I have a proyect using an Arduino mega rev3, with an ultrasonic sensor and a RGB LED to indicate the distance(calculated with the sensor), it is powered with a verbatin Powerbank, which is conected to the ground and to a resistor in the PCB, to drain the energy, which is the same as conecting to the VIN

Now I want the same but in a reduced size, for what I want to use an arduino nano, with the same equipment except the battery, but I do not know which battery to select (one without a turn on feature), and if i have to make a converted circuit or I can connect it to the VIN pin directly, or even to the USB.