Why portable GPS for my car is turning on for only few minutes?

Actually, you have done wrong by connecting your GPS device to the cigarette lighter. Because it could damage the power supply of the device and also corrupted the electronic circuitry of your GPS gadget. You should not do this act especially if you are not aware of the car power supply mechanism. The main reason being that the cigarette lighter is a high-voltage charger which has a high-current flow. It is used for burning the cigarette. However, it is not for any sophisticated electronic device like Garmin Nuvi. The high-current flow from the cigarette lighter burned up the sophisticated circuitry of the GPS device. Technically, the diodes in the GPS gadget damaged due to the high-current flow. In the car battery, the current flow is generally more than 20 A. So, the GPS device only withstands the current flow of 300 mA. Therefore, when you put your GPS device into the cigarette lighter, then it instantly damaged the diode and killed your gadget. However, you are lucky enough that your GPS gadget is not completely damaged rather it is partly damaged. So, take your GPS device to the technician who will change the diode and take necessary steps to restore your Garmin Nuvi.