What is battery memory effect?

Battery memory effect is about batteries remembering remaining charge if you don’t let them go all the way to zero too often. So a battery frequently charged from 20- to 80

Sounds crazy but that’s sort of true – but only for older nickel-based (NiMH and NiCd) batteries, not the lithium-ion batteries in your modern phone.

Lithium-ion (Li-ion) batteries don’t suffer the memory effect so you almost need to do the opposite: charge them often but not all the way throughout the day, and don’t let them drop to zero.