**Barebones BMS Design for Single 18650 Cell** **Objective:** Safely charge and protect a single 18650 cell using discrete, widely available components. --- ### **1. Charging Circuit (CC/CV)** **Components:** - **PNP Transistor (TIP42C):** Pass element for regulating current/voltage. - **Current Sense Resistor (0.1Ω, 2W):** Monitors charging current. - **Op-Amps (LM358):** Implements CC/CV control. - **U1A (CC Mode):** Compares voltage across the sense resistor to a reference (e.g., 0.1V for 1A). - **U1B (CV Mode):** Compares cell voltage to a 4.2V reference (Zener diode 1N4732A). - **Zener Diode (1N4732A, 4.2V):** Provides stable voltage reference for CV mode. - **Voltage Divider (Resistors R1, R2):** Sets CC reference voltage. - **Diode (1N4007):** Prevents reverse current. **Operation:** - **CC Mode:** U1A adjusts the PNP transistor to maintain constant current (e.g., 1A) until cell voltage reaches 4.2V. - **CV Mode:** U1B takes over, reducing current to hold cell voltage at 4.2V. --- ### **2. Protection Circuits** **Components:** - **Comparators (LM393):** For overcharge, over-discharge, and overcurrent detection. - **MOSFETs (IRF4905 P-Channel, IRFZ44N N-Channel):** Disconnect load/charger during faults. - **Zener Diodes (4.25V, 2.5V):** Reference voltages for protection thresholds. - **Sense Resistors (0.05Ω, 0.1Ω):** Monitor load current. #### **a. Overcharge Protection (4.25V Cutoff)** - **Comparator U3A:** Compares scaled cell voltage to a 4.25V Zener reference. - **Voltage Divider (R3, R4):** Scales cell voltage (e.g., 4.25V → 2.5V for comparator input). - **Hysteresis Network (R5, R6):** Prevents oscillation. - **Action:** Triggers MOSFET to disconnect charger when voltage > 4.25V. #### **b. Over-Discharge Protection (2.5V Cutoff)** - **Comparator U3B:** Compares scaled cell voltage to a 2.5V Zener reference. - **Voltage Divider (R7, R8):** Scales cell voltage (e.g., 2.5V → 1.25V for comparator input). - **Hysteresis Network (R9, R10):** Ensures clean shutdown. - **Action:** Disconnects load when voltage < 2.5V. #### **c. Overcurrent/Short-Circuit Protection** - **Comparator U4A:** Monitors voltage across load sense resistor (0.05Ω). - **Reference Voltage:** Set to 0.1V (for 2A threshold) via voltage divider/Zener. - **Action:** Disconnects load via MOSFET if current exceeds 2A. --- ### **3. Circuit Integration** - **Charger and Load Paths:** Use P-Channel MOSFETs on the high side for load/charger disconnect. - **Hysteresis:** Added to all comparators via feedback resistors. - **Power Supply:** LM358/LM393 powered directly by the cell (3.0–4.2V range). --- ### **4. Component List** - **Transistors:** TIP42C (PNP), IRF4905 (P-MOSFET), IRFZ44N (N-MOSFET). - **ICs:** LM358 (Op-Amp), LM393 (Comparator). - **Diodes:** 1N4732A (4.2V Zener), 1N4007 (Rectifier). - **Resistors:** 0.1Ω (2W), 0.05Ω (2W), 1.2kΩ, 100kΩ (voltage dividers), trimpots for calibration. - **Capacitors:** 100nF (decoupling). --- ### **5. Calibration & Testing** - **Voltage Thresholds:** Adjust trimpots to set 4.2V (CV), 4.25V (overcharge), 2.5V (over-discharge). - **Current Limit:** Tweak CC reference to achieve desired charging current (e.g., 1A). - **Hysteresis:** Tune resistor values to ensure stable switching (e.g., 50mV hysteresis). --- **Final Notes:** - **Thermal Management:** Ensure pass transistor and sense resistors are heatsinked. - **Reset Mechanism:** Manual reset (e.g., button) may be needed for latch-based protection. - **Efficiency:** MOSFET Rds(on) and sense resistor values impact efficiency; optimize for low loss. This design provides essential charging and protection functions while using only common discrete components.