Abstract: An energy storage device includes at least one block, the block being connected to an input voltage Vin and delivering a voltage Vout greater than Vin and including: n energy storage units, where n?2; a charge and discharge management circuit electrically linked to the n storage units and making it possible to alternately connect all of the energy storage units of one and the same block to one another, in parallel mode or in series mode; the energy storage units respectively having an end of charge voltage Vend of charge and an end of discharge voltage Vend of discharge; the block having a defined block voltage VBlock between the low potential of the first energy storage unit and the high potential of the nth energy storage unit, wherein the charge and discharge management circuit includes: first means for triggering the toggling from the parallel mode to the series mode, through the exceedance of a first threshold voltage (Vthreshold1) in one of the energy storage units, the first threshold voltage correspon