3.7V Battery Deep Discharge Cutoff Circuit using TL431 + PC817 + MOSFET
In this project I made a simple automatic cutoff circuit to protect a single 3.7V Li-ion or LiPo battery from deep discharge. When the battery voltage drops too low the load gets disconnected. When you recharge the battery the load reconnects automatically. No microcontroller or coding needed.
How the circuit works:
1. Voltage Sensing with TL431
The TL431 is used as a precision voltage reference and comparator. Two resistors form a voltage divider to set the cutoff voltage. When the battery voltage falls below this set point, the TL431 turns on and pulls its output low.
2. Isolation and Signal with PC817 Optocoupler
The PC817 optocoupler sits between the TL431 and the MOSFET. It isolates the sensing section from the switching section and also inverts the signal. When battery is low, the optocoupler turns on and pulls the MOSFET gate low to turn it OFF.
3. Switching with Logic Level MOSFET
An N-channel logic level MOSFET is used as the main switch between battery negative and load negative. When the battery voltage is good the MOSFET stays ON and connects the load. When voltage is low the MOSFET turns OFF and disconnects the load. Using a MOSFET instead of a diode gives very low voltage drop and less heat.
This circuit typically cuts off around 3.0V to 3.2V and reconnects around 3.6V. You can change the cutoff by changing the two resistors on the TL431.
Better components to use:
Voltage Reference:
TL431A or TL431B. Use 1% resistors for the voltage divider for accurate cutoff.
Optocoupler:
PC817, EL817, or LTV-817. All have same pinout and work fine at 3.7V.
MOSFET: Must be logic level so it turns fully ON at 3.7V to 4.2V
Recommended:
AO3400 – SOT23 package, 30V 5.8A, very low Rds(on) at 3.3V. Best for small loads.
IRLZ44N – TO220 package, 55V 47A. Good for motors and higher current, works at 4.5V gate.
IRLR7843 – SMD, extremely low Rds(on). Best for efficiency.
Note: Do not use IRFZ44N or other standard MOSFETs. They need 10V on gate and will not work properly with 3.7V.
Other parts:
10k resistor from MOSFET gate to ground
330 ohm resistor in series with PC817 LED
1% metal film resistors for the TL431 divider
Where you can use this:
DIY power banks, ESP32 or Arduino battery projects, solar lights, and any single cell 3.7V project where you need basic battery protection.
Comment your cutoff voltage and which MOSFET you used.
#BatteryProtection #TL431 #MOSFET #PC817 #LiIon #DIYElectronics #3.7VBattery