What Happens When a Lithium Battery Gets Too Old?

The most obvious sign that a lithium battery is getting old is that it simply does not last as long as it used to. That laptop which once ran for six hours might now barely make it through a two-hour meeting. Your phone that used to get you through a full day now needs a top-up by early afternoon. This is not in your head—it is the battery aging.

Capacity fade is the primary way an aging lithium battery announces itself. A healthy lithium battery gradually loses its ability to hold a charge, and once its usable capacity drops below 80% of the original rating, most manufacturers consider it at the end of its useful life. You will also notice that charging takes longer than it used to. An old battery might sit on the charger for hours and still not reach a full charge, or it might show 100% but drop to 90% within minutes of unplugging.

Another telltale sign is that the device powers down unexpectedly even when the battery indicator shows remaining charge. This happens because the battery’s internal resistance has increased. The battery management system thinks there is energy left, but the battery cannot deliver it reliably under load. If your phone shuts off at 15% or your laptop dies with 20% showing, your lithium battery is telling you something.

What Old Batteries Look and Feel Like

Physical changes are often the most visible warning signs. As a lithium battery ages, it may begin to swell or bulge. This happens because chemical reactions inside the cell produce gases that cannot escape. In pouch-style batteries—common in phones, tablets, and many portable devices—this swelling can be dramatic enough to push the screen or casing apart.

Heat is another red flag. An aging lithium battery tends to run warmer during charging and discharging. If your device feels unusually hot to the touch during normal use, or if the battery area becomes uncomfortably warm while charging, this indicates increased internal resistance and ongoing degradation. A battery that gets hot enough to be uncomfortable during routine charging needs immediate attention.

You might also notice a strange smell—sometimes described as sweet or chemical—coming from the device. This can indicate that the electrolyte inside the battery is breaking down and venting. While not always an emergency, it is a sign that the battery’s internal chemistry is no longer stable.

Why Lithium Batteries Deteriorate over Time

Lithium batteries age through two parallel processes: calendar aging and cycle aging. Calendar aging happens regardless of whether you use the battery. Even sitting on a shelf, the internal chemistry slowly degrades. Cycle aging happens with each charge and discharge cycle. Both processes add up over time.

At the chemical level, several things go wrong as a battery gets older. The solid electrolyte interphase (SEI) layer—a protective film that forms on the anode during the first few charges—continues to grow thicker over time. This growth consumes active lithium and increases internal resistance. Lithium inventory is gradually lost to side reactions. Active material in the electrodes can crack, detach, or become electrically isolated. The electrolyte itself decomposes slowly.

Temperature plays a huge role in how fast a lithium battery ages. Every 10°C increase in operating temperature roughly doubles the rate of chemical degradation. This is why leaving devices in hot cars or using them while they are charging (which generates additional heat) can significantly shorten battery life. Cold temperatures do not cause permanent damage on their own but can temporarily reduce performance and make charging unsafe.

Depth of discharge also matters. Draining a lithium battery all the way to zero before recharging puts more stress on the cells than topping it off from 50%. The voltage range you operate within affects degradation pathways as well—higher voltages accelerate cathode degradation, while very low voltages accelerate anode damage.

When You Need to Replace an Aging Lithium Battery

Most consumer lithium batteries have a practical lifespan of three to five years under normal use. This does not mean they stop working on their third birthday—it means their performance has degraded enough that replacement becomes worthwhile. Some chemistries, particularly lithium iron phosphate (LFP), can last significantly longer—often 10 years or more with thousands of cycles.

You should consider replacing a lithium battery when any of the following applies: the runtime has dropped so much that it no longer meets your needs; the battery takes excessively long to charge; the device shuts down unexpectedly with charge remaining; you notice physical swelling or deformation; or the battery gets unusually hot during normal operation.

For devices with removable batteries, replacement is straightforward. For sealed devices like modern smartphones and laptops, replacement requires professional service. In either case, do not continue using a battery that shows signs of swelling, excessive heat, or leakage—these are safety hazards, not just performance issues.

If you are unsure about your battery’s health, many devices have built-in battery health indicators. On iPhones, go to Settings > Battery > Battery Health. On MacBooks, hold the Option key and click the battery icon. For Windows laptops, many manufacturers provide diagnostic tools. A reading below 80% maximum capacity generally means it is time to start shopping for a replacement.

Safety Concerns with Older Lithium Batteries

The safety question is the one most people worry about, and for good reason. Lithium batteries store a lot of energy in a small space, and when things go wrong, they can go wrong dramatically. The good news is that most aging lithium batteries simply become less useful rather than dangerous. The bad news is that the risk is not zero.

As batteries age, the risk of internal short circuits increases. Dendrites—tiny metallic lithium structures—can grow inside the cell and eventually penetrate the separator between electrodes. This can create a short circuit that generates heat, which can trigger a chain reaction called thermal runaway. In thermal runaway, the battery heats itself to the point of catching fire or exploding.

Swelling is a particular concern because it indicates gas buildup inside the cell. While not all swollen batteries are immediately dangerous, a swollen battery is a battery under internal pressure. If the casing ruptures, the contents can ignite or release toxic fumes. Do not puncture, crush, or attempt to “deflate” a swollen battery—this is extremely dangerous.

Old batteries that are no longer in use also pose a risk if stored improperly. A lithium battery left in a drawer for years can still have enough charge to cause a fire if damaged. Before disposing of an old lithium battery, discharge it as much as possible (safely—do not short-circuit it) and take it to a proper recycling facility. Never throw lithium batteries in household trash.

The practical takeaway is this: respect your aging lithium batteries, monitor them for warning signs, and replace them when performance declines to the point of frustration or when physical changes appear. A five-year-old battery that still works fine is probably okay to keep using—just keep an eye on it. A three-year-old battery that is swelling or getting hot needs to go immediately.