The simple and low cost circuit shown below will cut the battery from the load and save it from completely discharge when the voltage of the battery will fall below the 12 volts. Therefore your batteries will last longer and save you a lot of money.
The circuit can be used with any project in which you use 12V batteries, for example solar plants, UPS etc. It can be used with any type of batteries like sealed lead acid, lead acid, NiCd, NiMH etc. It can be used with any type of ampere-hour batteries like from 1.2AH to 400AH. The relay can be used according to the load for example a 10A DPDT relay will not work if your load is above 10A, so select the relay according to your load.
The circuit require some calibration for the first time. For this purpose you have to use a variable power supply and set the output of that power supply to 11.9V and connect in the place of battery in the circuit. Now slightly adjust the 10K variable resistor until the LED goes off. Now check if the circuit is exactly calibrated to cut the load on 11.9V by increasing the voltage of the variable power supply to 13.5V and then again slowly decrease it to 11.9V and see if the LED and relay goes of on 11.9V. When these calibrations are done disconnect the variable power supply from the circuit can connect you desired 12V battery. After that you circuit will be ready to use.
Note: For good and long term performance of the circuit use a good quality variable resistor and sealed it with some kind of glue so it will not change its position / value.
A deep discharge or fully discharge is harm full for the rechargeable batteries. Every time a battery gets fully discharge it loses some percent of its capacity. Due to which the life of the battery become short. The rechargeable batteries are very expensive so taking some care will extend the life of
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