The invention essentially relates to an electric power storage device (1) for hybrid or electric motor vehicles, comprising: a high voltage power supply bus (2) to be connected to an electric traction system (7) and/or to an onboard electric power system, and a first electric power storage element (3) connected to said power supply bus (2). According to the invention, a second storage element (4) is connected to the bus (2) via an electric coupling member (5), said second storage element (4) being connected to the bus (2) in parallel relative to the first storage element (3). One of the two storage elements (3, 4) includes an electrochemical battery, while the other storage element (3, 4) includes an ultracapacitor.