the battery on my astra mark 4 tends to drain overnight. if its fully charged it will last a few days an then die. i bought a brand new battery and thought it was faulty so i took it back and got another one and have the same problem. my alternator is working fine so sumthing must be draining the battery. if i disconnect the battery overnight and connect it agen the nxt day it is fine any ideas what could the drain be????