Emergency Backup Battery Supply
Stay the module, keeps the circuit cleaner and you would need more complex curcuit to handle relay operations. 
If you are that worried these two options might help:


A. Insert a diode to block the oncoming current (to the battery from module) to keep the module from charging the battery (people do this in pinball ram mods). 

B. Replace the batteries with rechargeable batteries. 

3.use a ganged SPDT to replace the  one SPDT and simply connect the unused one as if it was a SPST that is in series with the backup battery and connects to the module.