Batteries can drain very slowly over a long period of time. If the battery is still new (less than 6 months) it should not drain out on it's own. If it does then you may need a new battery.
If I were you I would invest in a handheld voltage meter that you can check anytime and charge as needed. Amazon.com: MinnKota Digital Battery Meter: Sports & Outdoors
I have three batteries and I charge mine directly after using and let it sit for a week or two before I go out again and even when it gets cold my batteries are holding the charge.