If it lost the charge not hooked up to anything it's probably bad. Either it isn't taking a full charge or it has an internal short and is discharging overnight. If you have a warranty you would like the store you bought it from to honor you can have them charge it and hold it for you overnight. If it's dead in the morning they should warranty it out. Or you could just dump out the acid and refill it with water, instant "bad" battery.
12v is about 50% charged. Charging it half way will ruin the battery. It needs to be put on a trickle charger overnight and charged slow. Fast charges can also ruin a battery. Running it low like that will cause it not to hold a full charge and it will lead to sulfation over time.
Mine right now is showing signs of sulfation. I let the battery totally die a couple of times and in my ignorance I didn't charge it fully. Now it won't hold a full charge and it's slowly dying. It won't make it past another winter. The ZJ hates low batteries too. http://batteryuniversity.com/learn/a...to_prevent_it/