Why does battery life get worse over time?
The chemistry that can provide power from metals makes those metals degrade over time and there's not much we can do about it.
I'm sure you've noticed that you see a noticeable difference in how well a phone battery holds a charge after a year or so. If you keep a phone long enough, its battery may not even have enough charge to survive a whole day. Have you ever wondered why?
Batteries: How do they work?
Electricity isn't magical. In fact, it's a pretty boring subject for most of us and we only want it to be there when we need to use it. But to understand why your phone needs charged more now than it did when you first got it, you need to know a little bit about how a battery works. Don't worry, we're going to stick with the basics here.
Electricity, like any sort of energy, isn't a thing you can create. All the things we think of as "making" electricity are really only converting one form of energy into another, and a battery uses a chemical reaction (energy) to build an electrical charge that can be metered out over time. Different materials can be used to build this charge and they will produce different results. In our phones, we use lithium-based batteries because they provide a decent level of output for a reasonable cost.
Inside a phone battery, you'll find three components that are important for what we're talking about: a negative electrode (called an anode and typically made of graphite), a positive electrode (called a cathode and made from a mix of lithium and other metals), and an electrolyte solution. The chemistry between these three things is simple at its base and is why they can be used to store energy. When you apply a charge to the electrodes (from your charger) lithium ions are positively charged and are attracted to the negative electrode. When you pull a charge away from the battery these lithium ions lose their positive charge and are no longer attracted to the negative electrode. The longer you pull the stored energy away from a charged battery, the number of lithium ions that are no longer charged increases until there just isn't enough of them left to produce any output and the battery is dead. Plugging it into a charger resets this cycle.
"Cycle" is an important word here. Because batteries are designed to store a charge it's difficult to measure their usable life as a unit of time. A battery that lasts two years for you may only last six months for someone else because they are being used differently. So that we can have an estimate of how long to expect them to last, battery longevity is measured by charging cycles. A phone battery is typically designed to last around 500 to 600 cycles, and a cycle is defined as charging a completely dead battery to 100% then draining it to zero again. Charging a battery that has 50% charge left on it, then draining it back to 50% is a partial cycle, which is why you'll hear it recommended to charge your battery before it gets low and also hear that you shouldn't as ways to game the system and stave off that 500th cycle. Of course, it doesn't work that way because the battery doesn't actually count the number of charge cycles. Five hundred is just an estimate.
But longevity can be measured in cycles because of what happens when you charge a battery and how it affects future charging cycles, the among of energy that can be stored and the potential (think the number of volts) of the stored charge.
Oxidation and efficiency hate each other
Because electric vehicles are a real thing and the batteries they use are insanely expensive, plenty of studies have been done about why lithium-ion batteries degrade during their lifetime. Thankfully, this also applies to the less-expensive (but still expensive!) batteries inside our phones, and it's because of chemical changes that happen during charging the batteries.
We know that charging a battery positively charges lithium ions which are then magnetically (electricity is magnetism) attracted to the negative electrode. As more and more charged ions are attracted, the difference of potential between the negative electrode and positive electrode increases. That's how you measure voltage — the difference of potential energy between two electrodes. Once it reaches a specific reading the battery is considered fully charged. The opposite is true while discharging a battery and the difference of potential decreases until it reaches zero because no more positively charged ions are present at the negative electrode. But that doesn't mean the negative electrode is clean and the same way it was before you started.
Electrodes oxidize. The same way water and air can cause iron to rust (which is where the word oxidization comes from), lithium, graphite and electrolyte salts will cause an electrode to oxidize. When every positively charged ion is stripped away from the anode in a battery a microscopic layer of particles is left behind and has been chemically bonded to the graphite anode. These particles are made from lithium oxide (lithium bonded with oxygen) atoms and lithium carbonate (lithium bonded with carbon) atoms, neither of which has the same chemical or electrical properties as graphite. This layer interferes with the charge/discharge cycle and both the difference of potential (voltage) and the number of charged ions that can be attracted changes, and eventually, the changes are enough to notice. If you continue to use the battery and charge it as you normally would, a point happens where there isn't enough electrical energy being stored to power your phone.
Different types of lithium-compositions, as well as different salts used in the electrolyte solution, have an effect on how much of these deposits are left behind on the electrode, but the materials that make for a cleaner cycle aren't necessarily the best because they can't provide as much stored power. We want high-capacity, low-power batteries in our phones because they are safer than high-power batteries and cost less and we want them to provide power to our phone as long as they can. An electric vehicle can use high-capacity, high-power batteries because they are protected by a solid frame and aren't as likely to be damaged. A car also needs to be able to go long distances between charges. But the cost of a replacement battery for a Tesla Model S is $12,000, too. Part of that cost comes from the expensive materials used to build a lithium-nickel-cobalt-aluminum-oxide battery as opposed to the basic lithium-cobalt batteries used in a phone that don't last nearly as many cycles before they degrade.
One of the biggest factors that can influence how many cycles a lithium-ion battery will last is its voltage. Because phones and cars aren't the only things designed to run on rechargeable lithium batteries, in 2015 the U.S. Department of Energy spent a lot of money and time to see exactly what causes problems and how to mitigate them because satellites use lithium-based batteries and solar chargers. After the composition of the battery itself, the next biggest culprit that can affect battery longevity is the charging voltage and the voltage of the held charge.
The chemistry that makes a lithium battery work naturally degrades the anode, and that's what we talked about above. But if you charge a battery with more than 3.9 volts, or store a charge with a difference of potential higher than 3.9 volts the same sort of degradation happens to the cathode (positive electrode) which cuts the longevity of a battery in half. Charging voltage and held voltage are essentially the same thing because you're exciting all the components of a battery except that charging also introduces heat, and the higher the charging voltage the hotter it will be. Heat applied when a battery is excited higher than 3.9 volts worsens the degradation of the cathode.
In other words, the voltages necessary to power a modern phone and quickly charge its battery mean it's almost impossible to "fix" things. Anyone with a battery-powered drill has seen this in action. The 12 or 14-volt batteries used in a tool don't last nearly as many cycles as the ones in our phones. They store and operate at a higher voltage, charge at a higher voltage and much hotter, and can be noticeably affected after just a few charging cycles. They use the same basic lithium-based batteries as a phone because using the sorts of materials we see in a Tesla S battery would make them more expensive, and they just don't have a very long lifetime. Thank goodness we can recycle most of the materials in them and we're not drowning in a sea of discarded Makita and Porter-Cable batteries with lithium being more expensive than gold.
The good news is that all the companies who make lithium batteries are working on making things better. Whoever can come up with the first battery that lasts significantly longer will make a lot of money from it. All we can do is charge our phones when they need to be charged, and know that there isn't some conspiracy between battery manufacturers to get us to buy new products more often.
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I'm an RHCE and Electrical Engineer who loves gadgets of all kinds. You'll find my writings across Mobile Nations and you can hit me on Twitter if you want to say hey.