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SCHEMATIC: MPPT Solar Charge Controller Integration with BMS
TM-BMS-001  Rev A
Interface between solar MPPT controller (TM-PWR-001) and BMS protection stage
Charge current limit signaling  |  Temperature-adjusted charge setpoints
Charge enable/disable via BMS  |  Multi-source charging (solar + mains charger)
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REFERENCE: See TM-PWR-001 sch_solar_charge_ctrl.txt for MPPT hardware detail.
This document covers the interface between the MPPT controller and the BMS.

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SYSTEM ARCHITECTURE
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CHARGING SOURCE OPTIONS:
  Source 1: Solar panel(s) → MPPT controller → BMS input (B+/B−)
  Source 2: AC mains charger (LiFePO4 profile, 14.4V / 25A) → BMS input
  Source 3: Vehicle DC-DC (Orion, etc.) → BMS input

CHARGING PATH:
  Solar/Charger → BMS B+ / B− terminals
  BMS CHG FET: in series with B−
  Pack cells: B+ to B− direct (unswitched positive)
  Load path: B+/P− through BMS DSG FET

  NOTE: With common-port BMS (single B− for charge and discharge):
    Charge and discharge cannot occur simultaneously on separated ports.
    Source and load connect to same P− terminal.
    BMS manages whether CHG FET or DSG FET operates based on current direction.

  SEPARATE-PORT BMS (JK BMS with C- terminal):
    Charge connector: B+, C− (charge negative through CHG FET)
    Load connector:   B+, P− (discharge negative through DSG FET)
    Allows simultaneous solar charging and load operation.
    RECOMMENDED for field station: charge from solar while powering radio.

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CHARGE ENABLE SIGNAL INTERFACE
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The ESP32 BMS monitor controls the MPPT controller's output enable:

BMS FIRMWARE → MPPT CONTROLLER:
  When BMS detects: OV, OT (high temp), charging-below-0°C → disable MPPT output.
  Signal: ESP32 GPIO pin → MPPT controller CHRG_EN input.

CHRG_EN CIRCUIT (in MPPT controller, TM-PWR-001):
  CHRG_EN GPIO (ESP32 GPIO27 in MPPT) ──[RELAY COIL] controls relay K1:
    K1 NO contacts: in series with buck converter enable or CHG FET gate driver.
    When CHRG_EN = LOW → relay open → charging stopped.

INTER-SYSTEM SIGNAL WIRING:
  BMS ESP32 GPIO34 (output) ──── MPPT ESP32 GPIO27 (CHRG_EN input)
  This is a 3.3V logic signal; both ESP32 share the same GND reference.
  If ESP32 boards are in different enclosures: optocoupler isolation recommended.

OPTOCOUPLER ISOLATION:
  BMS_ESP32 GPIO34 ──[1kΩ]── PC817 LED anode
                              PC817 LED cathode → GND (BMS ESP32 GND)
                              PC817 transistor collector ──[10kΩ pull-up]── 3.3V (MPPT)
                              PC817 transistor emitter → GND (MPPT ESP32 GND)
                              Signal at MPPT ESP32 GPIO27 → LOW when charge enabled.
                              (Inverted logic; adjust firmware accordingly)

CHARGE CURRENT LIMIT SIGNALING:
  BMS sends target charge current to MPPT via UART or I2C:
    MPPT ESP32 Serial1 ←── BMS ESP32 Serial1 (3.3V UART)
    Protocol: simple ASCII: "ILIM=15.0\n" (set charge current limit to 15A)
    MPPT parses and adjusts PID setpoint.

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CHARGE CURRENT AND VOLTAGE SETPOINTS
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STAGE-BASED SETPOINTS (communicated from BMS to MPPT):

  BMS determines charge stage based on pack SOC and voltage:

  STAGE 1 — BULK (SOC < 80%, V_pack < 14.2V):
    Command to MPPT: "VSET=14.40 ISET=25.0"   (CV limit 14.4V, CC limit 25A)
    BMS monitors I_charge; if I > 25A, firmware tightens ISET.

  STAGE 2 — ABSORPTION (V_pack ≥ 14.2V, I_charge > C/20):
    Command to MPPT: "VSET=14.40 ISET=10.0"   (hold 14.4V, taper current)

  STAGE 3 — TERMINATION (I_charge < C/20 = 2.5A for 50 Ah):
    Command to MPPT: "VSET=0 ISET=0"           (stop charging)
    MPPT sets duty cycle to 0.

  STAGE 4 — FLOAT:
    Command to MPPT: "VSET=13.50 ISET=5.0"    (float at 13.5V, allow small trickle)

TEMPERATURE-ADJUSTED CHARGE VOLTAGE:
  BMS reads NTC temperature; calculates adjusted charge setpoint:
    V_adj = V_nominal - (T_cell - 25) × 0.012   (0.003 V/°C/cell × 4 cells)
    At T = 40°C: V_adj = 14.4 - 0.18 = 14.22V
    At T = 10°C: V_adj = 14.4 + 0.18 = 14.58V

  BMS sends updated "VSET=14.22" to MPPT controller each temperature update cycle.

COLD TEMPERATURE LOCKOUT:
  If T_cell < 0°C: BMS sends "VSET=0 ISET=0" → MPPT stops charging.
  BMS continues to monitor every 60 seconds.
  When T_cell > 2°C: BMS sends "VSET=14.40 ISET=5.0" → resume pre-charge rate.
  When T_cell > 5°C: BMS sends "VSET=14.40 ISET=25.0" → resume full charge.

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MULTI-SOURCE CHARGE MANAGEMENT
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When both solar MPPT and AC mains charger are present:

CURRENT SENSE ARBITRATION:
  BMS monitors I_charge (total, from INA226 on B−).
  If I_charge_total > I_max (25A): send ISET reduction to MPPT (mains charger
  does not take I2C/UART commands; it is self-limited).
  MPPT backs off to maintain total ≤ I_max.

PRIORITY ORDER:
  1. Solar MPPT: primary source (free energy)
  2. AC mains charger: supplemental when solar insufficient
  Both connect to BMS B+ / C− simultaneously.
  Reverse-polarity diodes prevent sources from charging each other.

SOURCE ISOLATION DIODE:
  Each source connects through a Schottky diode (MBR3045, 30A, 45V):
    MPPT_out (+) ──[D_solar: MBR3045]──┐
                                        ├── BMS C−/B+
    AC_charger (+) ──[D_mains: MBR3045]┘

  Forward voltage drop: 0.3V at 20A → 6W loss per diode.
  At full 25A charge: total diode loss = 12W — acceptable for field use.
  Better: use OR-ing controllers (LTC4413) for near-zero-loss ORing.

LTC4413 IDEAL DIODE CONTROLLER (optional, better than Schottky):
  Controls a P-channel MOSFET to act as ideal diode (very low voltage drop).
  V_drop = I × Rds_on = 25A × 0.005Ω = 0.125V → 3W loss (vs. 6W Schottky).
  Use for permanent installations where efficiency matters.

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CHARGE EFFICIENCY MEASUREMENT
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COULOMBIC EFFICIENCY = charge delivered to battery / charge drawn from source
ROUND-TRIP EFFICIENCY = energy discharged / energy charged (includes charge + discharge losses)

MEASUREMENT SETUP:
  I_charge: INA226 on BMS C− (charge path)
  I_discharge: INA226 on BMS P− (load path)
  V_pack: INA226 bus voltage

  Energy in:  Wh_in  += V_pack × I_charge × dt   (integrate during charging)
  Energy out: Wh_out += V_pack × I_discharge × dt  (integrate during discharging)

  Coulombic efficiency η_Q = Ah_out / Ah_in (should be 99%+ for healthy LiFePO4)
  Round-trip energy efficiency η_E = Wh_out / Wh_in (typically 92–96% for LiFePO4)

CHARGE EFFICIENCY BY STAGE:
  Bulk (CC): nearly 100% (low overvoltage)
  Absorption (CV): 95% (current decreasing; some I²R loss)
  Combined charge efficiency: ~97% for LiFePO4; ~95% for Li-Ion

FIRMWARE LOGGING:
  ```cpp
  void update_charge_efficiency() {
      float i_chg  = read_charge_current();   // INA226 channel 1
      float i_load = read_load_current();      // INA226 channel 2
      float v_pack = read_pack_voltage();
      float dt_h   = delta_ms / 3600000.0f;

      if (i_chg > 0.01f) {  // Charging
          g_ah_in   += i_chg * dt_h;
          g_wh_in   += i_chg * v_pack * dt_h;
      }
      if (i_load > 0.01f) {  // Discharging
          g_ah_out  += i_load * dt_h;
          g_wh_out  += i_load * v_pack * dt_h;
      }
      if (g_ah_in > 0.1f) {
          g_coulomb_eff = g_ah_out / g_ah_in * 100.0f;
          g_energy_eff  = g_wh_out / g_wh_in * 100.0f;
      }
  }
  ```

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WIRING DIAGRAM — COMPLETE BMS + MPPT INTEGRATION
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SOLAR PANEL ─── MPPT CONTROLLER ─────────────────── BMS CHARGE INPUT (B+/C−)
  [18V panel]    [ESP32 + buck]        [Schottky D]
                  │
               UART TX ──────────────── BMS ESP32 UART RX
               (charge setpoint commands)

MAINS CHARGER ──────────────────────── BMS CHARGE INPUT (B+/C−)
  [LiFePO4          [Schottky D]
   profile, 14.4V]

BMS OUTPUT ─────────────────────────── LOAD (radio, accessories)
  (B+ / P−)          [INA226]
                         │
                      ESP32 I2C (load current monitoring)

BMS ESP32:
  I2C (GPIO21/22): BQ76920, INA226×2, ADS1115
  UART (GPIO16/17): JK BMS or MPPT
  BLE: broadcasting pack status (see firmware)
  CYD display: cell voltages, SOC, temperature, charge/discharge efficiency

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PARTS LIST — MPPT INTEGRATION
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Item      | Qty | Description                               | Source
----------|-----|-------------------------------------------|-------------------
MBR3045   |  2  | Schottky 30A 45V TO-220 (source ORing)   | Mouser
INA226    |  2  | INA226 precision power monitor (ch. I2C) | Mouser
PC817     |  2  | Optocoupler (CHRG_EN isolation)           | Mouser
R_opto    |  2  | 1kΩ 0.25W (opto LED current)             | Mouser
TERM_BLK  |  1  | 4-position terminal block (wiring center)| Mouser

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