This circuit can be used to ensure that a 12V sealed lead-acid battery does not go into the condition of deep discharge. The circuit disconnects the battery from load when its voltage goes below a preset voltage (say 10.5V) level. The load remains disconnected until the battery is recharged.
Initially, when the circuit is switched on through switch S1, capacitor C1 momentarily acts as a short circuit. It connects the coil of relay RL1 to the battery. Relay RL1 energises and connects the load to the battery. At the same time battery terminal volt-age is divided by resistor R2 and the combination of preset VR1 and R3 that provide reference voltage. The battery voltage does not go below the reference voltage, preventing it from going into deep discharge.
The heart of the circuit is IC TL431 (IC1), which is a well-known voltage reference IC. The battery voltage is compared with the internal reference voltage of regulator IC1. As long as the voltage set by the preset VR1 is above the internal reference voltage, IC1 allows relay RL1 to energise, connecting the load to the battery. As the battery begins to discharge its terminal voltage decreases. When the battery voltage reaches the set voltage, say 10.5V, the cathode of IC1 goes high, which de-energies relay RL1 and disconnects the load from the battery.
Assemble the circuit on a general purpose PCB and set the low cut-off voltage (deep discharge voltage) of the battery. For setting the low cut off voltage, remove the battery from the circuit and connect the circuit to a variable 12V DC power supply. Also connect a digital multimeter across the variable power supply. Increase the voltage from 0 to 12V through the variable power supply and read the voltage in multimeter. Adjust preset VR1 in such a way that relay RL1 gets energised and load is connected to the battery. Now decrease the supply to 10.5V (low battery voltage setting recommended by manufacturers to avoid deep discharge). Move the preset VR1 slowly until the relay deenergises. You may try this adjustment more than once to get IC1 to switch at exactly 10.5V. After the adjustment is done remove the variable power supply and multimeter, and reconnect the battery and switch on S1.
After calibration, enclose the assembled unit in a suitable shock-proof cabinet. Fix on/off switch S1 on the front panel and a two-pin connector for load on the back panel. Keep the battery inside the cabinet.
