Charge e-bike to about 80% using Home Assistant

automation.yaml

alias: charge ebike to about 80%
description: ""
triggers:
  - type: turned_on
    device_id: 556a422a7ba455208c850f978429413d
    entity_id: 97d303fe6f6a9fdee99de120848bacaf
    domain: switch
    trigger: device
conditions: []
actions:
  - delay:
      hours: 0
      minutes: 10
      seconds: 0
      milliseconds: 0
  - wait_for_trigger:
      - value_template: >-
          {% set power =
          states("sensor.mini_plug_with_power_meter_electric_consumption_w")|float
          %}

          {# Check two different power ranges to distinguish Trek FX+ 2 from
          Orbea Diem #}

          {{ (power > 97 and power < 150) or (power > 191.0) }}
        for:
          hours: 0
          minutes: 2
          seconds: 0
        trigger: template
    timeout:
      hours: 2
      minutes: 30
      seconds: 0
      milliseconds: 0
  - type: turn_off
    device_id: 556a422a7ba455208c850f978429413d
    entity_id: 97d303fe6f6a9fdee99de120848bacaf
    domain: switch
mode: single

I use this Home Assistant automation to stop e-bike charging at about 80% charge. I use a Zooz smart plug that has on/off control, and power monitoring via Z-Wave. I plug the bike charger into that plug. (For safety always use the original charger to charge your e-bike!)

The automation is triggered when I turn the smart plug on by pressing the button on the plug. It waits for 10 minutes to make sure power draw has stabilized. Then when power draw exceeds a specified target (in watts), the automation turns the plug off. There is a 2.5 hour timeout to stop charging after a reasonable amount of time in case I messed up the power draw logic, or some such thing.

The key is to charge the bike to 100% at least once. Then look at the graph of power draw history in Home Assistant. As battery charge increases it gets gradually harder to get more energy into it. So power draw increases steadily. What's special about the 80% number is that with typical lithium-ion batteries the last 20% is a little extra difficult to charge. So at some point toward the end of charge you should see the power draw slope increase slightly. It's subtle, so it takes some guess-work. Look at the watt value at the point where that slightly steeper slope starts, and use that as your cut-off value.

I use the same plug to charge two different bikes. Fortunately their chargers have non-overlapping power ranges, so it's easy to write cut-off logic that works for both bikes. (>97 W for a Trek FX+ 2, >191 W for an Orbea Diem). I previously used the same setup with a Tern GSD Gen 2 where I set >185 W as the cut-off.