my boat eats batteries

[ksievers]

new here, looking for help!! I have an '03 Skeeter 21 footer, with the Yamaha 200 HP outboard. All is great, except (!) I have been through several cranking batteries. Way too many way too often and way too soon.

Yes, I store it out of the weather with the accessory power off. My problem is, when I take it out to go fishing, the cranking battery is usually totally dead. For a time I'd be able to recharge it in the driveway before heading out, but that got worse and worse. I've had the rectifier replaced already. (It has a 24v trolling set-up if that matters, onboard charger for that).

Most recently I was leaving it stored hooked up to a battery tender, but last trip out the battery was deader than a brick, and when I checked it was bone dry too (apparently the tender cooked the battery even though it is supposed to sense when it's charged and turn itself off)..

but what I'd love to find out is why it goes dead while being stored in the first place? How do I trouble-shoot that? If I just let it sit for a week or few it is very likely to be dead.

help??

thanks - KS