Adjusting a design already found on here
What you need is a resistive divider (to translate voltage to the comparison range,) a voltage reference (typically 1.25V) and a comparator (I'd suggest an opamp, using the positive and negative inputs.)

Use a cheap Rail-to-rail opamp (like the MCP601 perhaps.) Tie the voltage reference to the negative input. Tie the voltage sense resistor divider to the positive input. Tie the opamp output to whatever turns off your circuit when it's high.

The voltage divider is simple: decide on a total resistance based on what input impedance you need for the opamp (say, 100 kOhm, leading to approximately 0.13 mA draw at 13 V) and then divide it so that the opamp sees 1.25V when the set voltage (say 13V) is reached. Use 1% or better components, and watch temperature deratings.

So, with a 1.25V reference, and 13.8V set point (car battery,) you would want 13.8 * R1 / (R1 + R2) to equal 1.25V, and (R1 + R2) to equal 100 kOhm. Plug this into the calculator, and you get: R1 = 100000*1.25/13.8 = 9058 Ohms. (You can get +/- 90 Ohms and stay within 1%.) By elimination, the higher end will then be 89942 Ohms (you can get _/- 900 Ohms and stay within 1%.) Perhaps use a 90 kOhm resistor, a 9 kOhm resistor, and a 1 kOhm trimmer in the middle for fine adjustment.