Exercise: figure out how long my Heltec Wireless Tracker can run on a 2500 mAh battery


- buy or borrow a "USB power monitor" - they're common, cheap, and every geek should own one

- connect the Heltec Wireless Tracker to a USB power block, through the power monitor

- reset or take note of the current mWh measurement on the USB monitor
  (46396 mWh)

- take note of the date/time, and let it run at least an hour (a full day would be better)
  (9:15pm)

- make note of the time, and the mWh measurement
  (10:16pm, 47476 mWh)

- figure out the number of minutes between test end and test start
  (10:16 - 9:15 = 61 minutes)

- subtract the original mWh reading from current to get mWh consumed
  (47476 - 46396 = 1080)

- divide mWh consumed by minutes ran, and multiply that by 60 to get average mW power draw
  (1080 / 61 * 60 = 1062)

- divide average mW draw by 3.7 battery volts to get average mA draw
  (1062 / 3.7 = 287)

- divide battery mWh by average mW draw to get hours of runtime
  (2500 / 287 = 8.7)

answer: 8.7 hours runtime