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
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.
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