Overdischarge detection device and overdischarge prevention device for a chargeable electric cell, operable with a variable threshold voltage to prohibit discharge in accordance with relative weight of a load

An overdischarge prevention device for use in a cell unit including a chargeable electric cell having a cell voltage which is supplied to a load through positive and negative load current lines to pass a load current through the load, said overdischarge prevention device comprising:

• an overdischarge detection circuit connected between the positive and the negative load current lines, said overdischarge detection circuit including a voltage comparing section having an input terminal for the cell voltage of the chargeable electric cell and a threshold terminal for a controllable threshold voltage for prohibiting discharging operation, said voltage comparing section comparing the cell voltage with the controllable threshold voltage to produce a detection signal when the cell voltage falls below the controllable threshold voltage;
• a switch circuit, connected to said overdischarge detection circuit, for stopping, in response to the detection signal, supply of the cell voltage from the electric cell to the load;
• and
• an adjusting device, connected to said overdischarge detection circuit, for detecting a particular voltage of a specified part in the cell unit that corresponds to the load current varying in response to relative weight of the load, and for adjusting the controllable threshold voltage based on the particular voltage;
• wherein said adjusting device comprises an overdischarge detection hysteresis circuit for changing the controllable threshold voltage by supplying a control current to said overdischarge detection circuit based on the particular voltage, and for clamping the controllable threshold voltage when the particular voltage is higher than a predetermined voltage.