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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: Rear Glass Defogger Printed Heater
Repair procedures Inspection • Wrap tin foil around the end of the voltmeter test lead to prevent damaging the heater line. Apply pressure on the tin foil with hand and move the tin foil along the grid line to check for open circuits.Kia Stinger (CK) 2018-2023 Owner's Manual: Emergency fuel filler lid release
If the fuel filler lid does not open using the fuel filler lid opener button, you can open it manually. Remove the panel in the cargo area. Pull the handle out slightly. CAUTION Do not pull the handle excessively, otherwise the luggage area trim or release handle may be damaged. WARNING - Refueling Always remove the fuel cap carefully and slowly.