Modern batteries for such devices are Lithium-Ion batteries, which hold relatively lots of energy in a small package. These batteries discharge, and the rate of discharge is somewhat linear up until a certain point: here is a nice graph.
Your device needs [some] amount of energy to work, if energy is too low, it’ll not work. It depends on the circuitry, but I can assume most hardware is designed to not catastrophically fail somewhere below the “0%” voltage. The devices shut themselves down to prevent this energy level from being reached.
The batteries themselves also need protection. This page shows what happens when a lithium-ion battery is outside the safe range – it gets physically damaged. This is why most lithium battery packs also contain regulation circuitry with the package.
As you could see from the graph, the voltage drops very quickly after 3.6V or so, which is where most devices and batteries should say – “stop, no further”. When going further, you permanently damage the battery due to chemical reactions happening.
The “100%-0%” display you see is entirely up to who programmed your laptop or phone. You can set Windows, Linux or whatever desktop OS you have to sleep, shutdown or hibernate at any percentage you want. Depending on who made this function, the percentage meter can show the battery power percentage or the “usability” percentage, both on the upper and lower end. This is to save your work, prevent hardware damage due to a sudden shut-down due to the battery controller saying “no” and to make batteries just work longer.
tl;dr: Many reasons.
- OS level: Save work and data and let the OS shut down gracefully
- Battery level: Prevent permanent battery damage.
You can change when your laptop shuts down due to a low battery.
Go to Control Panel -> Power Options -> Change Plan Settings -> Change Advanced Power Options -> Critical Battery Level and change it to whatever percentage you desire.
Great tip! Never knew that.
I recommend not doing that.
Your computer needs enough power left over to safely shut down when the battery gets too low. Otherwise, you may lose information or corrupt files on your hard drive.
Most answers I’m seeing here contain a lot of true facts but omit an even more fundamental fact:
That percentage is a computed estimate.
You can’t directly measure “percentage used” from a battery. What you can measure is voltage. Battery voltage over time, even under constant load, is not a straight line but looks like this. As you can see, voltage stays about the same then drops sharply when the battery is almost depleted.
As you can see, the line is curvy and irregular, so you need to do some fancy math to tell you how much battery life you have left; it’s almost as complicated as estimating how many more minutes your battery will last. Also, notice from the graph that at which point the final drop starts varies from battery to battery (and probably varies a little over the lifetime of the same battery), so it can be hard to predict. This is why a lot of devices shut off around 5% and why a lot of devices seem to last at 1% forever.
Another way to measure battery percentage is to measure the amount of charge (in coulombs) that has passed through the wire and compare it to that of an earlier run. But the number of coulombs per battery cycle can vary and charge is “leaked” (or, depending on your perspective, the meter can be inaccurate) in various ways, so this method is also not effective at predicting battery life near the end.
When a flashlight starts to run out of battery, it just gets dimmer, but advanced electronic devices contain parts that can malfunction or break outside the voltage range it was designed for, so when the voltage goes below a certain point they need to turn off completely. So 0% battery is not 0 volts but rather the minimum operating voltage. Modern laptops might also account for the energy it takes to go into hibernation, or throw in some leeway for the energy required to sleep for a few minutes so it doesn’t have to go into hibernation if you charge it right away.
So to answer your question, there are two main reasons:
- The voltage curve is very steep and unpredictable when the battery is almost used up, so it’s hard to predict how much longer you can use it.
- The computer/battery/OS manufacturer’s definition of 5% and 0% might be different from yours or different from each others’. (Example: Computer manufacturer calibrates 0% to mean 0 volts, but you expect 0% to mean when the computer starts to hibernate.)
I work in IT, and the laptops I’ve dealt with here at work will go to sleep automatically at 5%. I am not a lawyer, as the saying goes, but I’ve always assumed that it’s a built-in feature so that your computer doesn’t go straight to 0% and you potentially lose everything you’ve been working on. You’ve at least got a chance to get it to a charger before it drains the rest of the way, and it would drain much slower being asleep as well.
As a battery changes its ‘charge level’ its characteristics change–discharge rates, charging rates, etc. The important thing here is that the charger can only output a certain voltage/current to your laptop battery. As we said before, remember that the battery characteristics change as it drains. When batteries are in a certain range(100% to 5-15% as you noticed) those characteristics stay the same, so those batteries can be reliably charged/recharged. The problem is that, when the charge level is dropped below that 5-15%, the characteristics start to change pretty quickly, with potentially disastrous consequences-the battery could be damaged or potentially even explode!
Fun fact! Your phone/laptop/whatever actually doesn’t actually know how much juice is left in the battery. It checks by monitoring the characteristics we mentioned before(discharge/charge rates). You can actually increase the accuracy of that battery level indicator by occasionally using your phone until it automatically shuts off itself.
Source-worked on power circuits for a CubeSat in college, the grad student in charge of our subteam explained all these concepts to us