Series and Parallel Circuits

Old-fashioned Christmas lights went entirely dark when one bulb burned out because the bulbs were wired in series. Modern strings stay lit because each bulb is wired in parallel: current has alternate paths, and one failure does not break the circuit. This difference governs how components share voltage and current in every electrical system.

In a series circuit, components connect end-to-end so the same current flows through each one. Voltages divide across components (V_total = V1 + V2), and resistances add (R_total = R1 + R2). One break stops all current. In a parallel circuit, components share the same voltage. Current divides among branches (I_total = I1 + I2), and total resistance decreases (1/R_total = 1/R1 + 1/R2). One branch can fail without affecting others.

Apply to solar arrays. Ten panels at 40 V each wired in series produce a 400 V string. Three such strings wired in parallel deliver 400 V at three times the current. This series-parallel topology matches inverter input requirements.

Apply to battery packs. A Tesla Model 3 uses roughly 4,000 cells arranged in series (to reach ~400 V) and parallel (to increase capacity and current). Series builds voltage; parallel builds capacity.

If one cell in a parallel group fails as an open circuit, what happens to the pack?

Parallel provides redundancy. The other cells in the parallel group continue operating. Current redistributes among surviving cells. In a series connection, one open cell would break the entire string.

Series and parallel configurations determine voltage levels, fault tolerance, and system design in solar arrays, battery packs, household wiring, and grid topology.

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