Changing VARV During Test Execution

As part of a test plan, I want to check that a device properly recognizes when its battery is low, and when it is fully charged. Here is a test plan that does this:

- title: VARV full battery
  steps:
  - command: power VARV 4.2

- ... do some other stuff...

- title: VARV low battery
  steps:
  - command: power VARV 3.75

When I do this, it seems like VARV does not change.

How do I adjust VARV after it is already on? Turning off VARV, then setting it to the new voltage, will not work in my use case.