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 shut down long before they reach an empty battery, mostly to preserve your work and system when the battery is low.
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.
It is an estimate and as batteries get older/weaker anything below 10% should be considered 1% unless you enjoy living on the edge.
Weird how it feels like the opposite with most cell phones (that I’ve owned). I’ll start using my phone at 5% just to kill it and be scrolling for a couple hours
Not sure if this is the correct answer but I heard batteries take longer to charge the more juice they have. It’s like stacking bricks, at a certain height.. it takes more time and energy to stack it there. So maybe the same goes for how the power is used and the last 10% deplete faster.
Probably bad algorithm to estimate battery life. Batteries tend to discharge in non-linear fashion, but it’s easier to estimate battery life linearly.
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.
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.
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!
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