================================================================================ SCHEMATIC: Lithium-Ion (Li-Ion / NMC / LCO) Battery Management System TM-BMS-001 Rev A 3S (10.8V nominal) and 4S (14.4V nominal) configurations Primary IC: BQ76930 (6–10 cell) or S-8254 (4S) or standalone discrete Protection: OV/UV/OC/SC/Temperature | Balance: Passive Max continuous: 20A (3S portable) / 30A (4S station) ================================================================================ LI-ION / NMC CELL PARAMETERS ─────────────────────────────── Li-Ion (NMC = Nickel Manganese Cobalt) is the most common lithium chemistry for portable consumer electronics. It has higher energy density than LiFePO4 but is more sensitive to overcharge and temperature. Nominal voltage: 3.60V per cell (NMC) Full charge voltage: 4.20V (NEVER exceed; leads to thermal runaway) Full discharge cutoff: 3.00V (capacity severely degraded below 2.8V) Optimal range: 3.0V–4.0V (maximize cycle life) Self-discharge: 2–3% per month LCO (Lithium Cobalt, phone batteries): Full charge: 4.20V; same as NMC More sensitive to overcharge; avoid for field use NCA (Lithium Nickel Cobalt Aluminum, Tesla cells): Full charge: 4.20V; higher energy density; wider temperature range IMPORTANT: LiFePO4 is a separate chemistry. Do NOT use this BMS with LiFePO4. Do NOT mix cell chemistries in a pack. All cells must be identical model/batch. PACK CONFIGURATIONS: 3S: V_nominal = 10.8V, V_max = 12.6V, V_min = 9.0V Suitable: QRP rigs (< 12V minimum), 12V USB chargers, portable lights 4S: V_nominal = 14.4V, V_max = 16.8V, V_min = 12.0V Suitable: 12V nominal ham gear (radios with 13.8V supply) NOTE: 4S Li-Ion max voltage 16.8V is above 13.8V standard — use voltage regulator or accept lower charge set-point (4.1V/cell = 16.4V for longevity) CAPACITY AND C-RATE: 18650 cell: 2.5–3.5 Ah (3.0 Ah typical, Panasonic NCR18650B, Samsung INR18650-30Q) 21700 cell: 4.0–5.0 Ah (4.8 Ah typical, Samsung 50E, Molicel P50B) Pouch / prismatic: project-specific Parallel cells (xP): V same, Ah multiplied. 3S2P (6× 18650): V = 10.8V, Ah = 6.0 Ah (two 3.0 Ah cells in parallel per group) 3S4P (12× 18650): V = 10.8V, Ah = 12 Ah Max charge rate: 0.5C–1C (protect cycle life; fast charge ≤ 2C for quality cells) Max discharge rate: 1C–5C (depends on cell; confirm cell datasheet) CELL-TO-CELL MATCHING: All cells in a series string must match: Capacity: within 5% of each other Internal resistance: within 10% of each other Age/cycle count: start fresh; do not mix new and used cells Mismatch accelerates imbalance and reduces effective pack capacity. ================================================================================ OPTION A: SEIKO S-8254 SERIES (4S LI-ION, SIMPLE) ================================================================================ The S-8254 is a simple 4-cell protection IC with no I2C — suitable for standalone protection without ESP32 integration. S-8254AAA: OV threshold: 4.250V ± 25 mV per cell UV threshold: 2.300V ± 100 mV per cell OCD threshold: 0.20V on sense resistor SCD threshold: 1.00V on sense resistor Output: CHG_OUT and DSG_OUT control external N-channel FETs LIMITATION: No I2C; no per-cell voltage readout to host; no coulomb counter. USE CASE: Low-cost protection-only BMS for fixed installations where monitoring is handled separately (e.g., add INA226 externally for current/voltage monitoring). S-8254 CONNECTION (4S): VSS (pin 1) ── CELL1− (GND) VDD (pin 20) ── CELL4+ (through 100Ω R_REG) V1 (pin 19) ── CELL1+ / CELL2− V2 (pin 18) ── CELL2+ / CELL3− V3 (pin 17) ── CELL3+ / CELL4− CS+ (pin 15) ── to P−; CS− (pin 16) ── to B− (sense resistor between) VM (pin 14) ── discharge mosfet drain; FO output at pin 13 VIN (pin 12) ── charge input detect CO (pin 10) ── charge mosfet gate drive DO (pin 9) ── discharge mosfet gate drive MOSFET STAGE (S-8254): B− ──[R_sense: 50mΩ]── DSG FET (source to R_sense; drain to CHG FET source) ── CHG FET (source at P−) Gate resistors: 100Ω each; 1MΩ gate-source bleeder (prevents float gate) ================================================================================ OPTION B: BQ76930 (6–10 CELL, CONFIGURABLE 3S OR 4S WITH I2C) ================================================================================ The BQ76930 is a higher-capability version of the BQ76920 supporting up to 10 cells. Configured here for 3S or 4S Li-Ion with full I2C monitoring. DIFFERENCES FROM BQ76920: - Supports 3–10 cells (BQ76920 = 3–5 cells) - Same register map; compatible firmware with minor address changes - Internal coulomb counter - 14-bit ADC, 1.22 mV resolution BQ76930 4S LI-ION THRESHOLD CONFIGURATION: OV_TRIP: 4.20V → (4.20 - 2.0) / 0.0506 - 1 = 42.5 → 0x2A (4.194V actual) For 4.10V/cell charge (extended cycle life): 0x27 (4.091V) UV_TRIP: 3.00V → (3.00 - 1.0) / 0.0506 - 1 = 38.5 → 0x26 (3.024V) PROTECT1: SCD_THRESH = 0b100 (200 mV), SCD_DELAY = 0b00 (70µs) With R_sense = 5mΩ: I_SCD = 200mV / 5mΩ = 40A PROTECT2: OCD_THRESH = 0b100 (100 mV), OCD_DELAY = 0b10 (160 ms) With R_sense = 5mΩ: I_OCD = 100mV / 5mΩ = 20A BQ76930 3S CONFIGURATION: VC1–VC3 connected; VC4–VC10 tied to VC3 (top of string). OV/UV thresholds same as 4S above. Pack V_max = 3 × 4.2V = 12.6V; pack V_min = 3 × 3.0V = 9.0V. ================================================================================ OPTION C: DISCRETE PROTECTION CIRCUIT (EDUCATIONAL/BACKUP) ================================================================================ DISCRETE 4S OVERVOLTAGE PROTECTION (one cell shown — replicate for each): V_cell ──[R1: 43kΩ]──┬── LM393 (+) input │ [R2: 33kΩ] V_+ = V_cell × 33/(43+33) = V_cell × 0.434 │ Trip: V_cell = 4.2V → V_+ = 1.824V GND V_ref ──[R3: 10kΩ]──┬── LM393 (−) input (V_ref = 1.8V derived from 3.3V + divider) │ [R4: 22kΩ] Use: TL431 set to 1.824V reference │ GND LM393 output (open-drain): LOW when V_cell > 4.2V → pulled HIGH by load R → drives P-channel MOSFET gate high → MOSFET opens charge path DISCRETE UV PROTECTION (comparator with hysteresis): LM393 with positive feedback for hysteresis (prevents chatter at threshold): Hysteresis voltage: ΔV = V_out_swing × R_hys / (R_hys + R_top) For ±50 mV hysteresis: select R_hys accordingly. At V_cell = 3.00V: comparator trips → UV flag set → DSG FET opens. Recovery: V_cell must rise to 3.05V (hysteresis) before DSG FET re-closes. COMBINED OV+UV LOGIC: OV from any cell → inhibit CHG FET (logic AND of all cell comparators) UV from any cell → inhibit DSG FET Both via: open-drain comparator outputs → wired-AND → FET gate WIRED-AND: multiple open-drain outputs on one pull-up resistor. Any output goes LOW → pulled node goes LOW → triggers protection. Simple; no logic ICs required. ================================================================================ PACK ASSEMBLY — SERIES/PARALLEL CELL GROUPING ================================================================================ 3S2P ASSEMBLY (12× 18650 cells, 10.8V / 6 Ah): GROUP 1 (at bottom): CELL_1A (−) ──┬── GROUP1_NEG (to BMS B−) CELL_1B (−) ──┘ CELL_1A (+) ──┬── GROUP1_POS (to BMS VC1 and GROUP2_NEG) CELL_1B (+) ──┘ GROUP 2 (middle): CELL_2A,2B in parallel (same as group 1 structure) GROUP2_POS = BMS VC2 = GROUP3_NEG GROUP 3 (top): CELL_3A,3B in parallel GROUP3_POS = BMS VC3 = B+ PARALLEL WITHIN GROUP: cells spot-welded or wire-bonded (pure nickel strip). Do NOT solder directly to cell terminals — heat damages the cell. Use 0.1 mm × 6 mm pure nickel strip; resistance-weld or spot-weld. SERIES BETWEEN GROUPS: nickel strips connect GROUP(n) positive to GROUP(n+1) negative. NICKEL STRIP SIZING: Current carrying capacity: 0.1mm × 6mm strip ≈ 4A (conservative) For 10A: use 0.15mm × 8mm or two strips in parallel For 20A: use 0.2mm × 10mm pure nickel CELL HOLDER OPTIONS: Plastic cell holder (18650, 2P3S arrangement): allows replacement without welding. Spring contacts: higher resistance (5–15 mΩ per contact); acceptable for < 5A. Spot-welded nickel: much lower resistance; permanent. Rule: use welded nickel for > 5A applications. ================================================================================ PROTECTION IC COMPARISON TABLE ================================================================================ Feature | BQ76920 | BQ76930 | S-8254 | Discrete -----------------|-----------|-----------|-----------|----------- Cell count | 3–5 | 6–10 (3S+)| 4S fixed | Any (design) I2C interface | Yes | Yes | No | No Coulomb counter | Yes | Yes | No | No (ext. INA226) Per-cell voltage | Yes | Yes | No | No (ext. INA226) Cell balancing | Passive | Passive | Passive | Passive or active Cost | ~$3 | ~$4 | ~$1 | ~$10 (components) Package | TSSOP-24 | TSSOP-24 | SOP-20 | Through-hole Accuracy | ±1% | ±1% | ±2% | ±5% (comparator) ESP32 compatible | Yes (I2C) | Yes (I2C) | No | Via ADC / GPIO RECOMMENDATION FOR FIELD USE: Primary protection: BQ76920 (LiFePO4) or BQ76930 (Li-Ion) + ESP32 via I2C Backup: Discrete TL431/LM393 comparators for independent OV/UV Current sense: INA226 (higher accuracy than BQ76920 internal ADC for coulomb count) ================================================================================ LI-ION CHARGING PROFILE ================================================================================ CC/CV CHARGING (standard for all Li-Ion): Stage 1 — Pre-charge (if V_cell < 3.0V): Charge at 0.1C until V_cell reaches 3.0V. Low voltage cells may be damaged; discard if pre-charge takes > 30 min. Stage 2 — Bulk CC: Charge at 0.5C to 1C until V_pack reaches charge cutoff. 3S: charge until V_pack = 12.6V (4.2V/cell) 4S: charge until V_pack = 16.8V (4.2V/cell) Stage 3 — Absorption CV: Hold V at charge cutoff voltage. Current decreases exponentially. Terminate when I < C/10 (some designs: C/20 for better capacity). Stage 4 — Charger disconnect. Li-Ion does NOT have a float mode like lead-acid. Leave charger disconnected after termination. Reconnect when SOC drops to ~20–40% for next charge cycle. CHARGE VOLTAGE OPTIONS (trade-off: capacity vs. cycle life): 4.20V/cell: 100% capacity; ~500 cycles typical 4.10V/cell: 88% capacity; ~1500 cycles 4.00V/cell: 78% capacity; ~3000 cycles For ham radio field use: 4.10V/cell (BQ76930 OV_TRIP = 0x27) recommended. TEMPERATURE LIMITS FOR CHARGING: Below 10°C: reduce charge rate to 0.1C (cold reduces insertion rate) Below 0°C: do not charge (lithium plating, same as LiFePO4) Above 45°C: stop charging (thermal runaway risk increases significantly) Above 60°C: immediate emergency disconnect (all FETs open) ================================================================================ PARTS LIST — LI-ION BMS ================================================================================ Item | Qty | Description | Source ----------|-----|-------------------------------------------|------------------- BQ76930 | 1 | BQ76930 3–10 cell BMS IC (TSSOP-24) | Mouser/TI (OR) S-8254AAA | 1 | Seiko S-8254 4S protection IC (SOP-20) | Mouser/Digi-Key IRLB3034 | 4 | N-FET 40V 195A (CHG/DSG × 2 each) | Mouser R_SENSE | 1 | 5mΩ 3W Kelvin shunt | Mouser NTC | 2 | 10kΩ NTC thermistor | Mouser INA226 | 1 | INA226 precision current sensor | Mouser 18650_HLD | 6 | 18650 cell holder for 3S2P | Amazon/AliExpress NICKEL | 1 | 0.15×8mm pure nickel strip, 1m | Amazon/18650space SPOT_WLD | 1 | Spot welder or nickel-strip tabs kit | AliExpress CELL_18650| 6 | 18650 cells, matched (Samsung 30Q) | Illumn/18650battstore PCB | 1 | Custom or perfboard BMS carrier | JLC/OSH Park ================================================================================