Battery health

OK. My previous question was really about how old the battery is to get an idea of how much it's been used. It might be useful to know how many cycles it has on it, which can be determined via an Apple diagnostic, as well as a few self-help diagnostic tools like coconutBattery for Mac. The nominal performance rating is 500 cycles before reaching 80% battery health. However, there are many factors that will lead to battery health going down, especially just charging and discharging it. It's a consumable part like tires on a car.

There's nothing unusual about 79% battery health in and of itself. However, 80% is the point that Apple has defined as a battery being depleted. The software may also determine that the battery can't support peak performance and thus go into a "throttling" mode where the processor speed is reduced in order to reduce the chances of the battery running out of power that risks an unplanned shutdown.

It looks like you can still use it, but at that point your battery is easily ready for a replacement.

It's probably the battery. Often the updates to battery health reporting happen all at once, even if the battery itself was steadily declining in capacity. When did you get it (or a new battery installed)?