Request: Custom User Item Formulas. ie. Volts X Amps = Watts

I'm starting to use Li-ion batteries which have much larger changes in voltage during the flight, from 4.2v to 2.7v. This makes Amperage and mAh used much less useful. I really need to see WATTS and Wh.
Since the batt % is based off of mAh used, when the voltage varies greatly like it does with Li-ion, the % used is no longer linear and can be misleading.
This would also help when switching between packs of different voltage.

The issue names no files, tests, or entry points. Start by locating Mission Planner's battery telemetry calculations and any user-configurable item definitions; done should include configurable voltage-based formulas that expose watts and watt-hours and make battery percentage meaningful for varying-voltage packs.

Written by the indexing model from the issue text.