flat battery

[npca-aussie bugger]

Hi guys, my battery has gone flat on me a couple of times and i have just discoverd that i have power being drawn from my battery when it is not in use, after searching for this power loss i discoverd that it is the alternator. Does anyone know how an alternator can suck power out of the battery when the truck is sitting there doing nothing?   email me at saal4@iprimus.com.au