Solar Controller

Info Frame (ID 0x20) - Status from Dometic Solar Controller

Frame Format: [ID=20] [Volt_LSB] [Volt_MSB] [Curr_LSB] [Curr_MSB] [Status] [FF] [FF] [FF]

Note: Solar controller appears to be monitoring-only (no control frame)

Frame Layout:

Bytes 0-1: Solar Panel Voltage (Little-Endian)

Format: 16-bit little-endian value

• Formula: ((byte[1] << 8) | byte[0]) × 0.01V
• Range: 0-819.1V (0x0000-0x1FFF)
• Resolution: 0.01V (10mV)
• Example: [0x4C, 0x04] = 0x044C = 1100 → 11.00V

Bytes 2-3: Solar Panel Current (Little-Endian)

Format: 16-bit little-endian value

• Formula: ((byte[3] << 8) | byte[2]) × 0.1A
• Range: 0-819.1A (0x0000-0x1FFF)
• Resolution: 0.1A (100mA)
• Example: [0x1E, 0x00] = 0x001E = 30 → 3.0A

Byte 4: Solar Controller Status

Bits 0-1: MPPT Mode

Status Flags:

• Bit 2: On/Off (0=Off, 1=On)
• Bit 3: Reduced Power (power limitation active)
• Bit 4: Fuse Error (fuse blown or protection fault)
• Bit 6: MPPT Status (MPPT algorithm active)

Bytes 5-7: Reserved

Currently unused - fixed at [FF] [FF] [FF]

Communication Example

Reading Solar Controller Status

Info Frame (0x20) Response:

[0x4C] [0x04] [0x1E] [0x00] [0x05] [FF] [FF] [FF]
Bytes 0-1: 0x044C = 1100 → 11.00V solar panel voltage
Bytes 2-3: 0x001E = 30 → 3.0A solar panel current
Byte 4: 0x05 = MPPT/Bulk mode + On + Reduced Power

MPPT Charging Phases

Bulk/MPPT Phase

• Controller operates at maximum power point
• Battery voltage increases
• Full charging current delivered

Absorption Phase

• Battery voltage reaches target (typically 14.4V for lead-acid)
• Current gradually decreases
• Held at absorption voltage for set time

Float Phase

• Maintenance charging mode
• Lower voltage (typically 13.8V)
• Maintains full charge without overcharging

Desulfation Phase

• Special recovery mode for sulfated batteries
• Higher voltage pulses
• Attempts to restore battery capacity