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
Laptops have a setting in Power options (MS Windows) where the laptop will automatically be put into hibernation mode at around 5% to prevent damage to the battery until you turn it back on and charge it. Additionally, the reason the battery percentage may drop slowly from 60% to 59% may depend on how it is being used. Light load work like word processing might cause the battery to drain way slower than if you were using YouTube and streaming videos or playing games.
Because batteries don’t die linearly. It’s easy to believe that a 12 V battery would go down linearly, so that, at 50%, it would be at 6 V, for example.
However, that would be a very crappy battery, as electronics need a pretty stable voltage. So, instead, an ideal battery would remain at, in our example, 12 V until depleted and then suddenly die.
But, ideal batteries only exist in fairytales (see Snow White and the Seven Ideal Batteries), so what a real 12 V battery does is that it usually starts a little bit above 12 V, say, 12.2 V. Then it slowly drains to just below 12 V, say, 11.8 V, at which point it starts to dive really fast. Look here for some examples of how the curve might look: [https://www.genstattu.com/blog/wp-content/uploads/2019/07/Constant-current-discharge-curve-of-3C-products-under-different-ambient-temperatures.png](https://www.genstattu.com/blog/wp-content/uploads/2019/07/Constant-current-discharge-curve-of-3C-products-under-different-ambient-temperatures.png)
Now, the method of measuring capacity is by measuring voltage and comparing to the discharge curve of that battery. But, as the voltage changes very little until it dives, it’s a quite fickle process. What makes it even harder is that as the battery ages, the curve changes, so that the flattish part becomes less flat and the dive happens earlier. Likewise, temperature also affects the curve.
So, basically, we are trying to measure tiny differences while using a mostly guessed reference curve to interpret those rather bad values. Bottom line is that it’s very hard to measure remaining battery capacity, and it gets a lot harder the lower the capacity remaining gets.
If a battery falls below a certain voltage, internal circuitry will think its damaged and cant be recharged. Because of this a system will do whatever it can to keep it from getting down to 0% even if that means shutting down your machine.
During the last 10% or so, the laptop starts saving your work and everything open so you dont lose your progress because of an empty battery.
Basically, the closer to 0 that a battery gets, the faster it depletes. Lithium batteries (like in your laptop and cell phone) have pretty flat discharge curves – meaning that they maintain their voltage under load for a long time, but then the voltage drops way off near the end of the charge.
beyond saving the your work and getting the machine to a good state, in general a battery that drops below a certain amount of charge, becomes almost impossible to recharge. There are ways to try and bring them back, but it takes certain electrical gear to do it. And most folks don’t have nor really need that kind of gear.
A battery’s charge is basically taken by measuring the voltage across the terminals of the battery. The problem is that a battery doesn’t drop voltage at a constant rate proportional to the discharge, because that’s not how battery chemistry works. Instead, the voltage drops during discharge with a pattern that looks like an algebraic equation, usually a steep slope followed by a long shallow slope followed by another rapid drop as the battery hits the end of its useful charge. [Here’s a site with some pictures explaining battery discharge characteristics – it’s not ELI5, but there are enough pictures there that you don’t have to have a doctorate in battery chemistry to get the idea.](https://www.mpoweruk.com/performance.htm)
Because of this non-linear discharge rate, your computer basically has to make an educated guess how much charge is still available based on the available voltage measurements. If the programmers who built the battery algorithm basically said that the battery’s voltage range goes from 4 to 3 volts, it’d be simple to say that 3.60v is 60% charge… except it’s more like 60% +/- 10%. And it’s going to take a while to go to 3.59v, because when you’re in that part of the discharge curve it’s not dropping as fast as when you’re at the end of the charge.
And also, as others have commented your laptop shutting down at 6-8% isn’t usually because it’s completely out of battery but instead is a usability function to allow the laptop to sleep/hibernate to protect its user data and gracefully recover. This is because users can’t be relied on to actually shut down or plug in when they’re warned that their battery is getting low and yet still get pissed off at the software people that their data gets lost when the computer runs completely out of power.
The reason is due to how batteries work. A typical laptop battery has an operating range between 10.1-12.9V. The voltage drops as it discharges and is restored by charging.
The remaining battery capacity is typically figured out from the voltage. So we can say a range of 2.8V each .1V will come out to about 4% of your battery capacity.
However there is more than just voltage at play. Power usage is measured in watts. Watts is equal to the voltage x amperage.
So as your battery discharges the laptop still needs the same amount of power to run. Let’s say your laptop draws 12 watts. So at 12V the laptop will pull 1 amp. At 11V the laptop will now draw about 1.1A. So because the laptop now draws more amps the voltage will drop faster thus causing your percentage to go down faster. Some computer companies know about this and calculate the remaining watt hours of the battery and report the percentage based on that. That is why some computers may have a very even discharge and others may discharge faster at lower percentages.
I haven’t seen the actual reason posted yet – it’s because battery characteristics change as they’re cycled, and over time. State of charge is usually determined by measuring the voltage of the battery, but batteries lose capacity over time. In addition, the voltage of a battery going from full to empty is not a linear regression. If you assume full is 5 and empty is 1, for example, the voltage may be at 3.5 at 90%, 3.0 at 30%, 2.5 at 10%, before falling 2.0 at 1%, and 1.0 at 0%.
It’s tough to measure that accurately.
Many battery indicators are not well calibrated. Battery monitoring circuits measure the voltage and the current coming from a battery, but do not directly see the amount of charge in the battery.
Software programs estimate that from the voltage and the current measurements, but doing so is somewhat of a black art. Good software algorithms keep records of past performance of the battery to estimate the charge state more accurately, but when a battery can be changed, it may take some time to recalibrate the algorithms.
Laptops (as with other devices) require a certain amount of power to operate, and when they sense the battery is nearly empty, attempt to shut down in an orderly way.
Older batteries may take longer than new batteries to charge up, and may not hold as much charge. Battery charging circuits may push a lot of current into a battery that’s nearly completely discharged, then reduce the current as the battery approaches maximum capacity.
Devices may also measure the temperature of the battery to best charge the battery without damaging it by overheating. (Off topic observation: Tesla cars use battery power to warm the battery itself, as well as a cooling system to keep the batteries from getting too hot.)
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