I've already checked the subject. I found some low power rf transmitters that have like 8-10mA current consumption when transmitting, which seems like a lot for a key that has the battery changed once a year or a couple of years.
Thanks for the answers. I didn't know about active rfid. I also found that there is an battery assisted passive rfid, although I don't find any diy guide. Everything related is only available as end product.