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 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.
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