This could be caused by several things, including:
* Weak or failing battery: This is the most common cause.
* Faulty alternator: The alternator is responsible for charging the battery while the engine is running. A failing alternator won't properly recharge the battery.
* Loose or corroded battery cables: Poor connections can prevent proper charging and power flow.
* Parasitic drain: Something in the vehicle's electrical system might be drawing power even when the car is off, slowly draining the battery.
You'll need to troubleshoot these possibilities. Checking the battery voltage with a multimeter is a good first step. If the voltage is low, even with the engine running, the alternator is likely the suspect. If the voltage is good while the engine is running but drops significantly when turned off, then a parasitic drain needs to be investigated.
