  Quantum batteries are devices made from quantum states, which store and
release energy in a fast and efficient manner, thus offering numerous
possibilities in future technological applications. They offer a significant
charging speedup when compared to classical batteries, due to the possibility
of using entangling charging operations. We show that the maximal speedup that
can be achieved is extensive in the number of cells, thus offering at most
quadratic scaling in the charging power over the classically achievable linear
scaling. To reach such a scaling, a global charging protocol, charging all the
cells collectively, needs to be employed. This concludes the quest on the
limits of charging power of quantum batteries and adds to other results in
which quantum methods are known to provide at most quadratic scaling over their
classical counterparts.
