What happens when my phone reaches 100% battery but is still charging?

Follow up question: what happens if I have a charger plugged into the wall and switched on but it isn’t connected to anything?


  1. Depending on how the charger is made, it can claim 100% when, in fact, is not yet 100%.

    A battery charge is done in several step.

    First, you charge the battery with a limited current (constant current mode). You charge until you reach the maximum voltage, for example 4.2V. The voltage will raise.

    Second stage: Constant voltage until the current drop bellow I beleive 4% of the capacity (3000mAH then 120mA charge current). During this stage the voltage is stable but the current drop.

    Third stage is optional: float charging (not for lithium) or maintenance charge (ex: charge for a few seconds). This keep the battery full. All battery self discharge due to the chemistry and build construction…

    What happend with the battery charge indicator? Well, it may be really monitoring the battery voltage: 3.0V = 0% and 4.2V = 100%. The voltage curve when you discharge it is kinda linear during that range, so very easy to simply go proportional and work well enought!

    A proper fuel gauge (that is the proper term somehow for the battery gauge) will not go with the voltage, but the actual energy that go in and out of the battery. However this is very complex to do: the battery self discharge in an unknown way, which is also dependent of the actual state of charge, temperature and aging, and of course the individual battery characteristics. The charge and discharge efficiency is also variable with the same factors. This is also why sometime the gauge say you still have power, and first thing you know is that it shut down hard on you: it think it still have power, but the low discharge voltage has been reached. This actually reset the fuel gauge and it start back to count the energy in the cell…

    Now, laptop battery are more complicated: there is several battery in series. Each have different characteristics, so may not fully charge equally. So what the battery pack do is: discharge a bit the battery that is full, then charge all, repeat until all are full. This can take a long while, hence why some pack get stuck a long while at 99%: it is balancing the cells.

    As for the charger: First, it is NOT a charger, but a power supply. The charger is inside your phone (same with a laptop). If disconnected from the device then it just waste a bit of power, only what the internal circuitry consume, which is very little, but do add on at the end of the year. Even if it is 0.1W … 0.1*24*365 = 876Wh, which is still only a few cents. Per charger…

    As to why it is not a charger: lithium is way too sensitive and require too much precision in charging. For example, a lithium polimer is 4.1V max, and it can catch fire at 4.4V. Your usb cable with the connectors can actually lose about 0.2-0.5V, depending on the quality and current. This mean that the charger wouln’t be able to sense proprelly the battery voltage and may under or over charge it. Also, the charger need to get the battery temperature and adjust the charge voltage, or stop charging. If it do not then a fire can happend! Also, remember the end of charge? How can it could figure out what is consumed by the battery and what is by the phone itself? And also each battery have a different capacity/size, and even some different chemistry, so different current and voltage limit (my cellphone batt is 4.35V when full, higher voltage than normal). It would get just too complicated. Better to put a charger inside the phone that take care of all that proprelly and safelly.

  2. Okay I don’t think anyone talked about your second question. Since there’s no consumption of energy, I believe it’s okay to leave the charger plugged in while not charging. It would only consume power if the phone is connected.

  3. Technically, the thing you plug into the phone is not a charger. It is a 5 volt constant voltage (CV) power supply. OK, some can put out different voltages, but they are still CV supplies.

    The charging is controlled by the phone. When the phone decides that the battery is fully charged, charging terminates, and the power supply continues to provide power for the phone.

    When the power supply is plugged in to AC without a phone, it still puts out a constant 5 volts.

    1. Nothing really “puts out” voltage, and chargers don’t put out any power when nothing is connected to them.

      1. When not connected to the phone the charger maintains 5V voltage on the output cable, but because the current is 0, its circuits just slowly leak some milliwatts, glow the LED if there is one and warm up. Even then this warmup is so small you won’t even notice it on touch.

    2. So why do phones overheat when left charging for long periods of time? If the charging terminates at 100% shouldn’t the phone not overheat?

      1. If you are using it while charging it can take on more heat than it can dissipate on its own through passive means. Starts to get hot, but probably not hotter than the phone can handle.

      2. You’re right, they shouldn’t. Li-ion/LiPo cells hardly increase in temperature at all when being charged at typical rates (1 hour full charge), most of the heat comes from inefficiency in the circuit that converts the 5V from USB to the natural 3.7-4.2V range of a Li cell. Also, if you charge a cell to 4.2V then immediately disconnect, the voltage will sag back down to ~4.1V after a couple minutes, but if you charge it to 4.2V then “hold” it there for a while, it won’t sag and you get the maximum capacity of the cell. It’s called trickle charging, and is usually what phones are doing when they’re taking so long to get from 90% to 100%. Your phone also uses power from the battery when still plugged in, so the charger has to keep adding a little power to the battery to make up for the power you’re draining by using the phone to keep it topped off.

      3. A lot of people are saying that you have a faulty phone and that the circuit is bad and, although this may be true, there is a much more plausible explanation.

        Smartphones can be potentially very power hungry. A vast amount of battery performance actually comes from the software of the phone being frugal with its resources. Tasks like backing up to cloud services, checking email, running background processes of any kind, are put off until the device has a constant and infinite (in practical terms) source of power. When this happens they heat up dramatically because the processor is under constant load. This is especially the case if you have a lot of information (takes a long time to back up) and you have social media or news apps that like to check in regularly or download in the background (Facebook is terrible for this)

        Either way; your phone almost certainly has a thermal sensor in it designed to shut down the device if it becomes too warm

      4. If your phone stays hot long after it has hit 100% then it probably has a crappy charging circuit that is still burning off power while the battery it’s not being charged. Or the battery is in poor health so the charging circuit is constantly “topping off.” If you are actually using your phone while leaving it plugged in at 100% then it’s probably the latter.

      5. Sounds like a faulty phone. Perhaps it is trying to charge the battery too agressively. Or maybe there is something running on the phone that intensively uses resources when plugged in causing the CPU to run hot.

        My phone gets warm when charging, but is currently at 100% charge and plugged in to the wall and is quite cool and will be (I know from experience) for a long time if I leave it to its devices. Of course there is some heat generated, after all the phone is on, but the heat isn’t more than one would expect from a sleeping phone in one’s pocket.

          1. Your phone gets warm when fast charging because it is being given a lot more electricity which causes the phone to heat up, but it slows down once the battery gets close to being fully charged.

          2. Your phone will warm up while charging because of the current draw and internal battery resistance. That will cease once it’s topped up.

      6. If the phone is overheating, that’s because you’re A) using a phone charger that isn’t allowing charge termination and it’s damaging your phone (very unlikely), B) the act of charging a battery takes more electricity than is actually stored on the battery and the excess energy not stored in the battery is expended as heat ([Read here](http://www.explainthatstuff.com/how-lithium-ion-batteries-work.html)) or C) the phone or battery is faulty and charging poorly or not terminating charge properly.

        B is the most likely. Phones generally don’t “overheat”, they just get hot.

      7. When plugged in, the phone will start various processes. Like automatic backups, photo upload, maintenance. All of these use too much power on battery. They will heat up the processor because it’s doing so much.

        The battery will also raise a few degree, but it is not the main reason the phone is warm.

      8. Pay more attention to what *part* of the phone is getting hot. It’s possible it’s not the battery at all. The CPU and GPU can get hot too if run for a long time at close to 100% use. It’s possible there is an app on your phone that runs your phone hot while plugged in or is just running wild all the time.

    3. So does this mean if the charger is plugged in without a phone , it consumes energy? If yes how much in terms of cost approximately?

      1. If it is warm to the touch after a few hours of being plugged in without changing, you’re looking at about 15-20 cents per year to run. If it is cool to the touch, even less

      2. Yes energy will be consumed by leaving it plugged in. Typically this would be referenced as a parasitic draw. Overall power use would depend on the charger but I’d wager less than 5 mW.

      3. The charger consumes enough energy to light up its little LED if it’s got one, and a little bit because nothing is completely efficient. When a phone is plugged in, it consumes enough power to charge and power the phone on top of that, up to its maximum current. A lot of chargers are rated for 2 amps, 5 volts, so that’s 10 watts. Run it for an hour and you’ve consumed 10 watt/hours. At $0.20 per kilowatt/hour, which is close enough to what I pay and a nice round number, that charger costs $0.002 (one-fifth of one cent) per hour to run. This is enough to make the charger noticeably warm, but not hot to the touch. When not powering anything, the charger is not warm, which should tell you where I’m going with this. Small LEDs are usually designed to consume less than 20 milliamps; that’s literally one percent of the charging current and the LED probably draws less than half that. The “nothing is perfect” current is usually even less, unless the charger is broken or *really* poorly engineered. (And a cheap charger is more likely to short out and send mains voltage into your phone, so you’ll have enough on your plate with the resulting fire to worry about your electrical bill.) If we assume that it adds up to an even 1% of the charge current, it’s $0.0002 (one fiftieth of one cent) per hour to leave it plugged in.

        Incidentally, if we assume your time is worth $5/hour, the minimum wage when I was in high school, and that you take five seconds to plug or unplug the charger, you’d be spending about $0.83 every plug/unplug cycle to save less than a penny a day.

        TLDR, it’s close enough to zero that you can just assume it’s zero.

        1. Your point is correct in that the numbers are too small to worry about, but if your time is $5 per hour, then it’s $0.014 for every 10 seconds of plugging/unplugging. So both cost and benefit are negligible.

          Here’s the rub, though. I’ve spent 5 minutes thinking about this and reading the comment stream. If my time is worth $5 per hour, and I waste $0.0002 every hour that my charger is plugged in, then I’ve already wasted the equivalent of 2,083 hours of disconnected phone charger time.

          1. But you’re building up your budgeting skills thinking about it, which is well worth the time.

      4. VERY little. They’ll only burn enough juice to power the charger’s LED (if it has one), and a milliwatt or two of leakage current. So likely $0.01-0.05 a year at most.

    4. > When the power supply is plugged in to AC without a phone, it still puts out a constant 5 volts.

      Is this why it’s said that unplugging unused devices saves money on one’s electric bill?

      1. No, because the charger doesn’t actually draw any power from the outlet unless connected to something, except for a miniscule amount of leakage power.

        Unused devices are more in the lines of a TV in standby. Although nowadays new electronics have pretty low power consumption in standby mode.

    5. >
      When the power supply is plugged in to AC without a phone, it still puts out a constant 5 volts.

      In the same sense your wall socket “puts out” 120V. However the wall socket does not draw current because it is passive whereas a charger might draw a tiny amount.

      In Australia libraries loan out plug-in power meters which are fun to test on household appliances.

    6. < When the phone decides that the battery is fully charged, charging terminates

      So it’s not necessary to unplug the phone when it reaches 100% ? One of my phones displays the message “Battery fully charged. Unplug phone.” It’s an older phone, circa 2012. My new phone doesn’t display that message.

    7. Does this mean I am still being charged for electricity by just a phone charger cord being plugged into the wall when I am away from home?

  4. Also building on to OP’s question: Why does a phone get hot if left on the charger too long? Could it explode? (I have a somewhat irrational fear of my phone exploding and killing me in my sleep because I leave it charging all night next to me on a pillow lol)

    1. If you leave it plugged longer, you will notice that it cool down. A battery tend to heat up at the end of the charge. Once the charger detect a full charge then it will stop charging and the battery will cool down.

    2. No charging circuit is 100% efficient, all of them without exceptions waste a little bit of power as heat. That’s why your phone feels warm. Lithium batteries (and most others) heat up when being charged.

      There have been a few cases where phones caught fire while charging, but those were just very rare manufacturing defects. Most phones will be perfectly fine.

      1. So it’s actually the charging circuit itself heating up that you feel? Or that in addition to the core of the battery itself? I always just assuming the heat was related to some chemistry going on in the battery itself.

    3. The battery is not connected straight to the USB/lightening cable. There is a circuit called a charge controller in the middle. Lithium batteries have strange behaviours and need to be looked after properly, so the controller makes sure that they are receiving the right voltages and not trying to output more current than they can safely. So when you leave you phone charging overnight, it’ll charge up to full, then the charge controller sees the battery can’t take any more, and stops drawing current (and will occasionally top up the battery once its been idle for a little while).

      1. Do laptops nowadays have a charge controller, or should I be disconnecting them to prevent battery degradation?

        1. all laptop have a charge controller, even the early ones. However, they are designed with a small hic: they assume that you will use the battery regularly, so they do some battery maintenance that is fine if cycled regularly, but deadly if you use it as a desktop and never ever use the battery…

          1. >deadly if you use it as a desktop and never ever use the battery

            Is there any evidence to your… Speculation? (I’m not calling it a theory because theories are supported by evidence of some sort.) Because if you can’t provide evidence, It would be wise to take what you say with a grain of salt.

          2. Yes there is, and it is actually a known issue and sometime a challenge. I sadly do not have a link for that and I forgot the details, but it can result in a drastic loss in capacity…

            Someone may be able to fill me in on the details, but at 2am I’m not gonna search, so feel free to think I’m full of it if you want.

          3. Some manufactures like dell let you control the settings in the bios ( there you can set your preference for battery or grid operation )

          4. the setting you talk about is usually a “do not fully charge the battery but charge to 80% instead and claim it to be 100%” because not using the full capacity actually make it last quite longer. This is what tesla do with their battery to make them last 10 years…

          5. lithium was made with cycle in mind, or happened to be. It is fine for most applications, but not for some… And since cycling is what most do then they don’t do anything to fix it. They can also claim misuse and sell you a new battery… So really, there is no reason for the manufacturer to fix it…

          1. Very much no.

            Lithium batteries have a finite number of charge/discharge cycles. If you disconnect the laptop from the charger you immediately start discharging the battery.

            Leave it connected to the charger as much as possible, this goes for pretty much any battery powered device. When the battery is charged it won’t try to charge it any further and can run off the power from the charger, thus not discharging your battery and extending it’s life.

          2. If I have a lemming and I attempt to impress on other lemmings do I need more or less lemmings again?

          3. I’m not sure how accurate this is, although logic does indicate that it could be true. My barely 2 year old gaming laptop does not power itself purely from the charger when it hits 100%. You can see the battery level dropping to something like 95-97% before it starts charging again. This has been quite common in laptops for a very long time.

            Hovering over the windows battery icon also says it is not charging, and it only starts up again at around 95-97%

          4. This does not account for the whole story. Lithium ion batteries don’t want to be at full charge long term. It reduces the total amount of energy they hold. Instead they should be kept around 2/3rds full as much as possible.

          5. Correct, Li-Ion has a better overall shelf life if left at ~70%

            This has become problematic due to the issues around Samsungs unfortunate battery incident. Many countries now require batteries to be below 10% charge during shipping and handling as below that it is not possible to have a thermal event. This makes logistics and storage harder as the life of the battery diminishes

          6. Is it possible that the device manufacturers take this into consideration and only charge the battery to 70% of its full capacity but report 100%. Or am I giving them too much credit?

          7. Not OP but no it wasn’t, but it’s good practice to disconnect a laptop from its charger once it’s finished charging. Having it plugged in all the time can degrade overall battery life over time, and it’s actually better for the battery to be completely drained then fully recharged every once in a while.

            Edit: Got my last sentence horribly wrong – was thinking of the old nickel cadmium batteries

          8. Disconnecting it once it’s fully charged? Pull the plug out.

            To drain it, just leave it on for an extended period of time. Change the power settings to not put your display to sleep, and maybe play a game or something until it dies.

            Edit: I think I just wooshed big time.

            How is it good for the battery? The process of recharging is a reverse chemical reaction (kinda, not really but its close). For it to be at its full effectiveness it should go through the whole thing, therefore it would be able to charge ‘more’ (again, not quite how it works but its close). Because it isn’t and never will be 100% effective, if you are only trickle charging it it will eventually lose out on the space that it is constantly recharging which happens over time to lithium batteries

          9. I only know some about this topic from like physics 2 but Li-ons wear out over time from oxidation and I believe heat can expedite this process. Leaving it plugged in means it will always be topped off, I’m unsure if being left at 100% causes more wear or if the process of topping off the battery just causes more heat/wear.

          10. Both. 100% by most company standards refers to 4.2V, but this chemistry is nominally 3.7V (formerly commonly 3.6V). For practical purposes we almost always want a full charge before heading out with a portable device, but if at a desk constantly plugged in, a lengthy full state of charge (SoC) can put pressure on the cell leading to reduced capacity long term.

            Heat is bad for batteries in general, and the heat formed due to the float charge of lithium batteries being inefficient compared to their bulk charge region is no exception. Due to the safe charge currents we use typically (under 1C), this is probably not as bad as a lengthy stay at 100% SoC.

          11. > Having it plugged in all the time can degrade overall battery life over time, and it’s actually better for the battery to be completely drained then fully recharged every once in a while

            people always say this but I always thought that using the device while plugged in is always better than on battery.

            eg. imagine you use a device 8hrs a day – for 4 of those hours you have access to have it plugged in.
            Best usage pattern should be to leave it plugged in for all 4 of those hours rather than running it on battery even if it has enough.

            but do let the battery get close to zero once every so often as you describe.

            There is debate however on if you should leave the device plugged in overnight and have the inconvenience of having a phone/laptop that isnt 100% when you need it

          12. I totally agree with you, I’m currently at my desk where my laptop is plugged in and has been for the whole 7 hours I’ve been at work so far, apart from the few meetings I’ve been to. This is where convenience takes over, and having to constantly remember to unplug your devices is an absolute pain.

            I’m pretty sure battery technology has progressed far enough that what I originally commented is nowhere near as much of an issue as it once was. I do believe however having a lithium battery plugged in and charging 24/7 is bad for it… Got no proof for that though!

          13. > it’s actually better for the battery to be completely drained then fully recharged every once in a while

            No, it isn’t. Fully draining the battery is sometimes recommended to calibrate the charge controller, so that battery charge estimates are more accurate, but fully discharging a li-ion battery reduces its charge capacity. It’s up to you whether you value an accurate estimate of remaining charge more than total capacity, but fully depleting the battery is not good for it.

          14. Thanks for this! I’m nowhere near an expert on this at all, mostly personal experience over anything else. This makes far more sense than what I wrote

          15. You were thinking of the old nickel cadmium rechargeable batteries. Those should be charged, completely drained on the first use, then charged again.

            Lithium ion on the other hand won’t recharge as much if you let it run super low a lot. It’s better to keep your phone charged as much as possible.

          16. I’m sure it is, never said it wasn’t. However because neither of us have provided any proof or external source, what they have stated makes more sense than what I wrote in my opinion.

            I’m not trying to detract from it at all, and am more than happy to admit that I’m talking out of my arse and am wrong. I try not to believe everything I read on the Internet without other sources (or if I have time I’ll research myself).

            Just trying to have a discussion, and learn as much as I can along the way

          17. I thought most electronics won’t let you fully deplete the battery. Like when your phone says it’s at 1% battery and shuts off it really isn’t but it’s shutting down to protect the battery.

          18. They’ll shut down before fully depleting the battery, but that’s to protect your data, not to protect the battery. Sudden power loss can corrupt many common filesystems.

          19. > but that’s to protect your data

            Which data does a phone have to protect, that isn’t already stored on its flash memory?

          20. Is it true that charging from ~50% to ~92-95% is the most effecient? I use an app that supposedly reports that as the case, and was wondering if that’s accurate or if it is complete nonsense?

          21. Depends on what you’re referring to. If you mean the charge process, no. After the bulk charge takes you to around 70%, you’ll see a somewhat reduced charging efficiency from then on to full charge. If you meant to ask whether not letting the battery go below 50% increases the battery’s lifespan in terms of number of cycles, then yes. In general, the lesser the depth of discharge (DoD), the more total charge cycles the battery will achieve. Deep discharges on the one hand and consistently leaving your battery at 100% both lead to reduced lifespans.

        2. Disconnecting the laptop from power will caused battery degradation because of additional charge/discharge cycles. Leave it plugged in.

    4. Because it performs indexing/updating/backup activities and the processor/radio gets a little warm doing that.

    5. Probably should be less worried of your phone exploding and more worried about your pillow catching fire. Shouldnt charge your phone on fabric, move in your sleep and cover it up or have it slip under the pillow and the little heat it does generate, will build up in the fabric. Hundreds of cases of bed sheets and pillows catching fire from teens sleeping with their phone charging user their pillows.

  5. Adding to this, it’s my understanding that phone batteries are damaged by being overcharged, especially while being used. Why is this an issue and why is it that phones don’t stop charging at 100% and wait till 95% to start again?

    1. This is usually what happens. However, your phone might still show 100%. Lithium cells have a range around their nominal charge where they work fine, going outside that range might damage the cell. This range isn’t fixed and the line between working fine and damaging isn’t very clear. The charge controller might change this range depending on numerous things. But yeah, lots of engineering work goes into maximizing usable energy storage and lifetime of cells. You can be sure they’re using all the tricks possible to get there.

        1. There are battery charging setups that are designed to do this; use external power to either charge a battery or directly power a load (or both?). They might actually do this? But it would be a size & cost disadvantage. Likely not worth it for the added complexity.

          1. I’d be surprised if it were that much of a size and cost disadvantage, though I wouldn’t be surprised if Apple and others decided not to do it for planned obsolescence reasons

    2. Phones will all have charging circuits built-in between the USB power connector and the battery itself. These circuits monitor the state of the battery and control the charging current and voltages.

      A good charging circuit will not allow overcharging or over-discharge of the phone’s battery. So almost all phones WILL stop charging at 100%, and then maintain the charge as close to 100%. Exactly how that charge is maintained varies a bit. One way is to stop charging and let the phone drop to 95% (like in your example), then charge back up. The problem is if the user disconnects at the wrong time, their phone will be at only 95% charge when they expected a full charge. So the chargers are usually set to keep as close to 100% as possible with some other tricks.

    3. More likely the battery charges to 100% and then stops charging. The phone is still on and drawing power, but it can run from the charger power instead of the battery, kepping the batter at 100% until it’s needed.

      1. >Phones no longer get damaged by being over charged now that lithium ion batteries are the stabdard

        Yes they do, li-ion certainly do die by getting overcharged.

        But both the phone AND the battery has features to prevent charge state to go over 100%.

        (I’m using slightly incorrect language here to keep it simple.)

  6. When a phone gets to 100% power to the battery is cut because your phone is programmed to cut power when it gets to that point. When the battery goes back down 1 or 2%, the battery will allow a charge back in refilling it to 100%.

    For the other question, a phone would get hot due to resistance and electricity. Battery power is electricity and electricity is hot, when charging a battery there is resistance which causes heat and and electricity flow which also causes heat.

  7. While you’re at it, I have a somewhat related question. Does putting an old lithium battery in the freezer ‘revive’ it?
    I’ve seen some video on youtube where a guy wraps his laptop battery in clothe then puts it in the freezer for a few days. He claims it revitalizes the battery on a cellular level and whatnot.
    Is it true or click-bait BS? And please explain too. Thanks

    1. It’s a misconception of an idea that the colder the temperature is, the better a conductor can conduct, hence superconductivity of metals at absolute zero (0k). This obviously can’t make a noticeable change at anywhere above -200c, and also it’s not plugged in, so any change in the atomic level would probably just be the discharge of ions within the battery pack, so it might stabilize, but then again this is possible without sticking a battery in a freezer.

      ^dont take my word for granted, I may have no idea what I’m talking about, I just knew this from chemistry class…

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