2012 jeep liberty dead battery.

Newbie question.

Just purchased a 2012 jeep liberty with the v6 engine. 38,000 miles. Automatic transmission. 4x4.

Truck has been sitting a few months. Battery was dead. When I put my battery jump starter pack on it, it starts right up. When I remove one cable off the jumper battery pack, the truck shuts off.

No alternator light or any other light on the dash.

Could it be a bad battery or is it something else that I should be checking.

Thanks in advance