1.8.2 START Battery Load Analysis¶
Scenario-based load analysis for START battery circuits (alternator-supplied).
START Battery Circuit Summary¶
All circuits powered by START battery (alternator charging):
| Source | Circuit | Typical | Peak | Duty Cycle | Notes |
|---|---|---|---|---|---|
| PMU Outputs | |||||
| OUT1+10 | iBooster | 0.25A | 40A | Seconds during braking | Non-adjacent combined |
| OUT2+3+4 | Radiator Fan | 20A | 53A | Variable PWM | 3× combined outputs |
| OUT5 | HVAC Blower | 10A | 20A | Continuous when on | Speed-dependent |
| OUT6 | GMRS Radio | 1A | 15A | Brief TX bursts | Standby vs transmit |
| OUT7 | Oil Cooler Fan | 0A | 15A | Temp-triggered | Off unless hot |
| OUT8 | PS Cooler Fan | 0A | 15A | Temp-triggered | Off unless hot |
| OUT9 | Dakota Digital | 25A | 25A | Continuous | Gauges always on |
| OUT11 | Wiper Controller | 0A | 15A | Intermittent | Rain only |
| OUT12 | Ham Radio | 1A | 13A | Brief TX bursts | Standby vs transmit |
| OUT13 | CT4 Controller | 10A | 10A | Continuous | Street lighting |
| OUT14 | DRL | 8A | 8A | Daytime only | Auto-off at night |
| OUT15 | Winch Trigger | 0A | 1A | Recovery only | Control signal only |
| OUT17 | A/C Clutch | 0A | 5A | Summer only | Seasonal |
| OUT18 | Horn | 0A | 5.4A | Seconds | Emergency only |
| OUT19 | iBooster Ignition | 5A | 5A | Continuous | Ignition signal |
| OUT20 | STX Intercom | 1A | 5A | Brief TX bursts | Standby vs transmit |
| OUT21 | Brake Lights | 0A | 3A | Seconds during braking | Traffic-dependent |
| OUT22 | Reverse Lights | 0A | 3A | Seconds in reverse | Parking only |
| BCDC Charger | |||||
| - | BCDC to AUX | 30A | 50A | Continuous | Charges AUX battery |
| Direct Loads | |||||
| - | Starter | 0A | 400-600A | 3-5 seconds | Cranking only |
| - | ECM | ~2A | ~5A | Continuous | Engine management |
| - | Grid Heater | 0A | 250A | 3-5 seconds | Cold start only |
Scenario Analysis¶
Scenario 1: Daily Highway Driving¶
Conditions: Highway cruising, moderate temps (70°F), radio on standby, climate control on
| Circuit | Load | Reason |
|---|---|---|
| iBooster (OUT1+10) | 0.25A | Idle - no braking |
| Radiator Fan (OUT2+3+4) | 16A | Low speed - highway airflow |
| HVAC Blower (OUT5) | 10A | Medium speed |
| GMRS Radio (OUT6) | 1A | Standby |
| Oil Cooler Fan (OUT7) | 0A | Temp normal |
| PS Cooler Fan (OUT8) | 0A | Temp normal |
| Dakota Digital (OUT9) | 25A | Always on |
| Wiper (OUT11) | 0A | Dry weather |
| Ham Radio (OUT12) | 1A | Standby |
| CT4 (OUT13) | 10A | Running lights |
| DRL (OUT14) | 8A | Daytime |
| iBooster Ignition (OUT19) | 5A | Always on |
| STX Intercom (OUT20) | 1A | Standby |
| BCDC Charger | 30A | Maintaining AUX |
| TOTAL | 107A |
Alternator Load: 107A of 270A capacity = 40% utilization Excellent
Scenario 2: Hot Day City Driving (Stop-and-Go)¶
Conditions: City traffic, hot day (95°F), A/C on, frequent braking
| Circuit | Load | Reason |
|---|---|---|
| iBooster (OUT1+10) | 5A | Average - frequent braking |
| Radiator Fan (OUT2+3+4) | 42A | High speed - no airflow, hot |
| HVAC Blower (OUT5) | 20A | Max speed for A/C |
| GMRS Radio (OUT6) | 1A | Standby |
| Oil Cooler Fan (OUT7) | 15A | Hot - active |
| PS Cooler Fan (OUT8) | 15A | Hot - active |
| Dakota Digital (OUT9) | 25A | Always on |
| Wiper (OUT11) | 0A | Dry weather |
| Ham Radio (OUT12) | 1A | Standby |
| CT4 (OUT13) | 10A | Running lights |
| DRL (OUT14) | 8A | Daytime |
| A/C Clutch (OUT17) | 5A | A/C on |
| iBooster Ignition (OUT19) | 5A | Always on |
| STX Intercom (OUT20) | 1A | Standby |
| Brake Lights (OUT21) | 1A | Average - traffic |
| BCDC Charger | 40A | Higher rate |
| TOTAL | 194A |
Alternator Load: 194A of 270A capacity = 72% utilization Good
Scenario 3: Offroad Trail (Slow Speed, Hot Engine)¶
Conditions: Technical trail, slow speed (<10 mph), engine working hard, radio use
| Circuit | Load | Reason |
|---|---|---|
| iBooster (OUT1+10) | 2A | Occasional braking |
| Radiator Fan (OUT2+3+4) | 53A | Max speed - no airflow |
| HVAC Blower (OUT5) | 15A | Moderate |
| GMRS Radio (OUT6) | 5A | Occasional TX |
| Oil Cooler Fan (OUT7) | 15A | Hot - active |
| PS Cooler Fan (OUT8) | 15A | Hot - active |
| Dakota Digital (OUT9) | 25A | Always on |
| Wiper (OUT11) | 0A | Dry |
| Ham Radio (OUT12) | 1A | Standby |
| CT4 (OUT13) | 10A | Running lights |
| DRL (OUT14) | 8A | Daytime |
| iBooster Ignition (OUT19) | 5A | Always on |
| STX Intercom (OUT20) | 3A | Occasional TX |
| BCDC Charger | 50A | Full rate - supporting SwitchPros |
| TOTAL | 207A |
Alternator Load: 207A of 270A capacity = 77% utilization Good
Note: Offroad lighting (SwitchPros) draws from AUX battery, not alternator.
Scenario 4: Emergency Braking in Rain¶
Conditions: Highway, sudden hard braking, wipers on, headlights on
| Circuit | Load | Reason |
|---|---|---|
| iBooster (OUT1+10) | 40A | Peak braking assist |
| Radiator Fan (OUT2+3+4) | 20A | Normal - highway airflow |
| HVAC Blower (OUT5) | 15A | Defrost |
| GMRS Radio (OUT6) | 1A | Standby |
| Oil Cooler Fan (OUT7) | 0A | Temp normal |
| PS Cooler Fan (OUT8) | 0A | Temp normal |
| Dakota Digital (OUT9) | 25A | Always on |
| Wiper (OUT11) | 15A | High speed |
| Ham Radio (OUT12) | 1A | Standby |
| CT4 (OUT13) | 10A | Headlights |
| DRL (OUT14) | 0A | Off - headlights on |
| iBooster Ignition (OUT19) | 5A | Always on |
| STX Intercom (OUT20) | 1A | Standby |
| Brake Lights (OUT21) | 3A | Braking |
| BCDC Charger | 30A | Normal rate |
| TOTAL | 166A |
Duration: iBooster peak lasts 2-5 seconds only
Alternator Load: 166A of 270A capacity = 61% utilization Excellent
Scenario 5: Parked with Engine Idling (Engine Warm)¶
Conditions: Stationary, engine idling, waiting, moderate accessories
| Circuit | Load | Reason |
|---|---|---|
| iBooster (OUT1+10) | 0.25A | Idle |
| Radiator Fan (OUT2+3+4) | 35A | Moderate - no airflow |
| HVAC Blower (OUT5) | 10A | Comfortable |
| GMRS Radio (OUT6) | 1A | Standby |
| Oil Cooler Fan (OUT7) | 0A | Temp stable |
| PS Cooler Fan (OUT8) | 0A | Temp stable |
| Dakota Digital (OUT9) | 25A | Always on |
| Wiper (OUT11) | 0A | Off |
| Ham Radio (OUT12) | 1A | Standby |
| CT4 (OUT13) | 10A | Parking lights |
| DRL (OUT14) | 0A | Off - parked |
| A/C Clutch (OUT17) | 5A | If summer |
| iBooster Ignition (OUT19) | 5A | Always on |
| STX Intercom (OUT20) | 1A | Standby |
| BCDC Charger | 30A | Normal rate |
| TOTAL | 123A |
Alternator Load: 123A of 270A capacity = 46% utilization Excellent
Summary¶
| 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 Realistic Case: 207A (offroad with hot engine) = 63A margin
Key Insight: All realistic scenarios stay well within alternator capacity. The 270A alternator provides adequate margin for all operating conditions.
Load Exclusions (Not Alternator Loads)¶
The following high-current loads are NOT supplied by the alternator:
| Load | Current | Source | Why Not Alternator |
|---|---|---|---|
| SwitchPros (all outputs) | 127A max | AUX battery | BCDC-fed, isolated |
| ARB Compressor | 90A | AUX battery | Via SafetyHub on AUX |
| Winch | 400A peak | AUX battery | Direct connection |
| BODY PDU circuits | 56A max | AUX battery | CONSTANT bus fed |
| Starter | 400-600A | START battery | Cranking only, engine off |
| Grid Heater | 250A | START battery | 3-5 sec cold start only |
Architecture: The dual battery system isolates high-current accessory loads (AUX battery) from engine/safety loads (START battery). The BCDC charger (50A max) is the only connection between batteries during normal operation.
Related Documentation¶
- Alternator Specifications - 270A capacity details
- PMU Outputs - Individual circuit assignments
- BCDC Alpha 50 - DC-DC charger specifications
- AUX Battery Load Analysis - Companion analysis for AUX battery