Phone chargers - all mobile devices that we use have a built-in battery. This means that your smartphone or tablet device has to be charged every so often. Smartphones and tablets now almost exclusively use lithium-ion cells. These require precise charging systems with control over current, voltage and cell temperature throughout the charging process. Most of these functions are fortunately built into the device itself, so phone, smartphone and tablet chargers only need to provide a stable 5 V voltage and adequate current capacity.