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
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 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.
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.
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.
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…
Because the battery is old.
**Edit: here’s a water analogy**:
The battery is like a tank of water connected to a pipe with a valve at the end. The laptop adjusts the valve in order to keep the appropriate amount of water flowing (power) to supply the laptop. As the tank drains, the pressure drops, and the laptop opens the valve further to keep the same amount of water flowing.
When the battery is old, the pipe is all rusted up and full of gunk. When the tank is full, the pressure is still enough to keep the water flowing at a sufficient rate. But as the tank drains, the pressure drops lower, and the laptop has to open the valve up more. Eventually, even though there is still water in the tank, the gunk in the pipe causes the pressure at the valve to be too low. Even with the valve open all the way, the water flow is insufficient to power the laptop, and it shuts down, even though there is still water in the tank.
If you use the laptop heavily (like playing games), you need more water flow, and it will shut down at a higher percentage because the tank can’t keep up even if it isn’t empty. If the laptop is idle and the display is at a low brightness, you need less water flow, and it will more likely drain smoothly until the tank is really empty (0%).
**Original explanation**:
The laptop measures the charge in and out of the battery in order to estimate how much remains; simple. They can even compensate for how batteries have less usable charge if you discharge them faster (some is wasted). However, that is not quite all that goes into how batteries work.
Batteries have a property called *internal resistance* that increases over time as they age. Internal resistance is public enemy #1 for batteries. The higher the internal resistance, the more heat is emitted by the battery when you use it, which means energy is wasted. But it gets worse: as internal resistance increases, the more current you use from the battery, the lower its voltage drops. And the lower the voltage drops, the more current you need to sustain a given power level. You might be able to see where this is going.
As the battery runs out, its voltage drops. To compensate for this, the laptop needs to pull more current from the battery to keep running – it can’t really decrease its power consumption, since that just depends on what you’re doing with it, so it needs to do whatever it can to sustain the amount of power required. Now, if the battery is brand new, this isn’t a problem: the internal resistance is low, and as the current increases, the voltage stays roughly the same. Cool.
But now you try this on an older battery, and there’s a problem. As the laptop pulls more current, the voltage drops. Then the laptop tries pulling more current to compensate, and the voltage drops more. It still can’t get enough power, so it pulls more current. Now the voltage drops below the minimum level at which the laptop’s power supply can run, and your laptop shuts down. Hard. No warning.
If the power management system is smart, it might be able to catch that this situation is about to occur before it becomes unsustainable, and then jump straight to indicating 0% charge, and trigger a proper shutdown.
Effectively, internal resistance sets a *limit* for how much power you can pull from a battery, and this limit varies depending on how charged the battery is. Your battery might have 20% charge remaining, but if it’s old and you try to pull 40W from it and it can only sustain 30W at that point, then it might as well have 0% charge remaining. You can’t use that extra charge.
You can see this by running down your battery to, say, 10%, and comparing how long it lasts with the laptop totally idle and the screen at a low brightness (low power), vs. with the laptop running some benchmark or something that uses a ton of power. If the battery is old, chances are it will still last a while and trickle down to 0% in the first case, while it’ll crash and drop to 0% (or die instantly) in the second case.
You can see why this is also a user interface problem. Even if the battery controller knows all about internal resistance, what is it going to tell the user? “If you use your laptop lightly, you have 10% charge remaining; if you try to max out the CPU, you have 0% charge remaining”? There’s no good way of indicating this problem to the user, because “how much charge remains” isn’t a single number!
Voltage drop under load and allowable voltage tolerances of devices.
ELI5: You’re drinking a juice box and super thirsty, but if you suck any air your head explodes. If the box is full you can drink as fast as you want. But if it gets down low you have to sip really slow or your head will explode. You’re going along just fine riding in a car but getting a little low on juice, but you’re oblivious because the scenery outside is awesome. You grab the juice box and drink like normal, forgetting you’re getting low, and your head explodes.
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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