Low-Power Boron LTE, using the EN Pin

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Trial #1 (Boron LTE, 0.5-watt Solar Panel, 2,000 mAH Li-Po, external Timer for EN Pin) required a modification to the Code during the experiment. Originally, the PMIC would not resume recharging the Li-Po if the input (solar) voltage dropped out completely. This means that charging would not resume after severe cloudy conditions, or night time. My solution was to add pmic.enableBuck(); inside the Setup() routine which runs each time the Boron Starts-up from the external Timer. The following PMIC settings were eventually used:

// Li-Po Power Management:
  pmic.begin();
  pmic.setInputVoltageLimit(5080);  //  for 6V Solar Panels   
  pmic.setChargeVoltage(4208);      //  Set Li-Po charge termination voltage to 4.21V,
  pmic.enableDPDM();  
  pmic.enableBuck();                //  enableBuck required when using EN Pin Shutdown.
  pmic.enableCharging();

I’m guessing that pmic.enableDPDM(); & pmic.enableCharging(); could be removed, but I haven’t confirmed that.

Once pmic.enableBuck(); was added, the daily Battery Profile became pretty boring, with no major unexpected results. The 0.5-watt Solar Panel can easily recharge the Li-Po when running a 5 minute Wake/Publish schedule. The Boron doesn’t terminate the Li-Po charge at 4.2V (SoC 100%) anymore, as with my previous tests using pmic.setChargeVoltage(4208). This isn’t necessarily a negative result, and I’m sure there is a explanation.
A typical 24 hour Graph looks like this:

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SoC ranges from 93% (daytime) to 85% (night)
A typical Startup/Connection/Publish uses 1 mWh.
Quiescent current (while Shutdown via EN Pin) uses 7.5 mWh per day.

Notes for Trial #1:

  • The 2,000 mAh Li-Po battery that ships with the Electron is rated at 7.4 Wh, however those tests are conducted from 4.2V to 3.0V which is not a realistic range for actual operating conditions when calculating your Power Budget.
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  • The Tiny enclosure used for this Solar Trial doesn’t leave any room for insulation to minimize the Heating due to the Sun. Obviously, the Black color isn’t optimal either :sunglasses:

  • The Results show it’s EASY to operate a Boron LTE with Solar Recharge, even with a Tiny Panel.
    Your Panel size will depend on the average daily power for your application.
    Your Battery size will depend on the maximum days that you want to operate without any sunshine.

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Trial #2 (Boron LTE, 3xAA, external Timer for EN Pin) is finally complete.
Energizer L91 AA batteries are used in this test. Fresh L91’s have an OCV of ~1.8V each, and 1.65V under load from Boron LTE. The AA L91’s are rated at 4.5 Wh at a constant 100 mA load.
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As you can see the below, the Battery Voltage has started a steep decline indicating the end-of-life is approaching soon. The Boron performed 9,869 Publishes with the 3xAA Batteries in Series.

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[Edit: Test Complete, final 12 hours shown below, 4.20V would be the Battery Alarm Threshold for 3xAA]

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The average MCU time for Startup/Cloud Connection/Publish was 12 seconds. This does not include the 2 Seconds of “wait-after-publish” before Shutdown.

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Notes for Trial #2:

  • The Boron LTE is truly a Cellular Module that’s capable of operating on AA batteries. This test far out-performed the calculations. I assume this is due to the cyclic nature using the EN pin (14 seconds on, 1 minute off) of this test. The 4.5 Wh published Rating of L91’s was under a constant 100 mA load.

  • Using AA batteries with a Boron LTE and the EN Pin opens up many possibilities.
    Specifically, for projects that may experience higher temps than we are comfortable with for Li-Po’s, or when the self-discharge of Li-Po’s prevent their use. A power budget for my location would be calculated using any publish schedule (1 mWh each publish) and the Quiescent current of 7.5 mWh per day + a significant Safety Factor.

  • The Boron LTE datasheet list the suggested Max Voltage on the Battery Connector at 6.5V.
    Increasing the batteries to (4)xAA Energizer L91= 6.6V, which is technically out-of-spec for the Boron, but well within the PMIC specifications.

  • 4S (4xAA in series) and 3S2P (2 parallel sets of 3xAA in Series) configurations will be tested in the near future for projects that would require more than 10,000 Publish events before changing batteries, or many years in Shutdown mode. Using the EN Pin, A Boron LTE publishing Every Hour for 1 year before changing AA batteries is feasible.

Any comments, suggestions, or questions are welcomed.

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