When should I charge my phone?

According to TechAdvisor.com, the golden rule is to keep your battery topped up somewhere between 30% and 90% most of the time. 

Top it up when it drops below 50%, but unplug it before it hits 100%. For this reason, you might want to reconsider leaving it plugged in overnight.

Pushing in the last charge from 80-100% causes a lithium-ion battery to age faster.

Maybe it's better to recharge in the morning instead, at the breakfast table or on your office desk. That way, it is easier to keep an eye on the battery percentage during charging.

iOS users can use the Shortcuts app to set a notification when the battery level reaches a certain percentage. This is done under the tab "Automation" and then "Battery Level"

Giving your phone a full recharge is not fatal for a phone battery, and it seems almost counter-intuitive not to do so, but giving it a full recharge every time you charge it will shorten its lifespan.

Likewise, at the other end of the scale, avoid allowing your phone battery to get below 20%.

Lithium-ion batteries don’t feel good about going too far below the 20% mark. Instead, see the extra 20% "at the bottom" as a buffer for demanding days, but on weekdays start charging when the warning for Low Battery level appears.

In short, lithium-ion batteries thrive best in the middle. Don't get a low battery percentage, but also not too high.