You’ve probably had it happen to you: your laptop, whose battery used to last for an entire plane flight, now runs down in the time it takes to watch a clip on YouTube. Then one day, your laptop stops running entirely unless plugged directly into an outlet. Why do your batteries keep dying?  And what can you do to make them last longer?

The mechanisms that reduce the amount of time your battery lasts after a single charge are different than those that cause it to fail entirely, so we will address these mechanisms separately.  Furthermore, the length of any given discharge depends in part on how you use the battery. Laptop users may notice that their batteries drain quickly if they run many programs simultaneously, or if their screen is set to full brightness.  In these cases, the computer is using a lot of power, so it is draining energy from the battery at a high rate.

Reducing screen brightness and closing background programs will make the computer last longer. Similarly, scanning through photos on a camera, or using apps on your phone, will run down a battery faster than if it is in standby.  The fewer programs you can use on any device, the longer it will last on an individual charge.

Batteries also never seem to last as long when it is cold out.  You may have pulled your camera out to snap a picture on a ski trip to see that the battery light is flashing red, even though it seemed just fine in the lodge that morning.

Two factors are at work here. The first is that when it is cold, the chemical reactions that govern energy storage in a battery require more energy.  When it is hot out, it takes less additional energy for these reactions to take place, so the battery lasts for longer.  (Note: being hot isn’t actually good for a battery. We’ll get to this later.)  The other problem is that your camera estimates how much charge is left in a battery based on the voltage of the cell. When it’s cold, the battery voltage drops and your camera thinks that it has discharged more than it actually has.   Next time you find yourself in cold weather with an unexpectedly discharged battery, try taking it out of your camera and heating it in your hand.  When you put it back in, it may have recovered somewhat.

No matter how carefully you use your laptop or camera, however, you will eventually notice that the battery doesn’t last as long as when you first bought it.  This phenomena is called “capacity fade.”

The actual mechanisms that cause your battery to degrade depend both on the battery chemistry and how it is used.  A lot of capacity fade is the result of the fact that the “reversible” chemical reactions in the battery are not always completely reversible. Irreversible side reactions build up over time.  These reactions reduce the surface area of the electrode, so the amount of electrode material available to store energy decreases with each cycle.

In the lead-acid batteries used to start most cars, for example, lead sulfate builds up on discharge and is typically broken down during charge.  Over time, however, the lead sulfate can form crystals that are hard to break down and block the surface of the electrode, reducing the active area; this “sulfation” is even worse if the battery is not fully charged on a regular basis.

Nickel-cadmium batteries, found in some portable electronics, also can suffer from a buildup crystals if they are not fully discharged on a regular basis. The lithium-ion batteries in your laptop or iPhone can suffer from side reactions, corrosion, electrolyte degradation, and other chemical and mechanical aging mechanisms common

Complete battery failure is usually a result of either battery mismanagement (i.e. running your battery down too low) or mechanical failure.  During charge and discharge, the movement of ions in and out of the electrodes increases the stress in the battery and can lead to mechanical failure.  This stress is greatest when the battery is fully charged, so most battery types should not be stored at full charge (the exception being lead-acid, which suffer from sulfation if it is left undercharged).

In other cases, the crystal growth mentioned above can build up into branch-like structures called dendrites.  A polymer separator typically keeps the electrodes in the battery from touching, but the dendrites are sharp and can pierce through this separator, connecting the electrodes and creating a short, rendering the battery useless.

While capacity fade in a battery is inevitable, you can reduce the speed at which it occurs by following a few of the following guidelines:

• Don’t keep your battery at high temperature.  Avoid heaters and hot cars.  High temperatures speed up the side reactions that damage the battery.

• If you leave a lithium-ion battery unused for a long period of time (say, a month), leave it only partially charged. Battery electrodes experience the most mechanical stress when the cell is fully charged, leading to failure. Furthermore, unwanted side reactions are more likely to occur near the battery’s maximum voltage, and can be slowed by maintaining the battery in a lower state of charge.  It is not good to leave the battery fully discharged either.

• Don’t fully discharge your battery on a regular basis. This guideline is a funny one.  First of all, most laptops and cameras will go into standby rather than let you discharge your battery entirely.  Also, it is important to occasionally discharge your laptop battery so that the laptop’s control system can recalibrate and improve its estimate of the battery’s state of charge.  Some types of batteries can also withstand “deep discharge” better than others.  Lithium-ion batteries, for example, are designed to be deep-discharged regularly, whereas many types of lead-acid batteries degrade very quickly when discharged all the way.  As a rule of thumb, however, the more often you run your battery down very low, the faster it will degrade.

• Do not overcharge or overdischarge your battery.  Your battery is designed to operate within a certain voltage range where desirable reactions are favored and undesirable reactions (like hydrogen evolution in an aqueous electrolyte) are minimized.  Overcharging and overdischarging your battery can increase the rate of irreversible side reactions. Most chargers protect against overcharge and overdischarge.  (Note: Some chargers for nickel-based batteries, rely only on the charging time to determine when the battery is fully charged, so they may overcharge the battery if it is not fully discharged first.  The reason for this charging technique is that unlike other battery chemistries, nickel-based batteries do not always reach the same voltage when they are fully charged.  If the charger cannot detect the subtle changes that indicate full charge, like the small dip in a voltage seen when the battery is finished charging, it may rely on charging time to determine its cut-off.  If the battery is not fully discharged and is placed on such a charger, it will overcharge the battery).

•  Charge and discharge your battery slowly when possible. If you’re running an energy-intensive computer program, it’s probably a good idea to run the computer from an outlet if you can. This is because demanding lots of power from a battery degrades it faster. If your battery has a “fast-charge” option, only use it when you really need to, and the rest of the time let it charge slowly.

• Don’t cycle your battery unnecessarily. The life of your battery is cycle dependent.  The more you cycle your battery, the sooner it will die. If you can plug your laptop into the wall while using it, do so, although never cycling the battery isn’t good for it either.  As for the oft-asked question of whether you should leave your computer or phone plugged in when you’re not using it: the answer depends on the type of control system governing the battery charger.  Sorry we can’t just give you the answer. Batteries are complicated.