When the applied nominal 12V supply drops below the trip set-point, the relay changes state. This can be used to switch off equipment or trigger an alarm etc. when the battery voltages drops too low.
To get this done without designing a new module, I used an existing design for a DIP-Switch settable pulse timer. Being micro based I had to write some new code for it.
It’s not a big job and relatively simple using the existing assembly framework that I base most of these projects on. The most complex part was sorting out what to do with the trip and restore points to provide some hysteresis and prevent relay chatter. The top end of the range is the problem.
More details here.