each of the plurality of battery cells comprising:

a transformer forming unit including an inductor and a core member, and configured to form a transformer between the transformer forming unit itself and an inductor and a core member of an adjacent battery cell; and

a potential detection unit configured to detect a potential of the battery cell;

the electrical energy storage device further comprising a control unit configured to control an energized state of the inductor in each of the plurality of battery cells;

wherein the control unit compares the potentials of the adjacent battery cells based on measurement results of the potential detection unit, and via the transformer in which the inductor of a battery cell having a higher potential is made to serve as a primary side inductor and the inductor of another battery cell having a lower potential is made to serve as a secondary side inductor, performs an equalization control to equalize the potentials of the plurality of battery cells, by supplying electrical charge from the battery cell having the higher potential to the battery cell having the lower potential;

each of the plurality of battery cells comprises a main body configured to charge and discharge the electrical charge;

the transformer forming unit is formed in a plate shape, and is arranged between adjacent ones of the main bodies;

in the inductor, windings are configured along a planar direction of a side surface of the main body;

the core member extends along the side surface, and is formed in a sheet shape so as to accommodate the inductor on an inner side thereof; and

the core member includes a central portion that is in contact with another core member of the adjacent battery cell, at a center of the inductor, and an outer side portion that is in contact with the other core member of the adjacent battery cell, on an outer side of the inductor.