System
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Normally the alternator supplies every load on the bus and trickle-charges the battery at the same time, the battery is a reserve, not the primary source. When the alternator fails, the battery becomes the only source, and it only has so many amp-hours to give.
This is a teaching simplification: one fixed battery capacity, no dual-alternator or dual-battery systems, no circuit breakers modelled individually. Always follow your own aircraft's electrical emergency checklist.
Controls
Quick check
Alternator down, battery counting
A typical light aircraft battery holds maybe 15–25 amp-hours, enough for a limited period of flight under load, not the whole trip. Its job is starting the engine and covering the few seconds until the alternator comes online, plus a defined emergency reserve. Once the alternator is gone, that reserve is all you have left.
Alternator vs battery: who powers what
In normal operation the alternator supplies every electrical load and simultaneously recharges the battery, so the ammeter reads a small positive (charge) value. The battery only steps in during engine start and briefly if load spikes beyond what the alternator can supply. It is a reserve, sized for a limited emergency, not a second power source you fly on routinely.
Recognizing the failure
The ammeter is the first tell: instead of a small charge, it swings to a steady discharge, because the battery is now supplying the entire bus alone. Many aircraft also have a low-voltage or ALT warning light. Bus voltage itself sags from the normal ~14V (28V) charging value toward the battery's own resting voltage, roughly 12V (24V).
Load shedding: buy back endurance
Turn off the highest-draw, least essential items first, typically the landing light, taxi light and pitot heat if you're clear of ice. Keep only what you actually need: one working radio, a transponder if required, minimum lighting if it's night or IMC. Every amp you shed extends the amp-hours you have left, directly and linearly.
Decision making, not just switches
Declare the situation early, while you still have a working radio, don't wait until the battery is nearly flat to ask for help. Land at the nearest suitable airport rather than continuing to a preferred one, especially at night or in IMC where losing electrical instruments and lighting is far more serious than in day VMC.