They absorb the most energy when they have the least in them. And as they get closer and closer to full, they can't absorb quite as efficiently and then there is some excess lost.

and

batteries degrades over time

Causes of degradation:

1. Heat ruins the battery (high impact)
2. Charging cycles
3. Time (slow impact)

How to charge the phone without generating extra heat ?

• Power breaks generating heat are now near the electrical socket
• the battery is divided by two
• adding more cooling hardware
  → heat problem is covered in normal situations

So what counts ?

• around 80% battery health after 800 charges (but we don't have long-term data about these new fast charges)

Announcement for 2022 from a tech company: 80% battery health after 1600 charges.