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.


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.

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.

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:


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