NOVELTY - Battery pack (I) (1) comprises: (a) a current control element which is inserted in a discharge current path of a battery cell, and in which a thermostat and a heat sensitive resistance element whose resistance value increases in response to an increase in temperature are connected in parallel; and (b) a detection unit that outputs an open signal indicating the opening of the contacts when it is detected that contacts of the thermostat are opened.    USE - The battery pack is useful in an electricity storage system, an electronic apparatus, an electrically driven vehicle, an electric power system, a control system (all claimed), electric drill and an electric screwdriver.    ADVANTAGE - The battery pack improves the safety of an apparatus.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a battery pack (II) comprising: (a) and (b) as above per se; a control unit that performs a control to realize a discharge prohibition state when the open signal is supplied; and a maintaining unit that maintains the discharge prohibition state, where in a case where the opened contacts return, the maintaining unit is controlled by the control unit to maintain the discharge prohibition state;    (2) an electricity storage system comprising the battery pack which is charged by a power generator that generates electricity from renewable energy, where electric power is supplied to an electronic apparatus connected to the battery pack;    (3) an electronic apparatus, where electric power is supplied from the battery pack;    (4) an electrically driven vehicle comprising a converting device to which electric power is supplied from the battery pack and which converts the electric power to a driving force of a vehicle, and a control device that performs an information processing related to a vehicle control based on information related to the battery pack;    (5) an electric power system, comprising: an electric power information transmitting and receiving unit that transmits and receives signals to and from another apparatus through a network, where a charging and discharging control of the battery pack is performed based on the information which the electric power information transmitting and receiving unit receives; and    (6) a control system comprising an external apparatus and the battery pack that is mounted in the external apparatus.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the battery pack.    Battery pack (1)    Positive temperature coefficient mounted thermostat (62)    Control field-effect transistor (63)    Control integrated circuit (65)    Detection circuit (66)