TL431 Low battery cut-off
If the battery voltage exceeds Vref*R2/(R1+R2) then the TL431 cathode goes low (to &lt; ~2V anyway), which turns Q1, a p-channel MOSFET, on. 

But this is not a very good disconnect. The value of R3 has to be quite low since the TL431 needs as much as 1mA to work properly, and the MOSFET should be off (for example, if 0.6V is the maximum voltage for the MOSFET to be acceptably off then R3 should be 600 ohms). 

Also there is no hysteresis in this circuit so it will tend to oscillate (because the battery voltage will rise with the load disconnected) or the MOSFET will get hot if there is a lot of current draw as you hit the cutoff value. 

The MOSFET needs to be fully on for the lowest 'on' input voltage and the TL431 at a couple of volts, so probably a logic-level MOSFET for a 12V battery. 

If there is any surge on the battery or reversal the circuit will die. The MOSFET gate will typically be rated for 20V or less, so even a small inductive load will kill it. 

Russell M. at least suggested a TLV431 which takes about 1/10 the current.