  One of the most fundamental tasks in quantum thermodynamics is extracting
energy from one system and subsequently storing this energy in an appropriate
battery. Both of these steps, work extraction and charging, can be viewed as
cyclic Hamiltonian processes acting on individual quantum systems.
Interestingly, so-called passive states exist, whose energy cannot be lowered
by unitary operations, but it is safe to assume that the energy of any not
fully charged battery may be increased unitarily. However, unitaries raising
the average energy by the same amount may differ in qualities such as their
precision, fluctuations, and charging power. Moreover, some unitaries may be
extremely difficult to realize in practice. It is hence of crucial importance
to understand the qualities that can be expected from practically implementable
transformations. Here, we consider the limitations on charging batteries when
restricting to the feasibly realizable family of Gaussian unitaries. We derive
optimal protocols for general unitary operations as well as for the restriction
to easier implementable Gaussian unitaries. We find that practical Gaussian
battery charging, while performing significantly less well than is possible in
principle, still offers asymptotically vanishing relative charge variances and
fluctuations.
