The curcuit uses a TLV431 low-power voltage-reference IC as a comparator, driving a high-side power P-MOSFET to switch the load voltage. The P-MOSFET is ON when the TLV431 Ref voltage is above 1.24V, and OFF when it is below 1.24V, as determined by the resistor divider network at that node. Pot U2 adjusts the trip point and R4 adjusts the hysteresis (difference between high and low voltage trip points. The simulation shows a lower trip point of 11.79V and and upper trip point of 13.1V for the 50% pot setting and resistor values shown. The circuit maximum current drawn from the battery is about 1/2 mA. The maximum load current is determined by the MOSFET used. To avoid the use of a heatsink, select a P-MOSFET with an ON resistance low enough to keep the transistor dissipation below about a watt for the maximum load current (Rds(on) ≤ 1 / Ild²). Circuit desigh by crutschow @ allaboutcircuits.com