I built a few very low SWaP (size, weight, and power) LoRaWAN sensors to be installed in a forest like area and now exploring solar solutions. I am hoping to hear experiences and solutions used for similar projects.
My sensor's worse case power consumption is 20mW continuous and, according to a generic rule of thumb, a panel wattage rating needs to be ~20x the continuous load. 20mW * 20 = 400mW. However, this is for direct sunlight, facing south, and at an angle (~30 degrees), but forest conditions will have minimal if any direct sunlight.
Using two SM141K05L solar cells (154mW rating x 2 = 308mW total rating) I measure 261mW draw from the solar cells in direct sunlight from an E-Peas AEM10941 energy harvester, which is about 85% of the solar cell's rating, not bad right? The AEM10941 looks to have a max charge current of 110mA and using an INA228 power sensor the charging current draw was about 85mA, so I am assuming the 261mW power provided is the best that can occur due to my location, daylight conditions, and solar cell.
In shade the same setup draws 5mW of charging current which is 1.72% of the solar cell's power rating, which is disappointingly low. Is 10% power production the rule of thumb for overcast days? If 1.72% production of rated solar cells is the best case, how much solar cell is needed? Topping off the battery requires ~60mWh for eight hours (a guess) which is: 60mW/0.0172 = 3488mW! (too large for my small sensors)
Similar results were also found using SPV1050 an BQ25570 solar charge controllers and similar panels.
My questions:
Is the low 1.72% power produced of the solar cell rating normal for shaded areas?
Is there a clear choice between energy harvesters: AEM10941, SPV1050, or BQ25570? In terms of charging capability regardless of other features like output DC-DC.
Are there other chip sets I should consider for low-power setups like this one?
Any other thoughts?
Thanks!