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 | |||||
| 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 |
| OUT13 | CT4 Controller | 10A | 10A | Continuous | Street lighting |
| OUT17 | A/C Clutch | 0A | 5A | Summer only | Seasonal |
| OUT18 | Horn | 0A | 5.4A | Seconds | Emergency only |
| 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 |
| OUT23 | DRL | 2.6A | 2.6A | Daytime only | Auto-off at night |
| Fwd Dist Bus (START-direct) | relocated off the PMU | ||||
| Fwd Bus | iBooster main | 0.25A | 40A | Seconds during braking | Brief peak |
| Fwd Bus | Radiator Fan | 20A | 53A | Variable (own controller) | Controller ⚠️ TBD #134 |
| Fwd Bus | Turbolamik TCU | 15A | 15A | Continuous | Must stay on |
| Ign bus | iBooster Enable | 5A | 5A | Continuous | Ignition-switched |
| 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 |
Forward Distribution Bus relocation — totals unchanged
The radiator fan, iBooster, and TCU now feed START-direct via the Forward Distribution Bus instead of the PMU. Because they were already powered by the START battery through the PMU, the battery and alternator totals are unchanged — only the distribution path moved. The TCU (~15A continuous) is now itemized separately; folding it into the scenarios below raises the worst realistic case from 201A to ~216A (80% of the 270A alternator) — still within margin.
Scenario Analysis¶
Scenario 1: Daily Highway Driving¶
Conditions: Highway cruising, moderate temps (70°F), radio on standby, climate control on
| Circuit | Load | Reason |
|---|---|---|
| iBooster (Fwd Bus) | 0.25A | Idle - no braking |
| Radiator Fan (Fwd Bus) | 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 |
| CT4 (OUT13) | 10A | Running lights |
| iBooster Enable (ign bus) | 5A | Always on |
| STX Intercom (OUT20) | 1A | Standby |
| DRL (OUT23) | 2.6A | Daytime |
| BCDC Charger | 30A | Maintaining AUX |
| TOTAL | 101A |
Alternator Load: 101A of 270A capacity = 37% 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 (Fwd Bus) | 5A | Average - frequent braking |
| Radiator Fan (Fwd Bus) | 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 |
| CT4 (OUT13) | 10A | Running lights |
| A/C Clutch (OUT17) | 5A | A/C on |
| iBooster Enable (ign bus) | 5A | Always on |
| STX Intercom (OUT20) | 1A | Standby |
| Brake Lights (OUT21) | 1A | Average - traffic |
| DRL (OUT23) | 2.6A | Daytime |
| BCDC Charger | 40A | Higher rate |
| TOTAL | 188A |
Alternator Load: 188A of 270A capacity = 70% 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 (Fwd Bus) | 2A | Occasional braking |
| Radiator Fan (Fwd Bus) | 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 |
| CT4 (OUT13) | 10A | Running lights |
| iBooster Enable (ign bus) | 5A | Always on |
| STX Intercom (OUT20) | 3A | Occasional TX |
| DRL (OUT23) | 2.6A | Daytime |
| BCDC Charger | 50A | Full rate - supporting SwitchPros |
| TOTAL | 201A |
Alternator Load: 201A of 270A capacity = 74% 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 (Fwd Bus) | 40A | Peak braking assist |
| Radiator Fan (Fwd Bus) | 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 |
| CT4 (OUT13) | 10A | Headlights |
| iBooster Enable (ign bus) | 5A | Always on |
| STX Intercom (OUT20) | 1A | Standby |
| Brake Lights (OUT21) | 3A | Braking |
| DRL (OUT23) | 0A | Off - headlights on |
| BCDC Charger | 30A | Normal rate |
| TOTAL | 165A |
Duration: iBooster peak lasts 2-5 seconds only
Alternator Load: 165A 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 (Fwd Bus) | 0.25A | Idle |
| Radiator Fan (Fwd Bus) | 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 |
| CT4 (OUT13) | 10A | Parking lights |
| A/C Clutch (OUT17) | 5A | If summer |
| iBooster Enable (ign bus) | 5A | Always on |
| STX Intercom (OUT20) | 1A | Standby |
| DRL (OUT23) | 0A | Off - parked |
| BCDC Charger | 30A | Normal rate |
| TOTAL | 122A |
Alternator Load: 122A of 270A capacity = 45% utilization Excellent
Summary¶
| Scenario | Total Load | Alternator | Utilization | Status |
|---|---|---|---|---|
| Highway Driving | 101A | 270A | 37% | Excellent |
| Hot City Driving | 188A | 270A | 70% | Good |
| Offroad Trail | 201A | 270A | 74% | Good |
| Emergency Braking | 165A | 270A | 61% | Excellent |
| Parked Idling | 122A | 270A | 45% | Excellent |
Worst Realistic Case: 201A (offroad with hot engine) = 69A margin (74% utilization). Folding in the now-itemized TCU (~15A continuous) raises this to ~216A (54A margin) — still within capacity.
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 | 409A peak | AUX battery | Direct connection |
| BODY PDU circuits | 54A 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.
Key-Off Parasitic Budget (START Battery)¶
Loads on the CONSTANT (always-on) feed continue to draw with the ignition off. The BCDC isolates the AUX battery but does not offload the START side, so these draws come straight out of the 68 Ah Odyssey PC15001 and set how long the vehicle can sit before cranking is at risk.
| Load | Source | Key-Off Draw | Notes |
|---|---|---|---|
| PBS-I keyless module | Critical Cabin PDU (CONSTANT) | ~50 mA2 | RFID auto-arm standby (arms 60 s after fob leaves range) |
| HDX + BIM cluster electronics | Critical Cabin PDU Slot 2 (CONSTANT) | not characterized | Dakota Digital keep-alive / sleep current — small, not yet measured |
| ECM keep-alive | START battery (direct) | not characterized | Cummins R2.8 ECM sleep current |
Characterized draw: ~50 mA (PBS-I) ≈ 1.2 Ah/day.
Time to 50% SOC (34 Ah usable1) from the PBS-I alone: ~28 days. The uncharacterized cluster and ECM sleep draws shorten this, so treat ~28 days as a ceiling, not a guarantee.
Recommendation: No mitigation needed for daily/weekly use. For storage beyond ~3 weeks, use a battery maintainer or a START-side disconnect — the ~50 mA floor alone reaches 50% SOC in about a month, and the unmeasured loads only add to it.
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
-
Odyssey PC1500 capacity 68 Ah (20-hr rate), 34 Ah usable at 50% DOD — see Batteries (Odyssey Extreme Series spec table, checked 2026-05-30). ↩↩
-
PBS-I ~50 mA standby is an engineering estimate, not vendor-confirmed (Digital Guard Dawg PBS-I, Keyless Ignition). Carried into this budget rather than chasing a vendor figure: at 1.2 Ah/day it changes no wire gauge or part selection. Field-verify with a clamp meter on the RED feed at first power-up if a precise number is ever needed. ↩