In the wintertime periods, when starting is hard and headlamps need usually be used, it is all too easy for a car battery to be discharged to a desperately low level, particularly if no long journeys are undertaken. Therefore this circuit, which implements continuous monitoring of the state of the battery, should verify extremely useful. The unit will indicate if the battery is discharged, or overcharged.
The schematic is based on the IC type TCA 965. This IC is a perfect window comparator, which will indicate whether the input voltage occupies between two preset reference voltages, is below the low reference voltage or above the high reference voltage. These three conditions are indicated by 3 LEDs, which are handled directly by the IC. The IC plus has a reference voltage output, which can be used to determine the upper and lower thresholds of the ‘ window’.
The circuit is powered from the 12 V car battery, and the battery voltage is also fed, via potential divider R1/R2, to the monitoring input of the IC. The reference voltage output is fed to the two threshold inputs via presets PI and P2, which are used to calibrate the circuit.
The lowest acceptable voltage for a 12V car battery is around 11.5 V, and P1 is arranged so that D1 lights when the input decreases below this voltage. If the battery voltage increases above 14.5 V it is overcharged, which indicates that the car voltage regulator is at fault. P2 is accordingly adjusted so that D3 is lit for input voltages above 14.5 V. Between 11.5 V and 14.5 V the green LED should light, indicating that the condition of the battery is satisfactory.
It will be noticed that the LEDs do not light and exterminate at the same voltage. This is due to a hysteresis of 6C mV which is incorporated into the IC to restrict the LEDs flickering when the battery voltage is close to the threshold levels.