Batteries die because the chemical reactions that produce electricity gradually become less efficient and eventually stop. In a single-use battery, the reactive materials simply get used up. In a rechargeable battery, the story is more complex: every charge cycle causes tiny, irreversible changes to the internal structure, slowly reducing how much energy the battery can hold. The average lithium-ion battery in an electric vehicle, for example, loses about 2.3% of its original capacity per year.
How a Battery Makes Electricity
Every battery works on the same basic principle. Two different materials (the electrodes) sit in a chemical solution (the electrolyte) that allows charged particles called ions to flow between them. When you connect a device, a chemical reaction at one electrode releases electrons, which travel through your device doing useful work, then arrive at the other electrode to complete a second reaction. The battery “dies” in the short term when those reactions can no longer sustain enough voltage to power your device.
In a disposable alkaline battery, zinc on one side reacts with manganese dioxide on the other. Once those materials are consumed, the battery is spent. In a rechargeable lithium-ion battery, lithium ions shuttle back and forth between electrodes. Charging pushes them one direction; using the battery sends them back. This is reversible, but not perfectly, and that imperfection is what kills rechargeable batteries over time.
What Kills Disposable Batteries
When you drain a single-use alkaline battery, the zinc electrode slowly dissolves and reaction byproducts build up on the surfaces. These byproducts increase what engineers call internal resistance: the battery’s own opposition to delivering current. As internal resistance climbs, voltage drops. That’s why a “dead” alkaline battery often still has some chemical energy left inside but can’t push it out fast enough to be useful. A flashlight dims and then goes dark even though the battery isn’t truly empty.
Even sitting on a shelf, disposable batteries slowly lose charge. Chemical reactions happen at a low level all the time, and electrode materials degrade with age. Alkaline batteries lose roughly 2 to 3% of their charge per year in storage, which is why they can sit in a drawer for years and still work. Rechargeable nickel-metal hydride (NiMH) cells lose charge much faster. Lithium-ion cells fall somewhere in between, losing about 2 to 5% per month depending on temperature and state of charge.
The Invisible Layer That Ages Lithium-Ion Batteries
The single biggest reason rechargeable lithium-ion batteries lose capacity over time is the growth of something called the SEI layer. The first time a new lithium-ion battery charges, the electrolyte reacts with the electrode surface and forms a thin film. This film is actually useful at first because it stabilizes the electrode. But it never stops growing. Every charge cycle adds a little more to it, and each addition permanently traps lithium ions that can no longer participate in generating electricity.
This SEI growth also increases internal resistance, meaning the battery delivers less power and generates more heat during use. The process is self-reinforcing: more heat accelerates further SEI growth, which traps more lithium, which reduces capacity further. It’s a slow, steady decline that explains why your two-year-old phone doesn’t last as long on a charge as it did when it was new.
Dendrites and Physical Damage
Beyond chemical film growth, batteries can also fail through physical damage at the microscopic level. During charging, lithium doesn’t always deposit evenly on the electrode surface. Tiny metallic spikes called dendrites can form at spots where the electric field is slightly stronger. Once a spike starts growing, it attracts even more lithium to its tip, creating a feedback loop that makes it grow faster.
In mild cases, dendrites reduce capacity by pulling lithium out of the useful reaction cycle. In severe cases, a dendrite can grow long enough to pierce the thin separator between the two electrodes, creating an internal short circuit. This can cause the battery to heat rapidly and, in rare situations, catch fire. Preexisting cracks or defects in electrode materials give dendrites a foothold, and high charging currents make them more likely to branch and spread.
Why Heat Is a Battery’s Worst Enemy
Temperature has an outsized effect on how quickly a battery degrades. High heat speeds up every damaging chemical reaction inside the cell. Research has shown that the degradation rate of battery capacity roughly triples at 70°C compared to normal operating temperatures. At 100°C, one study found capacity dropped by nearly 39% in just the first two charge cycles.
You don’t need extreme lab conditions to see the effect. Leaving your phone on a car dashboard in summer, or using a laptop on a soft surface that blocks its vents, raises internal battery temperatures enough to accelerate SEI growth and electrolyte breakdown. The unstable components of that protective film start decomposing between 80 and 120°C, and at even higher temperatures (200 to 300°C), the electrolyte itself breaks down, which is a key trigger for thermal runaway in catastrophic battery failures.
Cold temperatures are less destructive but create their own problems. Lithium ions move sluggishly through cold electrolyte, which temporarily reduces available power and can promote uneven lithium deposition during charging.
How Charging Habits Affect Lifespan
The depth to which you drain a battery before recharging it has a direct impact on how many total cycles it can survive. Draining a battery to 0% before recharging (100% depth of discharge) represents the worst-case scenario for cycle life. Shallower cycles, where you recharge at 30 or 40% remaining, cause significantly less stress per cycle. This is why most phones and electric vehicles are designed to keep the battery within a middle range rather than truly filling to 100% or draining to 0%.
Fast charging also takes a measurable toll. Vehicles that regularly use high-power DC fast charging above 100 kW experience degradation rates of up to 3.0% per year, roughly double the rate of vehicles that primarily use slower AC charging, which see closer to 1.5% per year. The higher current forces lithium ions to move faster than they can be absorbed evenly, promoting the kind of uneven deposition that leads to capacity loss and dendrite risk.
Car Batteries and Sulfation
The lead-acid batteries in most gasoline cars die through a different mechanism called sulfation. During normal discharge, lead sulfate crystals form on the lead plates. Recharging converts them back. But if the battery sits in a discharged state for a long time, or repeatedly fails to fully recharge, those crystals harden and become permanent. This “hard sulfation” coats the plates with an insoluble layer that blocks the chemical reactions the battery needs to produce current.
This is why a car battery that sits unused over a long winter often won’t start the engine come spring. The sulfate crystals have grown too large and rigid to reverse through normal charging. Once hard sulfation sets in, the battery’s capacity is permanently reduced, and replacement is usually the only practical option for most drivers.
How Long Modern Batteries Actually Last
Real-world data from hundreds of thousands of electric vehicles shows that battery longevity has improved substantially. Based on a 2026 analysis by fleet analytics company Geotab, the average EV battery is projected to retain about 81.6% of its original capacity after eight years. Drivers who primarily use slower charging methods and avoid extremes of charge see even better numbers, with projected retention around 88% at the eight-year mark. Based on observed degradation rates, the average EV battery lifespan is estimated at 13 years or more before reaching the point where it can no longer serve its original purpose.
For phones and laptops, the timeline is shorter because the batteries are smaller, cycle more frequently, and face tighter thermal constraints inside compact cases. Most phone batteries are designed to retain about 80% capacity after 500 full charge cycles, which translates to roughly two to three years of typical use. After that threshold, you’ll notice shorter battery life between charges, slower performance in some devices, and occasionally unexpected shutdowns when the battery can’t deliver enough current to match demand.