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1.4.5 PMU ARB Load Shedding Logic

Automatic load management during ARB compressor operation to preserve AUX battery capacity and maintain system voltage.

Dual Battery Architecture

Critical Understanding: ARB compressor draws from AUX battery, NOT directly from alternator:

  • ARB Compressor (90A): Powered by AUX battery via SafetyHub 150
  • BCDC Charging (50A max): Replenishes AUX battery from alternator
  • Net AUX battery drain: 90A - 50A = 40A during compressor operation

The alternator is NOT overloaded during ARB operation. The 50A BCDC significantly reduces net discharge rate, making extended air-up practical. Load shedding provides additional margin and maintains optimal voltage.

Purpose

Preserve AUX battery capacity during extended ARB compressor operation by shedding non-critical START battery loads. This maximizes BCDC charging rate and reduces overall system stress.

Secondary Goal: Maintain comfortable voltage levels (>13.0V at START battery) for consistent PMU and accessory operation.

Problem Statement

AUX Battery Depletion During Extended Air-Up:

ARB compressor draw:        90A  (from AUX battery)
Other AUX loads:             5A  (camera, radio, USB)
BCDC charging rate:         50A  (to AUX battery)
─────────────────────────────────
Net AUX battery drain:      45A

AUX battery capacity:       135Ah (Dakota Lithium LiFePO4)
Usable capacity (80% DOD):  108Ah
Time to 20% SOC:           ~144 minutes continuous (2.4 hours)

The Dakota Lithium 135Ah combined with 50A BCDC makes extended air-up a non-issue. Load shedding provides additional margin and maintains optimal voltage for electronics.

Impact Without Load Shedding:

  • Minor: AUX battery still has comfortable margin at 50A BCDC
  • START battery loads reduce BCDC charging efficiency slightly
  • Load shedding maximizes available margin for extended sessions

Solution Overview

Detect ARB compressor activation and disable non-critical START battery loads to maximize BCDC charging rate and maintain optimal system voltage.

Load Shedding Strategy:

  • Shed cosmetic loads first (DRL)
  • Shed comfort loads second (A/C)
  • Conditionally shed cooling loads if temperatures allow (oil/PS cooler fans)
  • Maintain critical systems (iBooster, HVAC blower, lights, CT4)

Load Shedding Logic

// Monitor SwitchPros OUTPUT-11 state via shared CAN bus or hardwired trigger
IF (SwitchPros_OUT11_ARB == ON) OR (BatteryVoltage < 13.0V AND EngineRPM > 1000):

  // Shed non-critical loads (priority order: lowest to highest impact)
  Out14_DRL = OFF              // -8A: Daytime running lights (cosmetic)
  Out17_AC_Clutch = OFF        // -5A: Air conditioning (comfort)

  // Conditionally shed cooler fans if temperatures allow
  IF (J1939_SPN175_OilTemp < 220°F):
    Out7_OilFan = OFF          // -15A: Oil cooler fan (temp-dependent)

  IF (J1939_SPN110_CoolantTemp < 210°F):
    Out8_PSFan = OFF           // -15A: PS cooler fan (temp-dependent)

  // Total load reduction: 28-43A depending on temperatures

  // Optional: Trigger dashboard indicator
  Out_ARB_Active_Indicator = ON  // Visual feedback to driver

ELSE:
  // Restore normal operation when ARB stops
  Out14_DRL = (Per DRL auto-off logic)
  Out17_AC_Clutch = (Per A/C request logic)
  Out7_OilFan = (Per oil temp thresholds)
  Out8_PSFan = (Per coolant temp thresholds)
  Out_ARB_Active_Indicator = OFF

Load Reduction Analysis

Load Shed Current Saved Impact When Disabled
DRL (OUT14) 8A Low - cosmetic only Always when ARB runs
A/C Clutch (OUT17) 5A Medium - comfort loss Always when ARB runs
Oil Cooler Fan (OUT7) 15A Low - if oil temp <220°F Temperature-dependent
PS Cooler Fan (OUT8) 15A Low - if coolant temp <210°F Temperature-dependent
Total Saved 28-43A - -

AUX Battery Impact Analysis

See AUX Battery Load Analysis for complete scenario details.

WITHOUT Load Shedding

ARB compressor draw:     90A   (from AUX battery)
Other AUX loads:          5A   (camera, radio memory, USB)
─────────────────────────────
Total AUX draw:          95A

BCDC charging:           50A   (to AUX battery)
─────────────────────────────
Net AUX drain:           45A

Time to 50% SOC:         45 minutes

Alternator Load (START battery): See START Battery Load Analysis

  • PMU + radiator fan + BCDC = ~165A typical
  • Alternator: 270A
  • Margin: +105A Alternator is NOT the constraint

WITH Load Shedding (Minimum - 13A shed from START)

Shedding DRL (8A) and A/C clutch (5A) from PMU reduces START battery load, allowing maximum BCDC output:

START battery:
PMU reduced:          93A   (was 106A, shed DRL + A/C)
Radiator fan:         35A   (moderate, stationary)
BCDC at full rate:    50A   (maximized)
─────────────────────────────
START total:         178A
Alternator:          270A
Margin:              +92A

AUX battery:
ARB compressor:       90A
Other AUX loads:       5A
BCDC charging:        50A   (full rate maintained)
─────────────────────────────
Net AUX drain:        45A   (unchanged - BCDC still maxed)

Primary benefit: Ensures BCDC maintains full 50A output even if START battery voltage sags.

WITH Load Shedding (Maximum - 43A shed from START)

When oil/coolant temps allow disabling cooler fans:

START battery:
PMU reduced:          63A   (shed DRL + A/C + oil fan + PS fan)
Radiator fan:         35A   (moderate)
BCDC at full rate:    50A
─────────────────────────────
START total:         148A
Alternator:          270A
Margin:             +122A   Excellent

AUX battery:
Net AUX drain:        45A   (unchanged)

Benefit: Maximum alternator headroom, stable voltage for all electronics.

Implementation Details

Detection Methods (Priority Order)

1. Hardwired Trigger (Preferred):

  • Wire from SwitchPros OUTPUT-11 to PMU digital input
  • Direct, reliable detection
  • Lowest latency

2. CAN Bus Monitoring:

  • If SwitchPros broadcasts output states on CAN
  • No additional wiring required
  • Slight latency

3. Voltage-Based Fallback:

  • IF (BatteryVoltage < 13.0V) AND (EngineRPM > 1000)
  • Triggers on ANY high load (not just ARB)
  • Universal protection regardless of cause

Temperature Safety Checks

Critical: Always verify temperatures before disabling cooler fans.

// Oil Cooler Fan
IF (J1939_SPN175_OilTemp < 220°F):
  Safe to disable Out7_OilFan
ELSE:
  KEEP Out7_OilFan = ON (engine protection priority)

// PS Cooler Fan
IF (J1939_SPN110_CoolantTemp < 210°F):
  Safe to disable Out8_PSFan
ELSE:
  KEEP Out8_PSFan = ON (engine protection priority)

Hysteresis: Use 10°F hysteresis to prevent rapid cycling when temps hover near threshold.

User Communication

Dashboard Indicator Options:

  1. Simple LED: ARB button backlight changes color
  2. Message: "ARB MODE - DRL/AC OFF"
  3. Dakota Digital Display: Custom message via BIM module
  4. No indicator: Silent operation (driver may not notice DRL/AC disabled)

Recommended: Backlight color change (green → amber when load shedding active)

Operational Procedures (Supplemental)

Best Practices for ARB Use:

  1. Increase Engine RPM: Run engine at 1500+ RPM during tire inflation
  2. Alternator output increases with RPM
  3. Better voltage regulation at higher speeds

  4. Faster tire inflation

  5. Monitor Voltage: Watch Dakota Digital voltage gauge during ARB use

  6. Normal: 14.0-14.4V (load shedding working)
  7. Marginal: 13.5-14.0V (acceptable, brief periods)

  8. Low: <13.5V (increase RPM or reduce loads)

  9. Hot Weather: Avoid prolonged ARB use at idle when ambient temp >95°F

  10. Radiator fan + ARB + heat soak = high total load

  11. Let engine cool between inflation cycles

  12. Avoid Simultaneous High Loads:

  13. ❌ ARB + winch (both 90A+ loads)
  14. ❌ ARB + all accessories at idle
  15. ARB + normal driving loads at 1500+ RPM

Testing & Validation

Initial Shakedown Testing

Test 1: ARB Load Shedding Activation

  1. Start engine, let idle stabilize
  2. Activate ARB (SwitchPros Button 11)
  3. Verify:
  4. DRL turns OFF
  5. A/C compressor disengages (if running)
  6. Dashboard indicator activates (if implemented)
  7. Voltage stays >13.5V

Test 2: Temperature-Based Fan Shedding

  1. Start engine cold (<180°F coolant)
  2. Activate ARB
  3. Verify:
  4. Oil cooler fan OFF (temp below threshold)
  5. PS cooler fan OFF (temp below threshold)
  6. Run engine to operating temp (>220°F)
  7. Activate ARB
  8. Verify:
  9. Cooler fans stay ON (engine protection priority)

Test 3: Load Restoration

  1. Activate ARB
  2. Verify loads shed
  3. Deactivate ARB
  4. Verify:
  5. DRL returns to normal logic (on with ignition)
  6. A/C returns if requested
  7. Cooler fans return to temp-based control

Data Logging

Enable PMU Continuous Logging:

LOG BatteryVoltage (1 Hz)
LOG TotalCurrent_PMU (1 Hz)
LOG EngineRPM (J1939_SPN190)
LOG Out14_DRL (state)
LOG Out17_AC_Clutch (state)
LOG Out7_OilFan (state)
LOG Out8_PSFan (state)
LOG SwitchPros_OUT11_ARB (state or trigger input)

Analysis:

  1. Export logs after tire inflation sessions
  2. Verify voltage stays >13.5V during ARB operation
  3. Confirm load shedding activates/deactivates correctly
  4. Identify any unexpected load combinations

Long-Term Monitoring

Track Over Time:

  • Battery voltage during ARB use (should be consistent)
  • Frequency of load shedding activation
  • Any low-voltage warnings or failures
  • Battery state of charge recovery after ARB use

Adjust if Needed:

  • Modify shed priority if testing shows different needs
  • Add/remove loads from shed list
  • Adjust temperature thresholds for cooler fans

Load Analysis:

Power Systems:

ARB Compressor:

  • Air System - ARB twin compressor specifications (90A total load)
  • SwitchPros - OUTPUT-11 control integration