Why does battery life get worse over time?

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.

The estimated life of a phone battery is just that — an estimate.

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 aren'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 it's 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 people telling you to charge your battery before it gets low and also hear people telling you the opposite 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 amount 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 exactly the same as 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. Eventually, the changes are enough to notice. If you continue to use the battery and charge it as you normally would, you reach the point where there isn't enough electrical energy being stored to power your phone.

Charging a battery essentially changes the composition of the electrodes and affects the way it will charge in the future.

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. They're necessary because a car 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.

Voltage matters

One of the biggest factors that can influence how many cycles a lithium-ion battery will last is its voltage. Phones and cars aren't the only things designed to run on rechargeable lithium batteries, and 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. Studies found that 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). This essentially 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, but 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 further worsens the degradation of the cathode.

There is no secret cabal of battery makers who are trying to fleece us; it's all chemistry.

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.

These Android phones have the best battery life

Jerry Hildenbrand
Senior Editor — Google Ecosystem

Jerry is an amateur woodworker and struggling shade tree mechanic. There's nothing he can't take apart, but many things he can't reassemble. You'll find him writing and speaking his loud opinion on Android Central and occasionally on Twitter.

  • Did you get an electrical engineering degree for this article?
  • I think he's already an engineer of some sort.
  • Well replacement batts aren't as expensive as they once were and much cheaper than getting a new phone. I rarely keep a phone that long anyway. One of the benefits of not using flagships.
  • Problem is, most phones nowadays don't have removable batteries. In IP rated phones, changing it yourself will undo the IP certification.
  • If only flagship phones still allowed user removable batteries. RIP the Note series after the Note 4.
  • Okay then, how should i charged my phone? I usually used it until 20% then fully charged it until 100%. Should i just keep with this method or change it? Im using s8+.
  • Agreed. Didn't really see a recommendation on best practice.
  • The very best thing you could do would be to charge the battery once it reaches it's shutdown state, somewhere around 5-10% but will say close to zero on your phone (that's be design so they never go completely dead) and charge it in the freezer . Buuutttttt. Keep doing what you're doing. Trying to min/max charging cycles doesn't make enough difference to notice. Batteries go bad no matter what you do to them, and fussing over it for two years to get one or two extra weeks of life doesn't seem worth it to me.
  • So, as much as possible, I incorporate a version of the 40-80 rule (you can find it online easily). The general idea is not to let the battery go below 40% or charge above 80%. I personally changed the 40% to 30%...and no less than 25%. I charge it to 100% once a week for calibration purposes. Otherwise, it doesn't get charged above 80%. These batteries don't like the "extreme" ranges. They prefer to operate in the middle range. This is why the software slows the charging rate when the battery gets to 80%. If you notice, when the battery gets really low. It takes a little longer to get back up beyond 25 or 30%. Also, when using the phone from 100%, it discharges faster from 100% down to 80%. I have a Note 8 and like the fact the Samsung allows the user to turn off fast charging. This reduces the voltage of course and, as a result, the heat generated. The difference in charging speed is not that significant in terms of time. If I'm not using the phone while charging with the fast charge setting off, I get 1% about every minute to minute and a half. Fast charging gets 1% in less than a minute. The great thing is that I can turn fast charging back on of I REALLY need to. Once a month, I allow a deep discharge to 5% or less. I also never leave the phone charging overnight, since it'll just get to 100% and continue trickle charging unnecessarily, which is also bad for the battery. I try to keep my wife from doing it but I'd have better luck talking to a wall. The avg consumer doesn't know about these methods or doesn't care to know. I do intend on keeping my Note 8 for a good while, though something tells me I'll be tempted by the Note 9. However, my wife had a S6 for almost 2 years and was complaining about her battery going down faster toward the end of her time with it. I told he she shouldn't be surprised since she'd charged it to 100% almost everyday, leaving it in the charger overnight several times a week.
  • "electricity is magnetism" Only if the electrons are moving ;)
  • You're right. I should have said magnetism is electricity, not the other way around.
  • Your article highlights a growing problem. What concerns me more than the absolute lifetime of a battery is the usable lifetime before it drops to an hour or two of life. After a year or a couple of years the average phone now needs to be recharged during the day. Your colleagues have defended - indeed lauded - thin phones with small batteries because they support fast charging to offset their day to day battery life - yet as you say this causes more degradation of anode and cathode. I do wish you could disabuse them of the notion that fast charging is a replacement for a larger battery. Those deposits from fast charging lessen the capacity of the battery to take a charge, meaning that before many months are up, it needs to be fast charged more frequently to remain usable. A bigger starting capacity would at least prolong the usability. Replaceable batteries would be good too - even if that meant fairly cheaply done at a specialist shop, or requiring tools. Samsung treated replacement batteries as a major income stream instead of offering them cheaply enough to make third party not worth the risk. The sad thing is that phone manufacturers have invested in factories and infrastructure focused on selling us new phones instead of long-lasting phones. Their investors require to see quarterly or even monthly growth in sales and profits, and woe betide the share price if that slows. The last thing that any mainstream phone manufacturer wants to sell us is a battery that outlasts the phone. They will do so only if it moves us from other manufacturers. Hence my Huawei came with a 4000 mAh battery which is a third as big again as a Samsung. Same thing with OnePlus, and others, because they are growing market share even if it is becoming a declining new phone market.
  • What about capacitors instead of batteries?
  • Capacitors charge super fast, but their self-discharge rate is very high. Best use would ba a super capacitor to take the initial charge, then have the capacitor charge a li-ion battery to power the device. However, that would double the amount of space needed and the phone would have to be pretty thick.
  • Great article and spot on. These are the kind of article I read to see if I might have missed something. (I hadn't, but you presented it in a concise, accurate, and readable way. Thanks.)
  • That's an interesting article, but this sentence : "electricity is magnetism" sounds odd to me.
    An electrical field is NOT a magnetic field.
  • Just a note, NiCd and NiMH batteries are affected by memory and the number of charge cycles. Li-ion and Li-Pol batteries are not affected by memory, and their life span is largely determined by total voltage that flows in and out of the cell. The worst practices are: Constantly topping off and trying to keep the battery at 100%, leaving the phone plugged in all night, and always using wireless charging.
    1. Constant topping keeps the battery way above the optimum voltage of 3.7 volts. 100% is 4.23 volts on the typical Android, 4.27 volts on iPhones. If you force your battery to stay full all the time, you are shortening it's life. The solution is to just let the battery levels follow their natural discharge, and recharge when it gets low.
    2. Leaving the phone plugged in keeps voltages high, and also forces a repeating charge/discharge cycle through the night. Your battery will be better off if you charge it before bed then unplug, or better yet: charge in the morning if your schedule allows. Most people don't have time in the morning, but if you do, it helps.
    3. Wireless charging. I'm expecting some flack for this, but wireless charging produces more heat than wired charging, and batteries don't like excess heat. It's also slower, meaning more time on the charger. And for those that care from an environmental perspective, it's grossly inefficient. The most efficient wireless chargers waste 41% of the electricity they use, and the more common ones waste 60%. Wireless charging is a well liked feature, but there are costs involved. What method consistently works well? Using a power bank which disconnects after the phone reaches 100%, and skipping days that the phone does not need charging, has been very successful for me. And lately, I've been able to quick charge via the wall plug in the morning because I have to get up early to get kids ready for school, but my work start time is flexible (unless I have a meeting!). Does this make a difference? I would say the evidence from years of Palm use, and watching patterns of 13,000 users at work, tend to support it. I was careful about charging with my M8, and it paid off handsomely by leaving the original battery still strong and able to handle a full day after three years. My wife's M8 was subject to all night charging and constant top-offs, and her battery had to be replaced after a year and a half.
  • By the way, battery life on the U11 was decent, and tended to get better with use as it learned your habits. However, the January update kind of ruined battery life. HTC released a new update last week, and boy, battery life is beastly now. How would like to finish the day with 76% left? How about still being above 50% after two days? How about completing more than three days with enough juice for a fourth day? Yes; 76 hours on one charge and moderate (not light) use. Don't know what HTC did, but AC may have to change their "Best for battery life" post. The 76 hour screenshot is below... https://photos.app.goo.gl/LF2sG9Bn80YD1p0J3
  • So many grammatical errors and missing words, I wonder if an editor actually gone over this article before clicking "publish"... 😒
  • "I wonder if an editor actually gone over" Pot. Kettle.
  • I have never paid any attention, to recharging my batteries ( since 1991) have all lasted more than 2 years at which point the phone gets replaced . I have a drawer full of phones fully charged and turned off sitting there taking up space... The Pixel 2 XL battery is in a whole class by itself compared to my other phones, or I should say it's soc efficiency, I charge it in a vehicle every day and do not even pay attention to it at home. It's been down to 15% once. For me the battery issue is solved...same with my power tools including my weed trimmer, and the next car is going to be full electric.
  • Sounds like Android Oreo is doing a great job.
  • This notion that random shutdowns after 1 year of use are "inevitable and natural and unavoidable because physics" is bullsh*t tho. This false notion only exists due to the Apple media engine, that is spinning it's poor engineering (an SOC that draws too high a current (to maintain a consistent 30-40% YoY performance increase), and under-capacity batteries (so they can increase the thin-ness of their devices every year). I have been using smartphones since 2007 and have NEVER had random shutdowns in the 1-2 year period. Then all of a sudden, Apple phones start having this issue and the tech media starts telling me that "this has always happened", and that "this is just physics". WTF... you can't just rewrite a history where this HADN'T been a standard thing that happens to all phones and say that it has. You can't just give Apple a pass and explain Apple's throttling as simply being "Apple being 'misunderstood'"... And that their only offence is not being more clear that they were doing it... But they always had customers at heart in all of this. Yes, there have been phones where this has occurred, but these have been outliers (like the Nexus 6p), and have stood out for this abnormal behaviour. Now we're told that this behaviour isn't abnormal, but the norm... You know what? F U tech media. Honestly.
  • « so they can increase the thin-ness of their devices every year).«  It’s been 3 years since the iPhone didn’t go thinner, all the opposite btw. Nice try Apple hater :)
  • If there was a battery cabal, they'd be pushing for removal batteries. Usually that's why I upgrade because my battery life has shortened to a point where it's useless. If I could just replace the battery (or add more memory) I'd definitely keep my phone longer. I
  • So when reviews can’t stop asking for fast charging and put it as a must have feature, it’s actually bad for the batteries??