As
@TheDeadlyShoe said, shields work simultaneously to recharge over the whole ship, not one at a time. Hence why shields are a more popular choice for defending lager ships because you get an exponential value the more you add. Ton for ton armor gives more raw defense, but shields give a better buffer as they constantly recharge and block shock damage. Also keep in mind you can repair shields mid-battle, and armor you cannot.
Edit; Forgot to mention they constantly regenerate. Even when you ship is hit and shields go down to half, they will still regenerate the next 5 second increment.