Many phones race from a low battery percentage to around the middle of the scale, then seem to take much longer to reach 100%. That change is deliberate. At first, the charging system can allow a relatively large current into the battery. A larger current transfers charge more quickly and can provide a high charging power. As the battery approaches full charge, its control system reduces the current. This limits heating and reduces chemical stress inside the lithium-ion cells. So charging speed is not constant. Engineers control current, potential difference and temperature to balance speed, safety and battery life.

Designers have to balance performance, safety and wasted energy. That is why phones manage battery temperature, chargers regulate current, power networks use particular voltages, and appliances carry power ratings. Looking at the label on a real device turns the idea into something measurable rather than abstract.

In a working circuit, current is the rate at which electric charge flows, while potential difference tells us how much energy is transferred per unit charge. Resistance controls how much current flows for a given potential difference. Real components are chosen so the circuit supplies the required current and power without wasting too much energy as heating.