My battery lies to me

I’ve had my laptop for just over three years now, and I’ve never changed the battery, so I guess it’s probably about time for it to die, but the thing that I find most frustrating is that my computer doesn’t know how to measure the battery anymore.

As I type this, I’ve been using my battery for about 45-50 minutes, and if I hover over the icon it tells me:

37 minutes (21%) remaining.

Now I know from experience that even though it might actually have 21% remaining, it’s going to last a lot less than 37 minutes. In reality I have more like 10.

You’d think my computer would base its guess on how fast the battery has been draining during the past hour. But I guess it’s just a pre-set algorithm. Too bad.