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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: Bulb replacement precaution
Please keep extra bulbs on hand with appropriate wattage ratings in case of emergencies. Refer to “Bulb Wattage” in chapter 8. When changing lamps, first turn off the engine at a safe place, firmly apply the parking brake and detach the battery’s negative (-) terminal. WARNING - Working on the lights Prior to working on the light, firmly apply the parking brake, ensure that the ignition switch is turned to the LOCK position and turn off the lights to avoid sudden movement of the vehicle burns to your skin or fingers, or an electric shock.Kia Stinger (CK) 2018-2023 Owner's Manual: Owner maintenance
The following lists are vehicle checks and inspections that should be performed by the owner or an authorized Kia dealer at the frequencies indicated to help ensure safe, dependable operation of your vehicle. Any adverse conditions should be brought to the attention of your dealer as soon as possible. These Owner Maintenance Checks are generally not covered by warranties and you may be charged for labor, parts and lubricants used.