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  • Author Author: Jan Cumps
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Solar Energy Harvesting with Epishine Flexible Solar Cell - Pt 4: Battery Backup

Jan Cumps

I received a preview of a light harvesting kit from Epishine and element14. Check my intro here.

In this post, I'm checking the behaviour when you provide backup coin cell.

 

 

The setup is identical to the previous tests: 1.8 V output and 590 Ohm load. But with a 3 V coin cell added to extend the uptime.

 

image

source: annotated draft product brief

 

What this achieves is that, when the supercap voltage drops to 3.6 V, the circuit will use the coin cell energy to recharge the cap back to 3.9 V.

That extends  the time that the circuit can keep your device powered, even without light source.

 

The Epishine example uses 3.3 V output. I did the test with 1.8 V.

image

source: draft product brief, with a 3.3 V output.

 

Here's my measurement. Completely as expected.

The e-Peas solar harvesting IC is doing exactly the same as in Epishine's setup.

When the VCAP drops to 3.6 V, the backup charger becomes active and pushes it back up to 3.9 V.

image

It keeps doing that as long as there's energy in the battery and VCAP hits one of the thresholds.

If you're wondering where these voltage levels come from, check the board configuration:

 

image

source: e-Peas AEM10941 datasheet

 

Related blog
Pt 1: intro
Pt 2: Circuit Analysis
Pt 3: Charge and Discharge
Pt 4: Battery Backup
Pt 5: test points
  • in reply to Jan Cumps

    Long term measurement, No coin cell involved..

    I have first put a load that can be sustained for hours.

    image

    Then at approx. 5500 seconds, I increased the load just over the tipping point so that the super cap energy was slowly consumed.

    That energy was consumed at around 8800 seconds. At that point the circuit switched off the 1.8 V output and started to focus on the regenerate strategy.

    The flat part between 9500 and 10500s is when I closed curtains because the lab became too warm image

     

    This is a 13000 seconds exercise, 216 minutes, more than 3 1/2 hours ...

    I recorded the currents I asked from the cicuit.

    It delivered  between 200 and 670 µA - and could sustain up to that 670µA inside a daylight room continuously. At the time you see the circuit tapering down (5500s), I started to ask 820 µA.

    I'll write a detailed blog

  • in reply to DAB

    The solar cell does it.

    In this special scenatio there's an additional coin cell that will help if there's not enough light available (or not long enough) to keep the device constantly powered.

  • in reply to Jan Cumps

    Hi Jan,

     

    I am just looking at your charts and watching the charge/discharge cycle with a sustained 3.3 v out.

    You say the battery recharges the supercap, but I would expect the solar cell to do that if the circuit has more power than the load uses or have I missed something?

     

    DAB

  • in reply to DAB

      wrote:

     

    Nice update Jan.

     

    From my interpretation of your results, it does not look like this configuration would capture enough energy to sustain operation once the sun went down or the light source was turned off.

    DAB

    It will, Don.

    - while there's enough light, the kit will deliver power to your design and charge the cap.

    - then the cap takes over.

    - only when you've consumed that energy to a certain level and the light hasn't returne yet:

    - the coin cell will help.

     

    What part of the review is your assessment based on?

  • Nice update Jan.

     

    From my interpretation of your results, it does not look like this configuration would capture enough energy to sustain operation once the sun went down or the light source was turned off.

    DAB