Linux

I assume this is an old laptop? If you're able to remove the battery, as in the old models, you can measure it's full milliamperes when at a 100%. It's probably going to show a lower value than what's reported by the user guides. From what you tell, I'd expect something as low as 1.5k... Beware that removing batteries with tools and so on might make them explode and is many times, just not an option.

Perhaps you can even see this in the BIOS. Some modern BIOS might even tell the number of charging cycles, allowing you to infer it's worn out. Chances are, you need a new battery. Which is tricky, since oftentime the "new" battery was sitting at some store but was manufactured by the company when the laptop model was fresh.