Battery dying when car not in use. Can it be my alternator?

Hi, this is Bo, an auto expert on RepairPal's Car Genius team! It sounds like you're dealing with a couple of issues here. Based on the voltage readings you took, it does seem like your alternator isn’t charging the battery properly. When the engine is running, you should see around 13.5 to 14.5 volts if the alternator is working correctly. Since you’re only seeing 11.5 to 12.5 volts, your alternator likely needs replacement or repair. Now, as for your battery dying while the car is sitting, that points to a parasitic draw—something in your car is draining power even when it’s off. The alternator itself usually doesn’t drain the battery while the car is off, but if there's a short or issue within the alternator, it could potentially cause a drain. You might want to check for any other accessories, lights, or modules that could be drawing power while the car is off. If you need any further advice or if you'd like any help finding a trusted RepairPal shop in your area, please feel free to call us at 877-913-1554!