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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: Maintenance services
You should exercise the utmost care to prevent damage to your vehicle and injury to yourself whenever performing any maintenance or inspection procedures. Should you have any doubts concerning the inspection or servicing of your vehicle, we strongly recommend that you have an authorized Kia dealer perform this work. An authorized Kia dealer has factorytrained technicians and genuine Kia parts to service your vehicle properly.(1) SPORT mode (if equipped) This mode displays Gauge, Lap Timer, G-Force. (2) Trip Computer mode This mode displays driving information such as the tripmeter, fuel economy, and so on. ❈ For more details, refer to “Trip Computer” in this chapter. (3) Turn by Turn mode (if equipped) This mode displays the state of the navigation. (4) LKA/SCC mode (if equipped) This mode displays the state of the Smart Cruise Control (SCC) and Lane keeping Assist (LKA).