CMOS (complimentary metal oxide semiconductor)
is a low power consumption field effect transistor
(ignore the above if you're a spell check victim)
Nearly all batteries are Lithium ion.
Many devices require a 3V Lithium
button cell for memory retention.
(BIOS, flash, etc.)
These are not normally charged, and must be replaced
periodically (by soldering with ESD precautions)
Micro USB charging ports become fouled with dust,
lint, and extraneous particulates, and require periodic dusting.
You can't assume your device's power supply is charging because
it's plugged in.
Batteries normally last years, but lost time and date are
symptoms of low battery output. Your device might
have a battery health trouble shooter.