[Solved] Need guidance on battery selection for small wearable device

Adam Wheeler Asks: Need guidance on battery selection for small wearable device
I need help in understanding the factors that affect my battery selection for a device. I’d like to make this generic so that it is useful for others but I will use what I am designing for examples and clarity.

The goal is to design a small battery powered device worn by a child with a wrist whos surface area on top is 40mm by 20mm with a height of around 5mm.

I believe that I need to know the voltage required by the unit, the amount of time that the power will be provided, and how long I want the battery to last in order to figure out the battery I need.

The chip specs have this:

  • Electrical Characteristics
    • Output Power:
      • Max. Power: 21 dBm
    • Consumption @ LTE Cat M1 (Typical):
      • Power Saving Mode: 3.2 μA
      • Sleep Mode:
        • 1.61 mA @ DRX = 1.28 s
        • 0.61 mA @ e-I-DRX = 81.92 s
      • Idle Mode:
        • 19.6 mA @ DRX = 1.28 s
        • 19 mA @ e-I-DRX = 81.92 s
      • Active Mode: 228 mA @ 21dBm, GNSS off
    • Consumption @ LTE Cat NB1 (Typical):
      • Power Saving Mode: 3.2 μA
      • Sleep State:
        • 1.54 mA @ DRX = 1.28 s
        • 0.66 mA @ e-I-DRX = 81.92 s
      • Idle State:
        • 15.8 mA @ DRX = 1.28 s
        • 15.3 mA @ e-I-DRX = 81.92 s
      • Active Mode:
        • 165 mA @ 21dBm, GNSS off

The device will need to connect to LTE-cat M1 or NB-iot, GNSS or gps to provide updates for location and geofencing.

I have two main points I am struggling with.

  1. I would like an update every 15 seconds or so if the devices physical position has changed. Assuming the device is in use 25% of the time (6 hours/day) I am not sure how to calculate the power requirements per day that I could then use to say “I need a battery with voltage X that has Y amp hours to last Z hours.”

  2. Does gnss off mean this does not account for positioning data? if so, how do I account for that?

I have a design docs for the chip I am connecting too and I am only linking to it in case there is something I have missed here that helps. https://www.quectel.com/wp-content/uploads/pdfupload/Quectel_BG77_LPWA_Specification_V1.4.pdf



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