Why does a laptop say it’s at 6-8% charge, and then it dies, but when it’s at a higher charge, going from 60% to 59% takes a while?
In: Technology
Most laptops like preserving your stuff before loosing power so they shut down before hand… BUT as your batteries age and the controller inside wears out the percentage reported is rarely accurate… and when the batteries cannot produce the amount required the system shuts down… this is also true for cellphone batteries… the older they are and the lower you go the harder it is for the controller to do its job so you might get unexpected shutdowns…
The laptop could give you an average charge rate to 100% indication, with each percentage progressing at the same rate. But if you unplug it before it reaches 100% then the indicator will have to change to the true percentage, so it can look like you’ve gained or lost a lot of percentage at once when you plug or unplug it while it needs a charge.
Because batteries don’t maintain their full capacity throughout their lifetime, and it is very difficult to determine how much charge they actually have at any given time. The battery level is only an estimate.
My phone batteries currently drop out with 4-8% reported left. New, they sat at 1% for the better part of an hour.
If it’s shutting down at higher percentages it’s likely just calibrated poorly because it’s an older battery.
Either way, you can trust that that displayed percentage is somewhat disconnected from what’s happening with your battery.
The battery charging ICs in your laptop very carefully monitor the battery voltage, charging current, and discharging current, and force a shutdown at a particular voltage regardless of what the OS says.
The best way I’ve heard it is imagine the battery like is like football stadium. Now when everyone tried to leave at once there is a lot of congestion sand it takes a long time for those first people (ions?) leaving to get out. When it’s at lower capacity all of the remaining people (ions?) Can get out easier and faster.
Knowing how much battery is surprisingly hard. The easiest way is by reading the voltage, but the voltage only varies when [almost empty and almost full](https://photos5.appleinsider.com/gallery/24198-31518-Li-ion-Discharge-Voltage-Curve-Typical-l.jpg). This means that between ~10-90% the computer has no easy way of knowing how much power is left.
To know how much battery is left between 10-90% a power counter is used, which literally counts how much power is consumed, since you already know how large the battery is and you know how much power has been used you know how much is left.
The problem is that batteries degrade with time, and the counting always thinks the battery is perfect, so there is a point where the counting thinks its above 10% (so the voltage should stay stable) but the voltage starts falling, so you either see the battery dropping very fast, or the computer over-reports how much battery is left.
It seems there are a lot of detailed explanations. Here’s the simplest I can think of it.
A battery is like a tank of water, you can suction out the water at one end and for a brief moment (Longer in batteries) the water level is lower on that end, and must settle to get an accurate reading on the water level across the entire tank.
In a battery, it’s very similar but much slower, so realistically there’s no real way to tell how much battery life is left. The reason it jumps so much is things that read the level are mostly guessing.
However, it’s in my personal theory, that for the energy in a battery to “Settle,” is near impossible, because batteries do slowly leak their charge. While we can get good estimates based on voltages, they can be easily thrown off or wrong.
Batteries don’t drain evenly. They drop off more quickly as they get more empty. Lithium cells are actually pretty good about staying flat and even throughout the discharge curve compared to old technology like alkaline or NiCad battery cells. But after your battery gets old, it really starts to drop off more dramatically. That explains what you’re experiencing.
Also, picture a scale. Power on one side, which is what the phone requires. On the other side of the scale, you have voltage blocks and current blocks. The voltage and current blocks are the same weight. Now let’s say 10 blocks is what it takes to balance the scale. If you remove 2 voltage blocks (meaning the battery is draining), you have to add two current blocks to keep the scale balanced. Well the problem is, current output drops as the battery gets closer to empty, and at a certain point, the battery cells just can’t put out enough current to meet the power requirements and it’ll just shut off. (in this analogy, it’s like running out of current blocks to swap for the draining voltage blocks, so you can no longer balance the scale, which is our equivalent to powering your device) This balancing act is what voltage regulators do.
[https://batteryuniversity.com/learn/article/discharge_characteristics_li](https://batteryuniversity.com/learn/article/discharge_characteristics_li)
^ this link has graphs where you can visually see what I’m talking about. If you scroll down a ways to the bottom you’ll see where they show older cells that really drop off dramatically as they near empty.
Also, worth noting that battery cells aren’t zero volts when they’re empty. Lithium ion cells are 3.8 volts, roughly, when empty. And when they’re full, they’re at about 4.2 volts. Going above 4.2 voltage will permanently damage the cell and can cause it to explode. Going below 3.7 volts can also cause permanent damage and most chargers won’t even attempt to charge it. (lithium fires are crazy, so chargers and voltage regulators for lithium batteries are pretty picky about battery health before they’ll attempt to charge them)
Batteries aren’t perfect in the way they charge and discharge. Lithium batteries, like the ones in almost all portable devices, are very delicate, and require special care in order to charge and discharge properly. Your phone never fully discharges, because if a lithium battery fully discharges it will never charge again because something inside the battery changes chemically. Your phone or laptop is constantly monitoring the exact voltage of your battery, and the voltage tends to be more consistent when the battery is above 10-15%, but when it drops below that, the voltage becomes inconsistent and often dips below and above the acceptable range, causing the software to shut off your computer
Don’t forget that it’s an estimate, this information is provided to you by the chip in the battery itself (in most, if not all, modern laptops), then further processed by the operating system. The battery (chip) can be calibrated to provide more accurate information, this is often done in the factory, but over time, as you use the battery, that initial calibration information becomes less useful because the battery degrades.
Your operating system also further processes that battery information and tries to estimate how long the battery will last on the current load, however, it can’t predict what you’re going to do next, whether you’re going to close all the applications or open something that requires lots of power.
Lastly, it’s unhealthy for batteries to completely run out of charge, so what you see isn’t the true battery charge level, so the operating system and your motherboard will usually try to protect the battery and kill your laptop before the battery itself fully runs out of charge.
I’m not an engineer, but there’s the whole issue with voltages, which further contributes to the problem.
N.B. a lot of this is based on my own experience, my computer science degree and some quick research, please forgive any inaccuracies.
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