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1.8 Load Analysis

Scenario-based load analysis for the dual battery electrical system.

System Architecture

The Jeep LJ uses a dual battery architecture with isolated power domains:

Battery Location Charging Primary Loads
START Driver wheel well Alternator (270A) PMU, engine systems, BCDC
AUX Passenger wheel well BCDC (50A) + Solar (6A) SwitchPros, SafetyHub, BODY PDU

Key Principle: The BCDC is the only connection between batteries during normal operation. AUX battery loads do NOT draw from the alternator directly.

Analysis Methodology

Realistic Scenario Approach

Load analysis uses realistic operating scenarios rather than theoretical worst-case sums:

  1. Identify which battery powers each circuit (START vs AUX)
  2. Analyze each battery separately with its own charging source
  3. Use realistic scenarios (daily driving, offroad, airing up, recovery)
  4. Account for duty cycles (brief peaks vs continuous loads)
  5. Consider mutually exclusive activities (can't brake hard while winching)

Why Not Sum All Loads?

Never create "theoretical worst case" scenarios:

  • Do NOT sum all loads at peak simultaneously
  • Do NOT combine mutually exclusive activities (braking + winching + radio TX)
  • Do NOT add AUX battery loads to alternator calculations
  • Do NOT flag alternator "undersizing" based on impossible scenarios

Example - WRONG:

PMU max: 253A + SwitchPros max: 127A + ARB: 90A + Winch: 400A = 870A
Alternator: 270A = CRITICAL UNDERSIZING ❌

Example - CORRECT:

START Battery Scenario (Offroad):
PMU typical: 115A + Radiator fan: 53A + BCDC: 50A = 218A
Alternator: 270A = 52A margin

AUX Battery Scenario (Night Offroad):
SwitchPros: 70A, BCDC charging: 50A
Net drain: 20A, Time to 50% SOC: 102 minutes

Load Categories

Category Characteristic Analysis Approach
Continuous Always on when operating Include in all scenarios
Intermittent Cycles on/off Use average or typical duty
Peak Brief bursts (seconds) Note duration, don't add to continuous
Seasonal A/C, heated seats Include in relevant scenarios only
Mutually Exclusive Braking vs accelerating Never combine

Load Exclusions

These loads are NOT included in running alternator calculations:

Load Current Reason
Starter 400-600A Cranking only (engine off)
Grid Heater 250A 3-5 sec cold start only
Winch 400A AUX battery (isolated)
ARB Compressor 90A AUX battery (isolated)
SwitchPros lights 100A+ AUX battery (isolated)

Summary Results

START Battery (Alternator-Supplied)

Scenario Total Load Alternator Utilization Status
Highway Driving 107A 270A 40% Excellent
Hot City Driving 194A 270A 72% Good
Offroad Trail 207A 270A 77% Good
Emergency Braking 166A 270A 61% Excellent
Parked Idling 123A 270A 46% Excellent

Worst Case: 207A (offroad) = 63A margin

AUX Battery (BCDC-Charged)

Scenario Total Draw BCDC Net Effect Duration Limit Status
Daily Driving 5A 50A +45A Unlimited Charging
Night Highway 38A 50A +12A Unlimited Charging
Night Offroad 70A 50A -20A 102 min Excellent
Air Up (5-10 min) 110A 50A -60A 10 min Excellent
Air Up Extended 110A 50A -60A 34 min Practical
Winch Recovery 265A 50A -215A 30-sec pulls Brief OK
Camp Mode 27A 0A -27A 76 min Limited

Key Insight: 50A BCDC enables night highway charging and 102-minute night offroad runtime.