Battery Memory effect: A battery frequently charged from 20% to 80% might ‘forget’ about the 40% that’s left uncharged (0-20% and 80-100%). 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 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.
Don’t charge your phone battery from zero to 100%: The rule with Li-ion batteries is to keep them 50% or more most of the time. When it drops below 50% top it up a little if you can. A little a few times a day seems to be the optimum to aim for. But don’t charge it all the way to 100%. It won’t be fatal to your battery if you do a full recharge - most of us are forced to do this every now and again in emergencies. But constantly doing a full recharge will shorten the battery’s lifespan. So a good range to aim for when charging a Li-ion battery is from about 40% to 80% in one go. Try not to let the battery drop below 20%.
When to full charge the battery? Experts recommend that you do a full zero to 100% battery recharge (a "charge cycle") maybe once a month only. This recalibrates the battery - a bit like restarting your computer, or, for humans, going on holiday! The same goes for laptops, by the way.
