A good quality battery monitor can also be made using a LM324 IC. Previously we have shown a battery monitor circuit that was using a LM3914 IC. LM3914 is also a good IC to make a quality battery monitor but its cost is higher than the LM324. LM324 is a low cost IC consists of four independent low power operational amplifiers. In this circuit we have used all the four operational amplifiers, each op amp is working separately as a comparator to monitor a certain voltage point set by the 10K variable resistors. The circuit is versatile and can be used to monitor just about any type and voltage of batteries, the tuning / adjustment for a desired battery can be done with the 10K variable resistors. For adjustments you will also require a adjustable power supply. For example you want to set the circuit to monitor a 6V lead acid battery. A fully charged 6V battery will show 6.3V after few hours of disconnected from the charger, and become deep discharge on 5.75V. It is always better for the life of the battery to not let it deep or 100% discharge. It is always better to recharge it when 50% capacity is left, for this purpose we will set the LED1 to become switch off when the battery voltage will fall to 6V. For this purpose set 6V in the variable power supply and adjust the VR1 Until the LED1 goes off. After that slightly increase the voltage of the adjustable power supply to 6.1V and slightly adjust the VR2 until the LED2 goes off. Now set the supply on 6.2V and adjust the VR3 until the LED3 goes off. At last set 6.25V in the supply and adjust VR4 till the LED4 goes off. After these setting again check all the settings by increasing the voltage of the power supply to 6.3V and slightly lowering it. After these adjustments your circuit will be ready for use.

P.S. I am not the original author of this circuit.

The source is: http://www.circuitdiagram.org/lm324-battery-monitor.html