Intermediate15 min 路 Updated Aug 2026

LM393 Battery Monitor Circuit: Low-Voltage Cutoff Design Guide

Build an LM393-based battery protection circuit for Li-ion, LiFePO4, or lead-acid batteries. Includes hysteresis calculation, MOSFET driver design, and complete schematic.

Why Use LM393 for Battery Protection?

  • Cheap: LM393 costs $0.10-0.20 in volume, much cheaper than dedicated BMS ICs.
  • Flexible: Works with any battery chemistry - just change resistor values.
  • Simple: 8-pin IC, no firmware, no programming required.
  • Reliable: Pure hardware - no firmware bugs, instant response.

Design Example: 1S Li-ion Low-Voltage Cutoff

Goal: Disconnect load when battery drops below 3.0V. Reconnect when battery rises above 3.5V. Battery voltage range: 3.0V - 4.2V (Li-ion).

Specifications

V_supply (Vcc)5.0V
V_ref (TL431)2.5V
V_threshold_low (cutoff)3.0V
V_threshold_high (reconnect)3.5V

馃挕 Tip: Use our Hysteresis Calculator to compute exact R1/R2/R3 values for your specific thresholds. This example uses a 4:1 voltage divider (R4:R5) to scale 3.0V battery to 0.75V at the comparator input.

Complete Schematic

BAT+ 鈹€鈹€鈹攢鈹€ R4 (30k) 鈹€鈹€鈹攢鈹€ LM393 Pin 3 (IN+)
       鈹?             鈹?
       鈹?             R5 (10k)
       鈹?             鈹?
       鈹?             GND
       鈹?
       鈹溾攢鈹€ R6 (10k) 鈹€鈹€鈹?
                      鈹?
       鈹屸攢鈹€ R7 (10k) 鈹€鈹€鈹粹攢鈹€ LM393 Pin 2 (IN-)
       鈹?
       鈹斺攢鈹€ Load (battery-powered device)
              鈹?
              Gate 鈹€鈹€ N-MOSFET (e.g., AO3400A)
              鈹?
              Source 鈹€鈹€ GND
              
LM393 Pin 1 (OUT) 鈹€鈹€ MOSFET Gate
LM393 Pin 4 (V-) 鈹€鈹€ GND
LM393 Pin 8 (V+) 鈹€鈹€ 5V

V_ref (TL431) 鈹€鈹€ 2.5V stable reference

When battery voltage drops below 3.0V, the voltage divider output falls below 2.5V reference. LM393 output goes LOW, MOSFET turns off, load disconnects.

Hysteresis Calculation

For this design, hysteresis prevents rapid on/off cycling when battery voltage hovers near 3.0V. Choose:

  • V_T- (cutoff): 3.0V battery 鈫?0.75V at IN+ pin
  • V_T+ (reconnect): 3.5V battery 鈫?0.875V at IN+ pin
  • Hysteresis width: 0.5V battery 鈫?0.125V at IN+ pin

Use our calculator with these values to determine R1/R2/R3.

PCB Layout Tips

  • Keep voltage divider resistors close to LM393 input pins
  • Use a 100nF decoupling capacitor between LM393 V+ and GND
  • MOSFET source should connect to GND with short, wide traces
  • Add reverse-polarity protection (Schottky diode or P-MOSFET)
  • Keep high-current traces away from analog signals

Testing and Validation

  1. Use a bench power supply set to 4.2V - verify load is ON
  2. Slowly reduce voltage to 3.5V - verify load stays ON (above reconnect)
  3. Reduce to 3.0V - verify load turns OFF
  4. Slowly increase back to 3.5V - verify load reconnects
  5. Test under load (e.g., 500mA draw) - verify MOSFET stays cool

Frequently Asked Questions

Can LM393 directly switch a battery load?

No. The LM393 output is open-collector (max 36V, ~20mA). Use it to drive a MOSFET gate or relay coil to switch the actual load current. LM393 is the decision-making IC that decides when to switch.

What battery voltages can I monitor with LM393?

Any voltage up to ~36V (LM393 absolute max). For higher voltages (48V battery packs), use a voltage divider to scale the input below 36V first. Common applications: 3.7V Li-ion (1S), 7.4V Li-ion (2S), 12V lead-acid, 12.8V LiFePO4.

How accurate is LM393 for battery voltage sensing?

The LM393 has an input offset voltage of 卤2mV typically, giving roughly 0.1-0.5% accuracy when used with a precision voltage reference (e.g., TL431). For most battery protection applications, this is more than sufficient.

Why use hysteresis in a battery monitor?

Without hysteresis, the circuit rapidly toggles on/off when the battery voltage is near the threshold. This wastes power and can damage the load. Hysteresis creates a clean voltage gap (e.g., disconnect at 3.0V, reconnect at 3.5V).

Calculate Your Own BMS

Use our Hysteresis Calculator to design the exact resistor values for your battery protection circuit. Adjust cutoff and reconnect voltages to match your battery chemistry.

Open Calculator 鈫?