A battery is an electrochemical device that converts stored chemical energy into electrical energy through spontaneous chemical reactions. When a battery runs out, it is either because the chemical reactants have been completely consumed or because the internal structure facilitating the reactions has degraded. Understanding these two failure modes explains why batteries ultimately cease to function.
The Fundamental Chemical Limit
The operation of a primary (single-use) cell relies on a reduction-oxidation (redox) reaction. The battery consists of an anode and a cathode separated by an electrolyte containing mobile ions. During discharge, the anode oxidizes, releasing electrons into the external circuit. These electrons flow to the cathode, where a reduction reaction consumes them.
To maintain electrical neutrality and complete the circuit, positively charged ions move through the electrolyte toward the cathode. This coordinated movement of electrons and ions generates electricity.
In primary batteries, the chemical reactions are non-reversible. The electrode materials are irreversibly converted into new compounds, similar to a fuel tank running empty. Once the chemical fuel is converted, the reaction stops.
When the active material at either electrode is fully consumed, the chemical potential difference collapses. Electron flow ceases, and the battery is considered dead. The finite energy capacity of a primary cell is determined by the initial quantity of reactants sealed inside.
Why Rechargeable Batteries Degrade
Rechargeable (secondary) batteries, such as lithium-ion cells, operate on a reversible reaction, allowing them to be refueled with electricity. They fail not due to permanent fuel consumption, but because the physical and chemical integrity of the internal components degrades over time and with repeated cycling.
Solid Electrolyte Interphase (SEI) Growth
A significant cause of capacity loss is the growth of the SEI layer, which forms naturally on the anode surface when the electrolyte decomposes. While a stable SEI protects the electrode, repeated cycling causes the layer to thicken and reform. This continuous growth traps and consumes active lithium ions, permanently removing them from the pool available for energy storage. As lithium becomes bound in this inactive layer, the battery’s capacity steadily fades.
Mechanical Stress and Erosion
Mechanical stress contributes to degradation through electrode erosion. The constant insertion and extraction of lithium ions causes the active material particles to swell and contract. Over many cycles, this volumetric change leads to particle fracturing and a loss of electrical contact.
Dendrite Formation
Another failure mechanism is the formation of dendrites, which are needle-like metallic lithium deposits that grow on the anode during charging. Dendrite growth is accelerated by fast charging or low temperatures. These spikes can puncture the separator, the physical barrier between the anode and cathode. A puncture creates an internal short circuit, which can cause rapid discharge, permanent cell damage, or thermal runaway.
How Batteries Lose Power Without Being Used
Even when idle, a battery experiences a passive energy loss known as self-discharge. This occurs due to slow, non-current-producing side reactions between the internal chemical components. These unwanted processes gradually convert stored energy into heat, leading to a steady drain on the charge.
The rate of self-discharge varies by chemistry; for example, lithium-ion batteries typically lose 3 to 5 percent of their charge per month at room temperature. This leakage is inevitable because the internal components are in a thermodynamically unstable state and naturally react to reach a lower energy state.
Temperature controls the speed of these internal chemical reactions. Storing a battery at high temperatures significantly accelerates self-discharge and general degradation. Conversely, a cool environment slows down these parasitic side reactions, helping to preserve energy and extend the battery’s shelf life.