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
You probably have your laptop configured to begin shutting down when it has some power left over (perhaps 5%), to avoid it losing power while doing something important. It shuts down when it’s hit that preset limit, not when it absolutely can’t run any more.
Because it’s an estimation. The computer doesn’t actually know how much charge is left. Batteries are direct current, that means as the battery discharges the output voltage changes. Through battery testing they’re able to determine a reproducible voltage curve. Then they assign percentage of charge to voltage. By measuring the voltage the computer can estimate the amount of charge left.
As the battery ages the chemistry changes making the voltage curve not as predictable as programmed. That’s why the battery will die at 6%. Then when you plug it in it says 0%.
Voltage curve:
https://images.app.goo.gl/aws4HdaGGfThUhYBA
In the image it starts at 1.5 V then gradually decreases to 1 V then suddenly goes to zero.
When rechargeable batteries age the sudden drop in voltage drifts to the left due to changes in the battery chemistry. So instead of suddenly dropping to zero at one volt it’ll do it at 1.2 volts. The computer thinks 1.2 V is 6%, so it says 6% battery life left. Then the sudden drop occurs and the battery dies.
Most laptops shut down long before they reach an empty battery, mostly to preserve your work and system when the battery is low.
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.
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.
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.
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)
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.
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.
The power supplied by a battery is non-linear.
We do our best to linearize it to make it intuitive to use but it isn’t easy especially not when you’re using the cheapest sensors known to mankind for the measurements.
Pretty impressive all of them don’t just explode.
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