You are right. I remember more details now. The problem occurs when you want to make regulated DC from a lower voltage DC supply. Step-down is easy.
For example, my battery is 12V, but my laptop needs 19V. AFAIK, you can't get regulated 19V supply from the 12v battery without a transformer (or maybe something equivalent).
Step-up is easy too. That's what boost converters (http://en.wikipedia.org/wiki/Boost_converter) are for. Most single-cell LED flashlights contain tiny boost converters to increase the 1.5v coming from the battery to above the drop voltage of the LED.
For example, my battery is 12V, but my laptop needs 19V. AFAIK, you can't get regulated 19V supply from the 12v battery without a transformer (or maybe something equivalent).