Should I keep my MacBook Air plugged into its charger whenever I can?

does this include a MacBook 2010?

i have 2, one kept plugged in running Mojave-battery fine

the other always unplugged running Mountain Lion

the MtLion seems to lose battery % when i turn on the MacBook.

sometimes a lot were 8% and sometimes 92% are available

which is a battery- settings i forget to configure before turning off

thanks in advance.

Yes.

With 10.15.5, Apple introduced Battery Health Management, and your Mac will run on battery occasionally even when connected to AC power, to maintain battery health.

+1 with neuroanatomist on this one.