Low Voltage Protection Module Review

In one of my projects, I needed a protection module to prevent a lead-acid battery from over-discharging. I picked up the first module I could find.

The module is built around the LM358 operational amplifier.

The intended purpose of this module is simple: when the battery voltage reaches a critically low level, it should disconnect the load to prevent further discharge until the battery is recharged. The cutoff threshold is adjustable; for old 7Ah batteries, I usually set it to 11.0V - 10.95V, though for today’s tests, I set it to 10.5V.

During testing, I discovered a significant “nuance.” The idle current consumption of the module with a “charged battery” is 7mA, which is perfectly fine.

However, what happens next is quite interesting. Once the voltage drops below the critical level, the relay clicks—and the current consumption jumps to 33mA.

The Problem: Instead of energizing the relay when the battery is healthy, the circuit does the opposite. When the battery is already discharged, the module starts “killing” it even faster by consuming more power.

Within a few days, it will drain the remaining ampere-hours. Once the voltage drops to around 8–9V, the relay will no longer have enough power to stay engaged. That becomes the “kill shot”: the relay releases the contacts and reconnects the load to the dead battery. After such an event, the battery is likely beyond recovery.

Conclusion

It is better to avoid using this specific module. It’s much easier to build a custom solution using a relay and a TL431 precision shunt regulator.