I asked about the negative input since that's the first issue - the Zener will do nothing at all until the supply rises above 12V, then it'll clamp the input and you'll have no sensitivity to further changes. Replace the Zener with a resistor to form a divider.
Then when the input rises above your reference voltage, the op-amp (acting as a comparator) will switch the output to the low rail and turn off the relay.
Next issue you have is that there's no latching on this - once the relay switches off, your battery voltage may fall back below the switching point and switch the relay on again. You could get around this by having a feedback from the op-amp output that pulls the reference lower, to provide either some hysteresis or latching action.