Direct current, batteries and circuit protection

Connect voltage, current, resistance and heat

Use voltage, current and resistance to calculate electrical demand, contact voltage drop and unwanted heating.

Mechanism and relationships

Voltage is electrical potential difference, current is the rate of charge flow and resistance relates them for an ohmic component: V = IR. Electrical power is VI; in a resistor it is also I²R. A series path carries the same current through its elements; parallel branches share the same node-to-node voltage while their currents add. A poor connection can become a significant local heater because current flows through unwanted contact resistance.

Reason through the evidence

A fuse or circuit breaker protects wiring and components against specified overcurrent behaviour. It is not a general detector of every dangerous fault and need not trip for a hot high-resistance connection with normal total current. Repeatedly resetting a tripped breaker can re-energise a fault; the actual reset policy belongs to the aircraft procedure. A voltage indication across an unloaded battery does not establish its capacity to supply a large starter current under load.

Apply the explanation to a different case

A parallel branch supplied by 28 V and carrying 7 A consumes 196 W. Adding another identical branch doubles source current while each healthy branch still has 28 V in an ideal network. Placing the same two resistive loads in series would create a different current and divide the voltage between them. This is why an electrical drawing must identify nodes rather than rely on the visual proximity of symbols. A short circuit bypasses intended resistance and can create large current; a poor high-resistance connection is a different fault that can heat locally without producing the same current signature.

Keep this distinction clear

A diode conducts preferentially in one direction within its ratings and can support rectification or isolation. It is not a perfect mechanical switch: voltage drop, heating and failure modes remain. A logic or protection design must account for those properties rather than assume ideal one-way flow indefinitely.

Treat battery endurance as an energy problem

Estimate endurance from a stated usable battery reserve and load, then identify the real conditions omitted by that estimate.

Mechanism and relationships

A battery converts stored chemical energy into electrical energy. Capacity is often expressed in ampere-hours under defined test conditions; usable capacity depends on temperature, discharge rate, age, state of charge and required terminal voltage. Lead-acid and nickel-cadmium batteries have different characteristics and servicing needs. Some battery faults can cause severe heating or thermal runaway; normal bus voltage alone is not a complete battery-health assessment.

Reason through the evidence

An ideal endurance estimate divides available ampere-hours by current, or watt-hours by power. It is an estimate only if the capacity genuinely represents the discharge condition. Load shedding reduces current and can preserve supply to essential equipment. An alternator or generator running indication and battery-current direction help distinguish generation from battery discharge. The lab deliberately shows an ideal upper-level budget; it cannot determine the actual reserve guaranteed by certification or an aircraft checklist.

Apply the explanation to a different case

Take a battery whose stated usable reserve for this exercise is 30 Ah. Supplying 15 A gives an ideal two hours; adding a second 15 A load reduces that budget to one hour. The second result is still conditional on the same usable capacity being valid at the new rate. Real batteries can deliver less usable capacity in cold or high-rate conditions, and equipment may stop working before chemical energy is exhausted because terminal voltage is too low. The useful lesson is to identify the load and applicable capacity rather than treating a nameplate Ah value as guaranteed flight endurance.

Keep this distinction clear

A generator-current indication may show supplied load while a battery-current indication shows charging or discharging, according to each instrument’s reference direction. The sign convention must be identified first. A zero reading has different implications depending on what path and direction the instrument actually measures.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.