Here is an interesting Car Battery Monitor circuit of a low power electronic dc voltmeter circuit that can be used with car electric systems that run on 12 volt batteries. The voltmeter is an expanded scale type that indicates small voltage steps over the 10 to 16 volt range for 12 volt batteries.
At the heart of the circuit is a ubiquitous dot-bar volt meter LM3914N (IC1). This IC is operated in the expanded-scale mode so that the circuit responds in the 10-16V range. IC1 outputs a steady voltage on pin 7 from the internal voltage reference. This is fed via voltage dividers VR2and R2 to the internal reference input pins(4&8) to set the range that the meter is sensitive to. The measured voltage is fed in on pin 5 via the voltage divider consisting of R1 and VR1. This divider scales the input voltage down to a range that is useful to IC1.
It is possible to set the meter to read equal steps across a variety of upper and lower voltages. Different colored LEDs can be used for the voltage level indicators(D1-D10). It will be necessary to have an adjustable regulated DC lab power supply and a good quality digital volt meter (DVM) to perform the calibration. Connect the external volt meter across pins 6 and 4 of IC1 and adjust VR2 for a reading of 1.2 volts. Center the settings of VR1 and VR3. If the span between the end points of VR1 is 4.5V, the circuit is ready to use as a 10.5 to 15V scale battery monitor.
Adjust the lab power supply from 9V to 15V and check where the meter is reading, it may not read at all until the potentiometers are near the right range. In this case, set the lab power supply to 12V and adjust VR3 until one of the green LEDs (D4-D7) light. Adjust the power supply until the first LED just comes on, measure that voltage. Adjust the supply up until the last LED just comes on, measure that voltage and subtract the first voltage, this is the span. Adjust VR1 and repeat the previous adjustment until the span is 4.5V. Now set the voltage to 10.5V and adjust VR3 until the lowest LED (D1) just turns on. Note that VR1 and VR3 interact so it may be necessary to perform the adjustments a few times to get it right.
After construction and calibration, fit the voltmeter unit in the dashboard and connect input leads across the 12 Volt vehicle battery terminals, with correct polarity. Now watch the LED display panel for a battery level indication. If the voltage is higher than the top step, the highest LED (D10) will remain on. If the voltage is lower than the bottom step, all of the LEDs will stay off.