48 V Low voltage alarm circuit for Battery rectifier / charger

If you use Google with the search terms: low voltage alarm circuit

...and then click on Google Images, you'll see quite a few such circuits.

You'll need to figure out what will power your circuit if the charger goes off line because of a power failure. You could perhaps use the battery bank in question, but you'll have to ponder the system / reliability implications of such an approach.

48 volts is towards the high end for many garden variety components, so you'd need to address the power supply side of the circuit if you plan to use the 48 volt battery as the power source.

Also, you might need a voltage divider on the input to bring it down to appropriate range.

PS: I'll bet eBay offers an off-the-shelf solution.

This link seems to offer such gadgets: (just an example)

You should double check to make sure that the "48 volt" range actually reaches the exact voltage you want (exact voltage as opposed to nominal voltage).