Why did my laptop battery get better after full discharge?

Factual Questions
1

I don’t have a good grasp of all things electrical, and seem to recall something about how some batteries don’t like to be fully discharged but…

My two year old laptop 9DELL suddenly gets tremendously better battery power after being fully discharged.

Over the last couple years the battery performance degraded until it was only good for 30 min or so. I chalked that up to the battery just getting old and wearing out.

Then last night I forgot to power off the computer and had something running in the background. Net result, the system did not go into auto-hibernate. it ran until there was no more juice and then shut down.

Now that I recharged the battery the meter says it is good for like 24 hours. I doubt it would last that long but it has been going 2 hours so far and still going strong.

TL;DR So why is my old laptop battery performing so much better after a deep full discharge and recharging?

2

There is no reliable, instantaneous way to measure how much charge is left in a battery - unless it’s full, or unless it’s almost empty. So, what most laptops do is to keep track of how much current has gone into the battery, and how much has come out. But these measurements aren’t 100% accurate, so if you never fully discharge it or never fully charge it, the errors add up over time. When you fully discharge the battery and then fully charge it, the charging circuit calibrates itself - now it knows exactly how much charge the battery can hold, and how much it’s holding now.

Here’s Apple’s instructions on how to do it, for example.

3

It’s not a bad idea to let the battery run down completely, or as far as you can, every month.

4

First, it IS a bad idea to let lithium ion batteries run down completely, so DON’T - that is old old advice taylored to nicad batteries. (How Lithium-ion Batteries Work | HowStuffWorks). Maximum lifetime on Lithium ion batteries is actually achieved if you never fully discharge or fully charge them, but instead run them between 30%-80% or something like that instead (which is what cars like the Prius do).

Second, I agree with the first guy - most likely was that the charge was always there, and your laptop was simply hibernating before the battery was even close to dead due to inaccurate calibration.