The batteries in our cell phones and laptops use chemistry to create power. By design, ions flow between an anode and a cathode.
When you charge your battery, ions are forced from the cathode into the anode. When you use your battery that flow is reversed.
Over time, heat, voltage, and the number of times you recharge your battery wears out the cathode. This wear accumulates at different rates for different users because the temperature of the battery, how deeply it is drained, and how it is recharged all factors differently into the life of an imperfectly manufactured product.
Even the highest-quality batteries will normally lose 25% of their capacity after being drained and charged 1000 times. Every time your battery is recharged, it loses a small amount of its power storage capacity.
Some people argue they leave their computer plugged in all the time so their battery will never degrade. Unfortunately, they are wrong.
A battery that is trickle charged or even removed from the device and stored will lose about 20% of its capacity annually. Again, ambient temperature is an important factor in how fast batteries degrade – heat is bad.
Additionally, larger, heavier batteries tend to live longer than the new, thinner pancake-style batteries that come in most laptops today. That is why your old laptop still fires up on battery after 8 years but your laptop from 24 months ago can have a bad battery. The technology is lighter but less robust.
No matter how you drain, charge, or store your rechargeable battery, it will not live forever.
The biggest sign of failure is reduced battery life when you are not plugged into a charger. However, there are other more worrisome issues when batteries fail.
Older large-cell batteries can leak when they fail to cause unpredictable voltage spikes that can damage your device. They can even explode or catch on fire.
Newer pancake-style batteries tend to swell as they fail. Because devices are getting thinner and thinner, there isn’t a lot of room to accommodate a swelling battery. You might notice you can’t click the physical buttons on your touch-pad or your keyboard surface might bow upward.
This swelling can cause physical damage to the internal components (i.e. the separator) that can cause a fire or explosion. Your battery is designed to hold a lot of energy and release it slowly.