Automotive Electrical and Charging Systems flashcards

$V=I\times R$: V in volts, I in amperes, R in ohms ($\Omega$). Rearranged, $I=\frac{V}{R}$ and $R=\frac{V}{I}$. Example: 12 V across an 8 $\Omega$ heater gives 1.5 A.

$P=V\times I=I^{2}R=\frac{V^2}{R}$, P in watts. A 12 V fan drawing 5 A uses 60 W; a 24 W accessory at 12 V draws $I=\frac{P}{V}=2$ A.

Series: $R_T=R_1+R_2+\dots$, one current, drops add to the source voltage, one open stops all. Parallel: each branch gets full voltage, branch currents add, $\frac{1}{R_T}=\frac{1}{R_1}+\frac{1}{R_2}$, so $R_T$ is below the smallest branch.

Runtime $t=\frac{C}{I}$ (hours, Ah, A); energy $E=V\times C$ in watt-hours. A 40 Ah battery at 2 A lasts 20 h; 12 V times 60 Ah is 720 Wh. Real usable capacity is lower.

Amp-hours (Ah): charge delivered slowly, often over a 20-hour test. Cold-cranking amps (CCA): current a new battery delivers for 30 seconds at 0 °F while staying at or above 7.2 V. Neither replaces the other.

The starter is a DC motor: battery electricity in, rotation out, only while starting. The alternator is a belt-driven generator: rotation in, electricity out (AC induced in the stator, rectified to DC by diodes) whenever the engine runs.

A fuse protects: its metal strip melts open when current exceeds its rating, guarding the wire. A relay switches: a small control current in its coil closes heavy contacts to run a big load. Never replace a fuse with a higher rating.

No. Resting voltage mostly shows state of charge, not state of health. A worn battery can read full yet collapse under cranking load; a load or conductance test judges health. Let surface charge settle before reading.