# eli5 Why does the phone seem to lose battery faster when it’s below 20%?

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Is it a psychological thing like how time seems to move faster when you’re late? Or is my phone lying to me?

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It very much is lying to you, yes.

Phone battery display is an estimation, and its accuracy depends on the state and age of the battery. The phone knows roughly whether the battery is full, half-empty or empty, but knowing that it’s exactly 19% or 20% is a good deal harder. You sometimes see a recommendation to change the phone fully and then let it discharge — this is not for the battery’s wellbeing. It allows the phone to gather better stats on the battery’s health and calculate the percentage better, because if the phone has spent 2 years going between 60% and 100%, it might not have noticed that the effective capacity of the battery got much lower meanwhile.

Also, yeah, the percentage display isn’t necessarily correct at all times. They know that people would complain if their phone was stuck on 98% (you can see that sort of thing happen to old devices), so the phone will often fudge things for display.

Well, your phone is lying to you (by some definitions) but it might be kinder to say your phone is guessing without telling you how confident it is in those guesses.

Battery percentage does not linearly mean anything measurable on the battery. Your phone can measure the battery’s current voltage, but that relates to how much time is left in a non linear shape (image search for ‘battery voltage over time’ will get you some nice graphs). If the shape was the same every time it would still be possible to calculate time based on voltage, but it’s not, so the phone needs to guess where it is most likely to be based on phone use, battery lifespan, and engineering tests from the phone designers. Even the 100% and 0% are really just points that the engineers have decided is good enough boundaries, rather then the physical limit (which is why 0% phones can still turn on long enough to tell you to plug them in).

Specifically the last 20%, I suspect there’s a decent amount of psychological bias affecting you, but it might be based on something as well. Most phones go into a low consumption mode to extend that last chunk of time, but if the programmers of this mode don’t anticipate how you use the phone it might have the opposite effect; I’d guess this would be the biggest factor.

Also, if you looked up those graphs, you can see that the voltage quickly falls after a certain point in time; the speed of this fall varies the most between charges and over time. It can make predictions less precise, to the point that the phone guesses it’s at 15% when it’s really at 10%.

Well, my phone seems to be doing the opposite; the last 10% seem to take 3 times longer than any other 10%.

The truth is, determining how much energy is left in a battery down to one percent’s accuracy is *difficult*. There are so many variables to keep track of, not to mention what’s accurate one day may be way off one week later because something changed. Your phone is basically just guesstimating, and for whatever reason it is guessing wrong in the lower range.

Or, like you say, it’s just because you get hyper aware of the battery percentage and it’s all in your head.

Either answer or both may be true, there’s not really a simple answer here.

As most have said, it’s difficult to monitor because most newer battery types have a very rapid discharge. Essentially, you don’t know exactly where you are before you drop off the curve.

https://electronics.stackexchange.com/questions/32321/lipoly-battery-when-to-stop-draining

There’s not really a good way to measure how much energy is in a battery other than completely charging it, and then discharging it until it reaches the cutout voltage, and counting how much energy was used and how long it took to do that

The phone usually just estimates it based on the voltage it is reading at the time, which is a poor estimate and since the discharge curve is not linear (not a straight diagonal line between 100% and 0%) then it seems to move at different speeds depending on the percentage shown

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It’s one of the most annoying things about battery devices, and it pisses me off that there isn’t an easy way to swap them out any more, or to connect to mains alone when a battery isn’t needed at that moment, or to choose the hold voltage

Battery charge loss is not linear – as in, it doesn’t degrade consistently across its capacity. It is also difficult for the electronic system to track properly.

Your phone doesn’t really know how much charge your battery has. It knows what battery was put into it, it knows how much charge that battery can hold, and it call tell if it is at a full charge or not, and can tell when it’s below the halfway point. That’s about it. What your phone does after that is keep track of its own power usage, compares that information to how much charge the battery can hold, and give you an estimated percentage (which it often lies about, anyway).

The problem with all of that is batteries lose their capacity over time, so depending on the age and how much use the phone has the battery may drain a lot faster than the phone thought it would, and by the time the battery starts showing signs of being at less than half a charge your phone thinks it should still be in the 70s. Your phone won’t just go, “whoops, better fix that” and drop your percentage, but it *will* have that percentage tick down a lot fast than it did before until things meet in the middle. And if you keep using it past that point, depending on the manufacturer, it’ll drop the percentage below what your charge actually is because *completely* discharging a battery without properly shutting down the software isn’t good for the device.

Meanwhile, your phone *should* be updating its power remaining referral tables based on the current data so that yout accuracy should be more up to date in the future.