Full project writeup is on my blog: Going solar: a maintenance-free redesign of my plant watering indicator

The system:

  • Harvester: AEM10941 - handles MPPT, cold-start, and storage management
  • Solar Cell: KXOB25-05X3F (31mW)
  • Storage: 110 mAh LiPo, used as a buffer rather than a primary source. The system rarely draws it more than a percent or two below full
  • MCU: STM32U031K6, programmed over SWD via a Tag-Connect TC2030 footprint
  • Sensing: the STM32U0's built-in TSC peripheral handles capacitive soil moisture measurement; the CPU stays in Sleep mode and is woken by the end-of-acquisition interrupt
  • Display: Ynvisible electrochromic bar-graph (semi-bistable, refreshes on the order of milliwatt-seconds)
  • Form factor: 110 × 20 mm 4-layer PCB, slim enough to slip into small pots

Measured performance:

Average current draw measured on a Nordic PPK2 over an hour: 1.2 µA. At that rate the LiPo alone (with no solar input at all) would take a little over ten years to run flat; long enough that self-discharge would empty it before the load did. In any usable indoor light the KXOB25 produces an order of magnitude or two more current than the load needs, so the cell stays effectively topped up.

Storage choice:
The default PCB configuration uses the AEM10941's dual-supercap preset with a tantalum polymer cap (not a supercap - but theoretically should have worked). Leakage currents and a limited voltage swing made it regularly brown out mid-display-refresh in anything but bright direct sun. I tried the LiFePO4 configuration next, with functional-but-fragile per-segment update strategy that took up to 35 minutes to refresh the display. In the end I bit the bullet and went with the LiPo as a buffer - bigger and more prone to degrade, but shallow-cycled enough that wear should be far too slow to matter, and orders of magnitude cheaper and smaller than a supercap stack of equivalent capacity.

Documentation
Full design write-up, energy-storage trade-offs, the supercap-vs-LiPo decision, board redesign, and a PPK2 capture of a single measurement cycle: Going solar: a maintenance-free redesign of my plant watering indicator