You will most likely be unable to start your vehicle if your key fob is damaged. On the other hand, some automobiles and vehicles will start by just pressing the dying key fob into the start button to start the engine. If your backup key does not start the car, it is not due to a low battery in the key fob. Instead, your keyless entry system is the source of the problem. There could be a problem with your entry system's computer programming. It's simple to figure out how to replace the battery in a Nissan car key. However, you must thoroughly read the instructions and be aware of the correct specifications for your battery. All Nissan models are nearly interchangeable with other vehicle key batteries from various manufacturers.