A battery that goes flat overnight usually means something is staying awake or drawing current after shutdown.
Here are the likely culprits on a Kia Cerato/Forte III and the checks a mechanic would do first.
Why a Kia Cerato/Forte III battery dies overnight
A healthy battery should still have enough charge after a night parked. If yours is dead by morning, you are dealing with a parasitic draw: a circuit that keeps using current after the car and modules should have gone to sleep. On the Cerato/Forte III, that can be as simple as a glovebox lamp or as involved as a body control module that will not power down. First, rule out the battery itself and the charging system; a weak battery can mimic a draw, and a failing alternator can leave you starting every morning on a low charge.
