  We discuss how the ideal rechargeable energy accumulator can be made, and
what are the limits for solid state energy storage. We show that in theory the
spin batteries based on heavy fermions can surpass the chemical ones by energy
capacitance. The absence of chemical reactions in spin batteries makes them
more stable, also they don't need to be heated in cold conditions. We study how
carriers statistics and density of states affect energy capacity of the
battery. Also, we discuss hypothetical spin batteries based on neutron stars.
