ADC Overvoltage Protection

I would like to use the BBB to measure the voltage on two battery banks (12V and 24V). The batteries can go well above the nominal voltages however to 15 and 30 volts at max respectively. In addition to using a resistive voltage divider to step down the voltages to the 1.8V required by the ADC is there a way to ensure that the voltage never excedes 1.8? I've been reading around about zener diodes and it seems like they would work, but I'm not quite sure about the implementation. Any advice?