How it works
A battery converts stored chemical energy into electrical energy, providing the electromotive force (voltage) that drives current through a circuit. Inside the battery, a chemical reaction between an anode, a cathode, and an electrolyte produces a steady DC voltage.
In circuit diagrams, batteries are often modeled as ideal voltage sources. However, real batteries have internal resistance, which causes their terminal voltage to drop when supplying heavy currents. The larger the current draw, the more pronounced this voltage sag becomes.
Connecting batteries in series increases the total voltage (e.g., two 1.5V AA batteries yield 3.0V), while connecting them in parallel increases the total current capacity (Ah or mAh) without changing the voltage.