Solar Panels

I have tested a range of solar panels, from a 136×110mm, 6V, 330mA unit to a 27×11mm, 5.5V, 30mA one. In the event, in my environment, a 5.5V, 70mA panel (85×35mm), paired with either a 14500 (AA) or 18650 Li-Ion or LiFePO₄ battery, keeps a battery at full charge while sending messages every 60 seconds from Arduino Pro Mini, CubeCell [Plus] and ESP32 processor configurations. I haven't tested battery life in any ESP-12 configurations to date, but I wouldn't expect them to be any more power-hungry than an ESP32.

Solar Panel Selection

On the CubeCell Dev-Board Plus configurations I have tested, a 40mA panel seemed to be too small to maintain the charge in the battery. In an effort to find the cut-over point, I have now tested a 5V, 60mA (67.2×34.7mm) panel, which kept the battery in the Weather Station Node fully charged, and I am now in the process of testing a 5V, 50mA (60×44mm) panel, which, to date, has also kept the battery fully charged, although it has not yet endured an extended period of heavily overcast conditions.

Of significance here is that while the 70mA panel fits comfortably in a 100×68 enclosure, it will not fit in an 85×58 enclosure. The 60mA panel will fit in the smaller enclosure, but is just a little too long to comfortably fit on our 85×58 PCBs. The 50mA panel is just a little too wide for these PCBs. The potential saviour for the smaller enclosure may be a 2 × 5V, 30mA (44×24mm) panel configuration, which should both fit comfortably on our 85×58 PCBs and provide enough power to maintain battery charge. The results noted above, using a 60mA panel, seem to suggest that this will be fine, but I will report here as soon as I have been able to confirm that this configuration is viable.

70mA	60mA	50mA	40mA	2×30mA

'Short Listed' Solar Panels

I have, nonetheless, had two CubeCell Dev-Board Plus Nodes operating in the open, exposed to all weather conditions, transmitting every 60 seconds for more than 2 years, and with a [5V] 70mA, 60mA or 50mA solar panel the batteries remain fully charged. The longest period of heavily overcast conditions that I noted during winter was four days. Even then the battery level barely got below 4.0V (some charging occurs in daylight hours, even in these conditions) and was fully recharged within hours of seeing some genuine sunshine.

Since I live in an area that boasts blue skies and sunshine for a significant proportion of the year, however, a configuration that works well here may not suit applications in other geographies. That might compromise the packaging of the overall solution, but should only require the configuration of the larger battery or a more appropriately sized solar panel.

Mounting Solar Panels

When I began my project, I assumed that I would mount my solar panel on the outside of the enclosure I used to house the processor and associated electronics. All of the configurations I had seen at that point used some form of 'external' solar panel arrangement and I was concerned that housing a panel inside an enclosure, with sunlight filtered by the enclosure cover, even if it were clear perspex, might reduce its efficiency.

In practice, the perspex covers on the enclosures I have used have little or no impact on the function of the solar panel. In this context then, two PCBs, generally used as part of a board stack, have been developed specifically for mounting solar panels—the 8558-SP for use within the smaller, 85 × 58 enclosure and the 10068-SP for use within the larger, 100 × 68 enclosures.

8558-SP	10068-SP

Solar Panel Support PCBs

It was more of a surprise to discover that even perspex covers that had been badly discoloured by extended [2 years] exposure to sunlight and general outdoor weather conditions still had no noticeable impact on the charging capacity of an enclosed solar panel.

Ageing Perspex Enclosure Cover

Further content pending...