voltage doubler वाक्य

My thinking is that when batteries are connected in parallel ( + ve to + ve and-ve to-ve ) the overall voltage is the same, but the current is doubled, whereas when they are connected in series ( + ve of one to-ve of the other ) the voltage doubles, but the current stays the same.
Some AC / DC equipment was designed to be switchable to be able to operate off either 110 V AC ( possibly with a voltage doubler ) or 220 240V AC or DC . Television receivers were produced which could run off 240 V AC or DC . The voltage was not high enough to power some circuits, so energy was recovered during the flyback period from the primary of the high tension ) supply.