TS I have a noob question.

My car battery produces 12V, and during engine running with alternator charging, after using headlights and aircon, alternator and car battery together produces around 13V.

While XiaoYi camera uses USB power rated at 5V, does it drain my car battery? since my alternator only produce extra 1V while using headlights and aircond.
If I use two cams, one front one rear, does it drain even more power?