Recently I bought new mobiles for myself and Nancy. These have huge batteries (5000mAh) which will last almost two days on a charge. Battery University has an excellent article on How to Prolong Lithium-based Batteries. One of the recommendations is to not full charge a lithium-ion battery, instead charging to 70-80%.

As luck would have it Ariela, the app I use for interconnecting our mobiles to Home Assistant, reports the current battery level back to Home Assistant as a sensor. Coupled with a TP-Link HS100 it became trivial to interrupt the charging of our phones at 85%.

- id: '1580492681958'
  alias: Switch:Adrian's Phone Charger:Turn Off
  description: ''
  trigger:
  - above: '85'
    entity_id: sensor.adrian_s_g7_power_battery_sensor
    platform: numeric_state
  condition:
  - condition: state
    entity_id: switch.adrian_s_phone_charger
    state: 'on'
  action:
  - entity_id: switch.adrian_s_phone_charger
    service: switch.turn_off

Some quick analysis of the sensor graphs shows that a charge to 85% takes around an hour. To maximise the phone charge during the day, I delay switching on the charger until the early morning (currently 04:00, but could be later).

- id: '1580492573046'
  alias: Switch:Adrian's Phone Charger:Turn On
  description: ''
  trigger:
  - at: 04:00:00
    platform: time
  condition: []
  action:
  - entity_id: switch.adrian_s_phone_charger
    service: switch.turn_on

There is probably some further tweaking and optimisation to be done here. Time will tell whether it actually has an impact on the long-term life of the phones.