Battery degradation is the gradual, irreversible loss of a battery’s ability to store energy and deliver power over time. Every lithium-ion battery, whether it’s in your phone, laptop, or electric vehicle, slowly loses performance from the day it’s manufactured. This happens through chemical reactions inside the cell that can’t be undone, and the rate depends on how you use, charge, and store the battery.
How Batteries Lose Capacity Over Time
A lithium-ion battery works by shuttling lithium ions back and forth between two electrodes. Over time, some of those ions get trapped in side reactions and can never participate in charging or discharging again. This is called loss of lithium inventory, and it’s the single biggest driver of capacity fade.
The main culprit is a thin film that forms on the surface of the negative electrode, known as the SEI layer. This film actually helps the battery function normally, but it keeps growing thicker with every charge cycle and during storage. As it grows, it consumes lithium ions and electrolyte (the liquid that carries ions between electrodes), permanently locking them away. The result: your battery holds less energy than it did when it was new.
Degradation shows up in two ways. The first is capacity fade, a straightforward reduction in how much energy the battery can store. A phone battery rated at 4,000 milliamp-hours might only hold 3,200 after a couple of years. The second is power fade, which is an increase in the battery’s internal resistance. Higher resistance means the battery struggles to deliver large bursts of current, leading to sluggish performance, voltage drops under load, and longer charge times. Most aging batteries experience both simultaneously.
Calendar Aging vs. Cycle Aging
Your battery degrades even when you’re not using it. This is called calendar aging, and it’s driven by three factors: how full the battery is (its state of charge), the temperature it’s stored at, and how long it sits. A fully charged battery stored in a hot environment degrades significantly faster than one kept at a moderate charge in a cool place. The SEI layer continues to grow during storage, and the electrolyte slowly breaks down through side reactions that don’t require any current flow at all.
Cycle aging, on the other hand, is the wear that comes from actually charging and discharging. Every cycle puts mechanical and chemical stress on the electrodes as lithium ions push in and out. The deeper you cycle the battery (going from nearly empty to completely full), the faster it wears. Studies consistently show that high depth-of-discharge values accelerate capacity loss regardless of other conditions. Charging and discharging at high rates also increases stress, generating more heat and driving faster degradation.
Temperature Is the Biggest Accelerator
Heat is the enemy of battery longevity. Research has found that the degradation rate roughly triples when operating temperature rises to 70°C (158°F) compared to moderate conditions. At 100°C, one study observed nearly 39% capacity loss in just two charge/discharge cycles. While those are extreme lab conditions, the principle scales down to everyday use: a phone left on a car dashboard in summer, or an EV battery without active thermal management, will age noticeably faster.
Cold temperatures create a different problem. Below about 0°C (32°F), lithium ions struggle to insert themselves into the negative electrode during charging. Instead, metallic lithium deposits directly onto the electrode surface, a process called lithium plating. This plated lithium is essentially lost from the system, causing rapid capacity fade. Worse, the deposits can grow into needle-like structures called dendrites.
Fast Charging and Lithium Plating
Fast charging pushes ions into the negative electrode at a high rate. When the charging speed exceeds what the electrode can absorb (generally above 1C, meaning a full charge in under an hour), lithium plating becomes increasingly likely, especially in cold conditions. The plated metal becomes a site for even more deposition, creating a snowball effect.
The plated lithium also reacts with the electrolyte, forming additional SEI and producing “dead lithium,” deposits that become electrically disconnected from the electrode and can never be recovered. This is why frequent fast charging tends to shorten battery life more than slower, gentler charging. Many devices and EVs now use software to limit fast charging speed when the battery is cold or nearly full, specifically to reduce plating risk.
When a Battery Is Considered “Degraded”
In consumer electronics, manufacturers typically rate batteries to retain 80% of their original capacity after a set number of charge cycles. Google’s Pixel phones (from the Pixel 3 onward) are designed to hold 80% capacity after about 800 cycles, with newer models like the Pixel 8a targeting 1,000 cycles. Apple’s iPhones follow a similar pattern: iPhone 14 and earlier models are rated for 80% at 500 cycles, while iPhone 15 and later models are rated for 80% at 1,000 cycles.
For electric vehicles, the industry has long used 70 to 80% remaining capacity as the end-of-life threshold, the point where the battery is considered no longer suitable for automotive use. However, research from the Journal of Power Sources found that EV batteries continue to meet the daily driving needs of most people well beyond that threshold. The 70 to 80% mark is more of a conservative benchmark than a hard cutoff, and many drivers won’t notice a meaningful reduction in their daily range until capacity drops further.
How Your Device Tracks Battery Health
Modern devices use a battery management system (BMS) to estimate state of health, usually expressed as a percentage of original capacity. The BMS tracks indicators like total charge throughput (how much energy has flowed through the battery over its life), internal resistance changes, charging speed patterns, and temperature behavior during use.
On phones, you can typically check battery health in settings. iPhones display “Maximum Capacity” as a percentage. Android devices vary by manufacturer, but many now surface cycle count or health data. EVs provide this information through the dashboard or companion app, often showing remaining capacity or estimated range relative to the original specification.
Safety Risks From Severe Degradation
Most battery degradation is a slow, predictable decline in performance. But in severe cases, it can become a safety concern. Lithium dendrites, the needle-like structures formed during plating, can grow long enough to pierce the thin separator between the electrodes. This creates an internal short circuit, allowing current to flow uncontrollably through the battery.
An internal short circuit generates heat through resistance. If that heat builds faster than the battery can dissipate it, it triggers a chain of runaway chemical reactions: the electrolyte decomposes, gases build up, and the battery can vent, catch fire, or in extreme cases explode. This sequence is called thermal runaway. It’s rare in properly managed batteries, but it’s the reason manufacturers build in multiple layers of protection, including temperature sensors, current limiters, and shutdown separators that block ion flow if the cell overheats.
Practical Ways to Slow Degradation
You can’t stop degradation entirely, but you can significantly slow it. Keeping your battery between about 20% and 80% charge for daily use reduces the stress of deep cycling. Many phones and EVs now offer a charge limit setting specifically for this purpose. Avoiding prolonged exposure to heat matters more than almost anything else, so don’t leave devices in direct sunlight or hot cars, and if your EV has a preconditioning feature, use it in both hot and cold weather.
Relying on standard charging instead of fast charging when you’re not in a hurry reduces plating risk and heat buildup. If you’re storing a device or vehicle for an extended period, a charge level around 50% in a cool environment minimizes calendar aging. These habits won’t make your battery last forever, but they can meaningfully extend the years before you notice a real drop in performance.