In a 2005 Buick Lesabre, the battery is underneath the rear passenger side seat. While there are terminals under the hood to make jumpstarting a dead battery more convenient, you’ll have to remove the back passenger seat to access the battery itself.
Most vehicle batteries can last between three to five years before they need to be replaced, so knowing your battery’s location can help you when you’re due for a new one. You can always replace the battery yourself to save a little money, or you can take your vehicle to a mechanic and have them inspect the battery for you.