You
always have to deactivate the option.
It depends on how much credit you had at the moment of the renewal of the option: if you had, for example, 10 euros, you would loose those 10 euros plus other 10 euros at the moment you refill. If you had 5, 5+15 and so on. If you had 30, you would loose 20 at the first renewal plus other 10 at the second renewal and other 10 once you refill. And so on.
It works this way in Italy...
Go and buy a new card, so you know how much you will pay.