This simple battery state tester which can simply check state of charge of the different types of batteries.
Tested battery must be connected using two connectors and with the help of three LEDs of different colors, battery condition is indicated ("full" - the green, yellow or usable LED and exhausted red LED).
When the battery is connected, the tester is automatically put into operation. The same thing happens, but when the battery is fully discharged. If we suspect if the battery is discharged we must to press the test button to be sure that it is indeed exhausted (where, with TEST button pressed, the red LED lights, means that the battery checked it is exhausted).
If the battery voltage exceeds a minimum value of V, both comparator outputs, pins 1 and 7 of IC1a and lC1b, are at high potential, making the red LED to light. When the battery voltage is between 1.0 V and 1.3 V output of IC1A remains in a H state, but that of IC1B passes in state L. As a result, the LED goes red and yellow LED lights . If the battery voltage exceeds about 1.3 V, IC1A basculate, so that only the green LED lights.