Solar powered weather station - Page 1
Forum section: electronics / projects

author: Axk
author_rank: Regular Contributor
timestamp: August 19, 2019, 10:11:50 pm
content: Looking at how easy it is to power a small basic "weather station" (hama ews-151, a thermistor with a 433MHz transmitter really) with a small solar panel from aliexpress.Out of the box the transmitter is powered with 2 AAA batteries and would work more than 4 months on these.The panel is a small 4x4cm thing:https://www.aliexpress.com/item/32814927225.html?spm=a2g0s.9042311.0.0.468d4c4dT9MHz0(I can go with a slightly bigger one if necessary).Assuming the panel would produce 1/10 of the specified power (140mW/10) on an overcast days it should still be enough to power the sensor/transmitter.The question is how difficult it is to hook this up to a small 100mAh 3.7V li-po battery for energy storage and power the transmitter from this setup.My understanding is that the panel's voltage would go down to 0.2V on overcast days (from its max of 2V) so the boost converter/charger should allow input voltages down to 0.2V.Then I'll need to charge the battery and power the device at the same time during daytime.I'm wondering if there's a standard chip/module for this sort of job.

author: james_s
author_rank: Super Contributor
timestamp: August 20, 2019, 12:46:01 am
content: I think for what you're doing just a diode in series with the panel connected straight across a pair of NiMH AA's will work fine.The voltage out of a solar panel is actually fairly constant once any significant amount of light is on it. It's the current it can deliver that varies with intensity.

author: Axk
author_rank: Regular Contributor
timestamp: August 20, 2019, 11:05:06 pm
content: So in my understanding the 2 series-connected NiMH batteries can be roughly modeled by a voltage source in series with a small (100mOhm) (internal) resistance.I suppose I'd need a 3V panel which with a 0.1V diode would give me around 2.9V or 1.45V max per cell.Given that the panel can provide enough current the charging voltage should "stabilize" somewhere between 2.9 and the 2 cells' combined voltage (1.2V or lower), considering the low current capabilities of these small panels it should be very close to the cells' voltage.Is my reasoning correct?

author: Axk
author_rank: Regular Contributor
timestamp: August 20, 2019, 11:08:54 pm
content: I suppose that my problem can be that if I have a very low charging current that could be not enough to overcome the NiMH batteries' self-discharge current.

author: Axk
author_rank: Regular Contributor
timestamp: September 08, 2019, 11:14:42 am
content: With the panels from Aliexpress (2 and 3V small guys) the voltage is quite illumination-dependent.It would only give the rated voltage in direct sunlight.Gonna try 2 panels in seriers connected to 2 NiMH batteries through a 0.2V drop diode when the batteries arrive.

author: Dmeads
author_rank: Regular Contributor
timestamp: September 10, 2019, 03:28:38 am
content: Quote from: james_s on August 20, 2019, 12:46:01 amYa in my experience the voltage is pretty constant even on overcast days my 22V panel still gets about 20V, but the current is really low. Maybe the ones from Aliexpress have a crap clear coat or something as long as an induvidual cell in the panel isnt covered you should get close to the rated voltage. If an induvidual cell is covered it can severly decrease voltage especially if there are no bypass diodes. Maybe try getting a high voltage panel and using a buck converter?

author: floobydust
author_rank: Super Contributor
timestamp: September 10, 2019, 03:41:29 am
content: I took apart some Ikea solar garden lights, each one using AA or AAA NiMh battery.One (Solvinden pendant) uses 4 cells, and the other (Solvinden table lamp) uses 8 cells. That would be 2V or 4V solar array for a 1.2V battery plus Schottky diode.Using a much higher solar cell voltage seems to be for greater charging in dim light?

author: Axk
author_rank: Regular Contributor
timestamp: September 12, 2019, 10:54:44 am
content: Quote from: floobydust on September 10, 2019, 03:41:29 amCan you measure the charging currents on an overcast day and in direct sunlight?