a flashlight.
One way to charge your cards is to place them in direct moonlight or sunlight. The sun and moon have different energies. Watch the moon phase as you charge your cards. The sun has no phases, so you wouldn't really have to watch it. Leave your cards in direct sunlight/moonlight for a few hours and theyre charged.
Another way to charge your cards is to place them in order, right side up, and put the deck back together. Think of it as if the cards were family. Through all the shuffling you've done, the family has lost eachother. When the cards are placed back together, they are linked and connected.
You could also lay out your cards on a reading cloth and sprinkle sea salt on them.
Hope this helped :)