Alternator Charging Load Balance
Whether the alternator keeps up with the electrical load at idle and cruise.
Example
You enter
- Total continuous electrical load (A) 65
- Alternator rated output (A) 120
- Output fraction at idle (0-1) 0.5
- Output fraction at cruise (0-1) 0.9
You get
- Idle balance a -5
- Cruise balance a 43
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Scopeit makes only ~50% of its rated output at idle and ~90% at cruise, so balance = output - total continuous load. A 65 A load on a 120 A alternator runs a 5 A deficit at idle (battery drains at stoplights) but a healthy +43 A at cruise; a 160 A alternator turns the idle balance to +15 A. A screening aid; the alternator's actual output curve and the real duty cycle govern.
Formula or tableidle_out = alternator x idle_frac; cruise_out = alternator x cruise_frac; balance = output - total_load; idle_frac ~0.5, cruise_frac ~0.9.
Source and editionAutomotive/marine alternator charging-balance practice (alternator output-vs-RPM curves) by name; first-principles load balance.
Free accessThe output-fraction rule of thumb and the load-balance arithmetic are standard published automotive-electrical results.
What governsEstimate. AHJ and licensed professional govern.
API field namestotal_load_a, alternator_a, idle_frac, cruise_frac, idle_balance_a, cruise_balance_a
- Output fractions ~50% of rated output at idle, ~90% at cruise (rotor-speed dependent)alternator output curves
- Load balance balance = output - total continuous load; negative at idle drains the batteryautomotive-electrical practice
- Curve governs the alternator's actual output-vs-RPM curve and the real duty cycle governscope of this tile