For load currents, up to 4 A, the author has used a bistable relay to disconnect the load on a 12 V battery to avoid deep discharge. How can we provide the same function at higher levels of load current?
The solution here is to use a P-channel HEXFET power MOSFET as a semiconductor relay to disconnect the load. The very low RDS(ON) of these devices is not much greater than a relay’s contact resistance. The device used here is the IRF4905 from International Rectifier . The IRF4905 has an RDS(ON) of 0.02 Ω and can handle an ID(MAX) of 74 A. It is used in the circuit to pass a current of up to 20 A and disconnect the load when the battery voltage falls below a preset threshold. On a practical note make sure that all interconnect cables between the battery and load have a sufficient cross-sectional area to handle the expected load current. The transistor must be mounted on a suitable heat sink in order to dissipate the power (approximately 4.5 W at 15 A) developed in the transistor.
The current consumed by the circuit itself is in the order of 0.5 mA which is really insignificant in comparison to the battery’s inherent self-discharge rate. P1 adjusts the falling voltage level trigger point for the circuit to disconnect the load. The load remains disconnected even when the battery voltage has risen again after recharging. Pushbutton S1 is used to switch T1 back on and reconnect the load. Ensure that any unused inputs of the 40106 hex Schmitt inverter chip are tied to the ground.