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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: Side Sill Molding
Repair procedures Replacement Put on gloves to protect your hands. • Use a plastic panel removal tool to remove interior trim pieces without marring the surface.(1) Power liftgate open/close button (2) Power liftgate handle switch (3) Power liftgate close button ✽ NOTICE If the engine start/stop button is ON, the power liftgate can operate when the automatic transmission is in P (Park). WARNING Never leave children or animals unattended in your vehicle. Children or animals might operate the power liftgate which could result in injury to themselves or others, or damage the vehicle.