Fluorescent lamp or tube lamp (TL) has good efficiency. This high efficiency feature make it suitable for battery operated lighting application. To drive the tube lamp, we need high voltage to make the gas inside the tube get ionized. Here is the schematic diagram of the fluorescent lamp circuit:

12v tube lamp circuit schematic

To trigger the ionizing process, we need high voltage as high as 200V or more. After the gas become ionized, the voltage will drops to about 40 volts. In the ionized state, the tube act like a short circuit if the voltage is higher than 40 volts. Large current will flow inside the tube lamp, and this make the voltage drops since the transformer has internal resistance. We can say the tube  lamp works like a zener diode (but bidirectional), stabilize the voltage around its working voltage. You must provide enough heat sink for TR1 transistor. You can use a standard 12V-230V step down transformer for this circuit, although we use it as a step-up transformer. The 555 oscillator frequency can be adjusted to get the brightest fluorescent lamp luminance.