Unlike handhelds such as eTrex devices, automotive devices like his nuvi are meant to be kept externally powered constantly when on in a vehicle... so by design will last for a very short period on battery-only power, like maybe only half an hour if actively navigating even with a heathy charged battery - but much, much longer in idle standby. Actually, it can be weeks. Even when the device is awakened but then shuts itself off spontaneously due to low power there will still be a residual charge in the battery, so to quicken the process it must be constantly powered on occasionally until the battery is at 0 volts to ensure all current is drained from the PCB. Disconnecting the battery is the surest way to achieve that with certainty but i hesitate to encourage users to do so due to the difficulties opening the case.