Low Voltage disconnect

It probably depends on the voltage and Amperage. I have cobbled some together from programmable voltage meters ($30) and higher amperage relays or contactors.
For 12 volts, Elk make a simple device for alarm panels.

freddieoliveras said:

I need it for a 48V battery bank and want to activate a relay

So, to clarify, you already have a relay or contactor capable of safely disconnecting the max possible current flowing from the bank to the connected load?

If so, charge controllers often include aux ports which can send a relay trigger signal (typically 12v)  based on various conditions, including low voltage.  The trigger is generally very low current, just enough to activate the relay coil. If charge controller or whatever aux output isn't available, there are lots of other pretty cheap ways of sending the trigger.

The load side of the relay has to be rated to disconnect the load current though, which is where it can get expensive.  Contactors capable of disconnecting 48vdc safely and repeatedly at higher amperages can get spendy.