When a battery is left installed on a device for an extended period of time, they will begin to self-discharge and in more serious situations, leak. If this happens to you and the battery leakage affects the LED flashlight in any way, you may be out of lock from the manufacturer who is selling the flashlight. For example, Led Lenser, a manufacturer who made an appearance on this list, states on their FAQ page that their warranty does not cover damages from battery leakage.
So, you may be out of luck with the LED flashlight designer. However, as Led Lenser also states, many times the brand name of the battery will warrant their batteries and the devices that are being powered by them. In this case, they may reimburse you for both.