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
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!
Latest Answers