A true piezo device needs no battery. It is mechanical. It is the same principal that allows crystals to be used as fixed frequency sources for radios and computers.

Now, that said, there are other devices out there that use batteries... (my Weber has one) that spark rapidly when a button is pushed... different electronic principal. Those are multivibrator circuits that have the output sent through a step up transformer to make current that sparks... sort of like a mini taser.

The difference is a piezo starter requires one push for one spark, the battery device puts out a series of sparks for as long as the button is held and the battery is good.