Adjust the value of R2 to change the final battery voltage and change C1 to change the time delay. The circuit may be modified for other battery voltages. When the base goes above 0♶V, TR1 conducts and takes the base of TR2, which is normally held high by resistor R3, low. This is used to charge the capacitor C1 and is fed into the base of transistor TR1. it is greater than 0♶V for a good battery). Resistors R1 and R2 make up a voltage divider, which needs to be set so that the output is just under 0♶V when the battery needs replacement (i.e. briefly when the battery is good but gives a permanent warning of a bad battery, and it does not place a large current burden on the battery either. The power supply to the 741 op amp is unipolar 12V regulated DC.įig.It also has advantages over many other similar designs as it only lights the l.e.d. Resistor (0.5K, 9K, 2 watt) (9K if not available may be split to three 3K ) The power rating of the resistors should be above 2 Watt. only 3V.So the extra 9V should be dropped across somewhere. Let the value of each resistor be R.īut we need to monitor voltage from 9V to 12V i.e. There should be six identical resistors across each op amp. The value of the resistors is calculated as:Īssume current ‘i’ flowing through the voltage divider is 1 mA. If the voltage goes down below 9V, the transistor will be activated and the buzzer will start indicating LOW VOLTAGE Similarly if the battery voltage is 9.2V only the LED at the bottom will be ON and other 5 will be OFF. The series of resistors connected at the inverting terminals of each op amp makes a voltage divider and exact comparing voltage is supplied to each op amp.įor example if the battery voltage is 10.7V, the upper 2 LEDs in the circuit will be OFF and the lower 4 LEDs will be ON. Comparison is done between the two terminals of each op amp and accordingly output is shown. So 6 op amps are used to compare and display the appropriate level of voltage. Here for the development of the circuit, the 12V to 9V range is divided into 6 parts as 9V, 9.5V, 10V…11.5V. My point to say all these is that to monitor a 12V battery, it is sufficient to monitor the battery voltage from 12V to 9V range and a voltage below 9V should give a warning for LOW BATTERY VOLTAGE. Any inverter designed to use with a 12V battery should be comfortable to produce a constant AC output voltage during the fall of battery voltage from 12V to 9v approximately. So it is generally considered that if the output voltage of a 12V battery changes from 12V to 9V approximately, it is constant and after the voltage falls below 9V, it is considered to be discharged and need to be charged again. Practically, it is not possible and the output voltage of the battery decreases gradually as load connected to it draws current from it. Generally a 12V battery is expected to produce a 12V dc voltage all the time until it gets discharged. The series of resistors connected at the inverting terminals of each op amp makes a voltage divider and exact comparing voltage is supplied to each op amp. Here for the development of the circuit, the 12V to 9V range is divided into 6 parts as 9V, 9.5V, 10V.11.5V. My point to say all these is that to monitor a 12V battery, it is sufficient to monitor the battery voltage from 12V to 9V range and a voltage below 9V should give a warning for LOW BATTERY VOLTAGE.
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