Minutes elapsed on an entity event

I am attempting to write a template to give me the time in minutes since a switch has been turned on. So far, this works in the template tool and reports the time in minutes that the switch has been turned on (repeater_7 is a simple zigbee switched outlet):

{{ relative_time(states.automation.repeater_7_auto_off.attributes.last_triggered) }}

But,
when I look at the entity created by the following template:

- platform: template
  sensors:
    trimmer_charge_time:
      friendly_name: Trimmer Charge Time Remaining
      value_template: "{{ relative_time(states.automation.repeater_7_auto_off.attributes.last_triggered) }}"

it looks good until after 1 minute and then reports a 0.

The whole point of this is to implement this automation to keep my battery from being overcharged:

- id: 'repeater_7_auto_off'
  alias: 'Repeater_7 auto off'
  description: 'Repeater off after 2.5 hours'
  trigger:
    platform: state
    entity_id: switch.repeater_7
    to: 'on'
  action:
  - delay: '02:30:00'
  - service: switch.turn_off
    entity_id: switch.repeater_7

The automation works but the template I’m trying to create won’t display the proper elapsed minutes.
What have I done wrong here?