You can do it using a voltage comparator
here is an example. Instead of driving a LED, you can connect it to your relay trough a transistor. So when voltage drops below the point set by R2 (see text and schematic), the led lights, and your relay turns off
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Circuit 04: An Op-Amp based battery voltage monitor
An ECU must have a way to monitor battery voltage. Here is a simple op-amp based circuit which will illuminate the LED when the battery voltage drops to a certain level. The turn-on point is set with R2. You must be able to vary Vcc (usually with a good power supply) to set the circuit. Decide at what voltage you'd like the LED to illuminate, and apply this at Vcc. Adjust R2 until the light just illuminates. Use an LED which will light at the desired voltage level, and determine R3, using ohm's law (E=IR), to keep the current flowing through the LED and op-amp to around 10mA.
The LM324 will draw 0.8 mA with the LED out. With the LED on, current will increase to that determined by your selection of R3, with lower being better, of course.