A dying flashlight doesn't switch off — it dims. A phone at one percent doesn't cut out instantly — it slows, stutters, and fades. That gradual decline isn't a quirk of the electronics; it's a direct consequence of chemistry. A battery doesn't actually have one fixed voltage. Its real output shifts continuously as the reaction inside it proceeds, described precisely by a relationship called the Nernst equation.
A fixed number that isn't actually fixed
Every battery chemistry has a standard electrode potential — a "textbook" voltage, measured under specific reference conditions of concentration, temperature, and pressure. That number is what gets printed on a spec sheet, and it's a genuinely fixed property of the chemistry involved. But a real, working battery isn't sitting at those standard reference conditions for long. As the reaction runs, reactants get consumed and products build up, and the actual concentrations inside the cell drift steadily away from the standard state the textbook number assumes.
Why voltage droops instead of dropping off a cliff
The Nernst equation is what adjusts the battery's real, instantaneous voltage for that drift, based on the current balance of reactants to products at any given moment. As a battery discharges, reactant concentration falls and product concentration rises, and the equation translates that shift into a smooth, continuous decline in actual voltage output — not a sudden switch-off once some threshold is crossed. That's exactly the dimming, slowing behaviour familiar from any device running on a fading battery: the chemistry doesn't hit a wall, it just keeps recalculating a slightly lower number.
What we're still unsure about
Modelling a real battery's voltage precisely with the Nernst equation gets complicated fast once you leave the idealised case. Real cells have internal resistance, temperature sensitivity, unwanted side reactions, and concentration gradients that vary across the physical geometry of the cell — none of which the basic equation captures on its own. That's why battery engineers lean on empirically measured discharge curves and more elaborate models layered on top of the Nernst relationship, rather than the equation alone, and predicting precisely how a given battery chemistry will age and degrade under real-world charging and discharging patterns remains an active area of applied electrochemistry research, not a fully solved calculation.
This sits inside Electrochemistry & the Nernst Equation, one of seven topics in Physical Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.