Battery state indicator

Figure shows a simple battery state monitor circuit. Green LED indicates a good battery and red for low voltage or discharged condition. When a 12 V battery is at its good working condition its out put voltage may be in 11 V or above. If its out put is below 11 V we can say it is discharged and require charging. The potentiometer in the circuit is used to set the input voltage of Q1 at its V_be at its active region when battery is above 11V and cut off when below. Set the potentiometer first.

If the battery is under good condition the transistor Q1 goes to ON and green led lights up, at same time Q2 cut off. If the battery under observation is discharged, its o/p comes below 11V so the transistor Q1 cuts off and Q2 become ON. So the red LED begins to glow indicating an urgent charging is needed for your battery.