Apparatus and method for indicating state of charge of a battery

A device for indicating the state of charge of a battery in an apparatus powered by the battery, the device comprising:

• a solid state display unit having a reference voltage input and first and second address inputs, each address input being connected to respective first and second adjacent analog display elements;
• an amplifying circuit having an input and an output, the circuit including means for producing a voltage at the output that has a predetermined characteristic corresponding to values of the battery voltage applied to the input in a preselected range of battery voltages under load that relate directly to the state of charge of the battery;
• an input switch for connecting the input of the amplifying circuit to the battery when the battery is supplying power to the apparatus;
• an output switch for periodically connecting the output of the amplifying circuit alternately to the first and second address inputs when the battery is connected to the input of the amplifying circuit;
• a regulated-voltage generating circuit having an input connected to the battery through the input switch and an output connected to the reference voltage input of the display unit for supplying a reference voltage to both display elements of the display unit when the input of the amplifying circuit is connected to the battery;
• and
• means connected between the output of the amplifying circuit and the first display element for storing at least one of the alternately applied output voltages so that the first display element shows a previous value of the state of charge of the battery when the second display element shows a current value of the state of charge of the battery, whereby the difference in displayed values by the two display elements provides an optical indication of the rate of change of the state of charge of the battery.

A method for providing an optical presentation of the state of charge of an electrochemical cell on an analog liquid crystal display unit having a pair of plates spaced apart from one another to form a gap therebetween and a liquid crystal material disposed in the gap, at least one of the plates being substantially transparent, and the unit having a reference voltage input and at least one address voltage input, wherein the method comprises:

• deriving a reference voltage having a predetermined fixed value from an output voltage of the cell within a preselected range corresponding to a range of charge states to be optically presented on the display unit;
• deriving an address voltage from the output voltage of the cell within the preselected range, the value of the address voltage having a predetermined relation to the state of charge corresponding to the output voltage of the cell and falling within the most negative and most positive values of the reference voltage;
• applying the reference voltage to the reference voltage input of the display unit;
• and
• applying the address voltage to the at least one address input of the display unit so as to cause the liquid crystal material to provide an optical presentation corresponding to the state of charge of the cell.