Adding voltage cutoff to a circuit?
In all the following a TLV431 1.25V reference is specified.
 This requires &lt; 100 uA minimum regulation current compared to about 500 uA for the 2.5V TL431.

When on the TLV431 on voltage is about equal to the reference voltage - NOT 0V.

TLV431 current is a battery load even when output is off. At about 100 uA this drains about 2.5 mAh/day. 

Voltage sensing divider is also a battery load. This can be small. 

Hysteresis is not used in any of the following circuits - except the one copied from internet.  A whiff of hysteresis could be used to stop battery on/off cycling when load is removed. Ask if unclear how to do this. 



(1) Opamp based
 P Channel FET high side switch.
 TLV431 1.25 V reference.
 R2/R3 divide Vmin to = 1.25V.



(2) N Channel FET, low side switch.
 TLV431 1.25V reference.
 FET Vth &lt;&lt; Vmin



(3) P Channel FET, high side switch.
 TLV431 = 1.25V reference.
 FET Vth &lt;&lt; (Vmin-1.25)V



(4) From web - similar to my N Channel low side circuit.
 From This discussion page.  

Here R6 adds hysteresis.