Il m'a toujours semble qu'il existait des cartes purement magnetiques sans qu'aucun courant electrique ne soit necessaire. Je me permet de citer une discussion en Anglais d'un autre Forum:
A magnetic card contains a magnetized strip that aligns magnetic
tumblers into the exact positioned needed to open the door. It is
essentially the same as a regular housekey, i.e., it uses no
electricity. It looks like a thick plastic credit card. They also make a
kind that looks like a credit card sized piece of Swiss cheese: the
tumblers fit into the holes. Same idea.
The elctric variety work on the same principle as an ATM card; an
electric reader identifies the card as the one that activates the
lock.
source
http://www.ottmall.com/mj_ht_arch/v15/mj_v15i67.html#COY