Electrical Power Budget

Canonical Definition

An electrical power budget is a documented comparison of source capability, stored energy, load demand, conversion efficiency, voltage, operating duration, engine speed, temperature, and reserve margin for each relevant vehicle and audio operating mode.

The Four Time Scales

Time scale	Typical question	Primary constraints
instantaneous	What happens at waveform peaks?	amplifier rails, capacitor ESR, local wiring, battery impedance
short burst	Can the bus support a bass hit or burp?	battery power, capacitor energy window, cable drop, protection
average use	Does voltage and state of charge remain stable over songs or demos?	alternator hot output, vehicle load, battery recharge, amplifier efficiency
thermal continuous	Can every component survive the sustained condition?	alternator, conductor, fuse holder, battery, amplifier, enclosure and cooling temperature

Core Calculations

Amplifier input power ≈ audio output power / efficiency
Current ≈ input power / bus voltage
Current margin = generation - vehicle load - audio load - recharge load

These are planning equations, not substitutes for measurement. Amplifier efficiency, bus voltage, program crest factor, limiter behavior, load impedance, and temperature change the result.

Define Operating Modes

engine off tuning or demo
cold start and crank
hot idle with cooling fans and HVAC
normal driving with realistic accessories
sustained music at the user's maximum level
test tone, competition burp, or endurance test
recovery and battery recharge after the event

A system can pass one mode and fail another. Hot-idle generation does not determine engine-off runtime, and a strong battery burst does not prove the alternator can recover it repeatedly.

Battery Current Can Reverse

With the engine running, the alternator and battery share the bus according to voltage, impedance, regulation, and load. The battery charges when current flows into it and discharges when the system demand exceeds available generation or the battery becomes the stronger source. Measure current direction instead of assigning every engine-running amp to the alternator.

Reserve Margin

Reserve is not wasted capacity. It allows for hot derating, accessories, battery recharge, aging, measurement uncertainty, future changes, and transient recovery. A design that balances only at one cold test point can spend the rest of its life cycling the battery and operating the alternator at its limit.

Verification Record

source and amplifier operating state
bus and amplifier-terminal voltage
alternator, battery, and load current with direction
engine and alternator speed
vehicle accessories and charging-control state
battery state of charge before and after
alternator, cable, fuse-holder, battery, and amplifier temperature
first limiting or protection condition