A typical mobile device begins to consume power as soon as you start using it. But there's another approach to e-writing that conserves battery life and power consumption. It all relies on a special type of liquid crystal display (LCD).
Here's how it works: The devices, such as Improv Electronics' Boogie Board e-writer, have a resistive screen. This screen has layers of electrodes with a layer of liquid crystals sandwiched between them. The liquid crystals' appearance changes depending on how they align within the layers. Normally they're transparent. But when you put pressure on the resistive screen, the gap between the electrodes decreases and the liquid crystals flow into a new alignment, becoming reflective.
Put a dark background behind the screen and presto! You've got yourself a writing surface. As you put pressure on the screen with a stylus, your finger or any other object, the crystals will flow and change into a reflective layer. The reflective layer stands out against the dark background. And there's no need for the e-writer to supply power to the screen -- you can just keep writing until you run out of space.
On a basic e-writing tablet with an LCD screen, the only time you'd need to tap into the battery's power is when you want to erase the screen and start over. This will prompt the device to send a small electric charge through its electrodes and coax the liquid crystals back into their normal state.
Such a basic tablet does have a major drawback -- there's no way to save your notes. It's akin to writing information on a pad and then tossing each piece of paper away once you're done. But by including a couple other elements you can preserve your thoughts for posterity.