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The battery is in the trunk under the compartment cover.
When replacing the battery, disconnect the negative (-) cable (1) and remove the positive (+) battery fuse box (2).
Remove the battery mounting bracket (3).
WARNING - Touching metal surfaces in the trunk under the compartment cover

Do not touch metal surfaces in the trunk under the compartment cover while the engine is operating or hot. Doing so could result in serious personal injury. Turn the engine off and wait until it cools down or wear gloves to replace the battery from the luggage room.
Your vehicle has a maintenance-free, calcium-based battery.
When recharging the battery, observe the following precautions:
Reset items
Items should be reset after the battery has been discharged or the battery has been disconnected.
Kia Stinger (CK) 2018-2023 Owner's Manual: Engine Room Under Cover
Repair procedures Removal and Installation Engine Room Front Under Cover. 1. Remove the engine room front under cover (A). Tightening torque : 7.8 - 11.8 N·m (0.8 - 1.2 kgf·m, 5.8 - 8.7 lb·ft) 2. Install in the reverse order of removal. Engine Room Rear Under Cover.Repair procedures Removal 1. Turn ignition switch OFF and disconnect the negative (-) battery cable. 2. Remove the luggage side trim. (Refer to Body - "Luggage side trim") 3. Disconnect the ECS control unit connector (A). 4. Loosen the ECS control unit bracket nut and then remove the ECS control unit (A).