Battery Voltage Indicator --
June 17, 2012
battery powered systems a user would like to know when it was time to replace the
batteries before they were completely discharged, causing the device being powered to
fail completely. Alkaline batteries have an open cell voltage of about 1.5
volts. As they are discharged, the voltage slowly drops. When the cell
voltage reaches about 1.25v, they have delivered about 90% of their stored energy.
The circuit below monitors the voltage of two alkaline cells in series and begins
flashing an efficient red LED when the total battery voltage drops below 2.5v,
alerting the user that the batteries need to be replaced.