With a new (to you or otherwise!<G>) boat, you really need to scope it out, stick your nose in every inch from bow to stern. With the
windlass, start by removing any cover plates or upholstery below it and check for corrossion or bad wiring. Follow the cables--they should be battery cables--from the
windlass and you'll find out what powers it. With a 415, there "should" be a dedicated battery up front to power the
winch, because there are only two ways to get high amperage up to the
windlass.
One is to have a battery located up front, so you only need battery cables from it to the
winch, and you can use lighter cables running aft for charing.
The other is by running heavy cables (possibly 0 or 00 gauge) all the way aft to the DC supplies.
The third option--using undersized cable/wires, is simply no good.
When you say "the battery" was drained right away...Were you able to check the main batteries? Is there a manual safety cutout for recharging a
winch battery, perhaps?
There's no telling what the PO's have done, don't assume any wire that "starts here" actually "gets there" in one run.<G>