MacBook Pro used as desktop model; never using battery.

Batteries degrade over time through use, environment, and time. Your battery has been constantly degrading and 8 years is a looooong time for a battery. If you intend to keep the Mac, I'd have the battery changed. Or, put that money toward a newer model.

GingerKoko wrote:

Okay, thank you! But the laptop works perfectly. Since I never need the battery, can't I just continue to use it only as plugged in? It won't cause problems will it ... such as catching fire, etc?

If it ain't broke don't fix it...

lithium-ion battery can overheat if it has too much or too little charge. Battery designers use a computer chip to control the charge level. When your device’s battery is reading 5 percent it’s not almost entirely out of juice. If the battery were to discharge way more, the possibility of dangerous chemical reactions can occur.

The rule of thumb is if you are near the mains leave it plugged in, if you need the portability run on battery only—this is how you prolong the life of the battery.

I would not spend any money on an outdated 2013 MBP—as mentioned above 8 yrs is a good amount of time under any circumstance.

You should have the battery serviced. Sometimes batteries just fail, other times they fail spectacularly. If the battery starts to swell it not only becomes very dangerous, but also begins to displace the mechanicals, casing, and electronics. Apple generally supports battery replacement for 10 years from when the device was last sold and parts for 7 years. So you'll need to decide if the investment in a new battery is worth it to you.