Lithium-ion batteries pack an amazing punch for their size. They’re robust enough to run our laptops for hours on a single charge, at the core of the latest smartphones, and even serve as the power plant behind cutting-edge electric vehicles like the Tesla family. But lithium-ion batteries do have their limits, and given how powerful laptops have become in the last few years, we're relying on our machines for longer than ever. And that has implications for batteries: They're in service for longer than ever, too. And sometimes, that means they show the limits of the technology.

You may well have had this happen to you: A laptop or phone you’ve had for years suddenly stops working, or maybe just starts showing some sign of internal physical swelling. The screen of your phone starts bulging out, or its seams open up, or your laptop keyboard grows an unsightly bulge. That could be due to a big bubble that may have gradually, or suddenly, arisen in your battery. But why does this happen, and what can you do about it?

We talked with some engineers and experts from computer manufacturers and related companies to dig into what you need to know about swollen batteries, and what to do if you have one.

What Causes My Battery to Swell?

Swollen batteries are the result of two things: energy density and heat. The swelling is the result of too much current coursing, in a noncontrolled fashion, inside a cell of the battery, which causes a build-up of heat and gas. This happens as the materials inside a battery decay or are subjected to stress or physical damage over time.

A given laptop battery is made up of several discrete cells, and issues can arise in just one, or more than one, of them. You'll often see laptop batteries described as, for example, four-cell or six-cell, indicating the internal structure of the battery. (Some laptops, usually business models, give you a choice of battery capacity/cell count at time of purchase, but that is less common than in the past.) The fact that laptops seldom come with removable batteries anymore (in the sense of, you can swap them in and out without opening up the chassis) exacerbates the problem.

“The reason why these batteries swell is because the electrolyte that separates the layers between the wrap degrades, and when that degrades, it changes into gas,” says Arthur Shi, lead technical writer at iFixit.com. “Now that the electrolyte is degraded, things are no longer insulated and so they start shorting, and that causes a hot spot later. That can eventually cause it to do a thermal runaway, which means it can even start a fire.”

The most common cause of a swollen battery is overcharging. Keeping your battery at a high state of charge, according to Shi, can stress it out, allowing it to degrade faster.

“In an application where you have a system plugged in 24/7, after a number of years your likelihood of getting a swollen battery increases,” says Phil Jakes, principal engineer and director of strategic technology at Lenovo. “The other thing that drives it is heat. Batteries don’t like to be hot, and there’s a chemical process that gets kicked off when a battery gets over 100 degrees.”

Another common cause is mechanical damage to the battery. That might be caused by a blow to the laptop itself, or damage done when changing out it (or messing with other components around it). Striking a hard surface and denting the casing can cause a swelling condition, as can exposure to excessively high temperatures. Also, damage inflicted to the exterior of your device can transfer to the battery and cause it to overheat and swell.

Signs of a Swollen Battery

Now, the signs may be subtle, or impossible to ignore. As the battery expands, your device might slowly change shape. For example, your laptop keyboard could start protruding, or you may notice your device is suddenly wobbly when you lay it on a flat surface, with the lower part of the chassis a bit distended if you look at it on edge. Or it might be something simply as mystifying as certain keyboard keys becoming hard to push.

If your laptop is easy to open (in other words, it has simple screws on the underside of the chassis), you can take off the bottom cover and, in many cases, check out the battery's physical state that way. If it looks puffy or rounded, that likely means it's swollen. Most important: When inspecting if your battery is swollen, proceed with caution, as the cells are under pressure. Wear eye protection and don't poke or pry at the cells.

A swollen battery isn't always immediately obvious when it happens, or at the onset. It depends on how much room it has to expand inside the laptop chassis. With thin laptops, that's usually little or none. The size of swelled battery can vary from a small bump to one large enough to turn your laptop into a seesaw, or even pop out the touchpad. (Yes, we've seen that.)

How Do I Fix and Prevent a Swollen Battery?

Once a battery starts bulging, it may well continue working, at lesser efficiency. Or your laptop may simply keep working when on the AC plug, but die quickly off-plug or power down as soon as the cord is pulled. Ultimately, don't ignore it; sooner or later, the battery will no longer work properly, and once swelling starts, it cannot be reversed. Your only solution is to replace the battery.

Of course, there are ways, according to the experts we talked with, to prevent this from happening again, or in the first place:

• Don’t keep your device plugged in all the time. Batteries are cyclical and have to discharge and recharge to work effectively.

• Keep your laptop in cool, dry environments. Hot and humid weather conditions put more strain on batteries and can shorten their operating life over time.

• When buying a new battery, buy from reputable manufacturers. It's generally best to buy a replacement from the original laptop maker than the cheapest compatible option from a third party.

• Replace your battery if it becomes old or exhausted, and if the laptop makes it possible to swap it out yourself. Manufacturers test their batteries to last up to three or four years, and in a perfect world, batteries wouldn’t degrade, but they do. If you start to see the signs of a damaged, swelling, or exhausted battery (in the last case, it only holds charge for a short time), replace it now rather than later.

It’s not all doom and gloom. Manufacturers such as Dell and Lenovo have been working on smart battery technology in their laptops for years now, allowing the battery to keep track of its usage throughout the day to avoid overcharging.