Arduino solar & supercapacitor - no battery

Thread Starter

sam_uk

Joined Jun 19, 2019
6
I'm looking to power these ESP32 boards on rooftops: https://www.aliexpress.com/item/4000396836096.html

They have a significant start up load, 70mA continuous, 200-300mA peak, so I'm looking to add a capacitor to help booting.

I'm thinking to use a 30w 18v solar panel.

To connect the panel to this 18v capacitor bank https://www.aliexpress.com/item/33010692724.html

And then power the device with this 12v >5v regulator that has low voltage disconnect to add hysteresis https://www.aliexpress.com/item/32854250040.html

Is this likely to work?
 

gramps

Joined Dec 8, 2014
80
The ESP32 needs 3.7v max. Looks like the psu outputs 5.0v. Not good. You'll need a 3.3v regulator. Is the ESP going to run 24/7 or boot up periodically, transmit data then sleep for a while?
 

Thread Starter

sam_uk

Joined Jun 19, 2019
6
These particular boards have a 5v micro usb input which is how they are powered.

The idea is that they boot shortly after sunrise and shutdown slightly before sunset.
 

gramps

Joined Dec 8, 2014
80
The device that you linked to appears to operate on 1.8v - 3.7v and has a TP4054 charger built in to it. Why not use that to charge a 18650 cell and a 5v solar panel to power it all. That just seems simpler to me.
 

Thread Starter

sam_uk

Joined Jun 19, 2019
6
Why not use that to charge a 18650 cell and a 5v solar panel to power it all. That just seems simpler to me.
It would certainly be easier in the short term. On a hot roof in a box I'd have to replace the batteries after a couple of years which could be significant effort if there are 20+ nodes spread around a city.
https://www.nature.com/articles/srep12967
Hoping capacitor will do 10+ years
 

oz93666

Joined Sep 7, 2010
736
It's not obvious that you need a capacitor ... max (noon) current from that panel is nearly 2A .... even on a very overcast day it should still deliver 200mA

Capacitor won't hold enough energy to operate device at night ...

I agree with gramps , go for lower voltage , it will reduce costs whether you go for batteries or caps or neither ...

Edit ... just seen your device operates on 3V start up 0.2A that's only 0.6W ....!!!

You only need a 3V 6W panel ...very cheap on EBay ... no need for cap or battery or regulator or anything !!

This is all about finding the right panel (voltage and Watts) ...experiment ..... a 6W panel will give 0.6W as sun is rising or setting

Your operating voltage is 1.8 to 3.7 .. a small panel with 3.5V open circuit is perfect ,they are available... just connect directly to your device
 
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Thread Starter

sam_uk

Joined Jun 19, 2019
6
I understand this won't work at night.

Without capacitors it can enter a state where there is not enough power to boot, but the power consumed crashes the panel voltage.

Between 70mA > 300mA where there is enough power to _operate_ the device, but not enough to provide the 300mA needed to _boot_ the device.

I have tried a 15w panel directly connected to the USB, and it does boot and run in sunlight or light cloud, It's insufficient for heavy cloud cover. https://ia601508.us.archive.org/20/items/solar_201912/solar.jpg

I also tried a supercap across the onboard charging circuit and it is better with that, but still not good enough off the 15w panel for gloomy days. https://archive.org/download/supercap/supercap.jpg
 

oz93666

Joined Sep 7, 2010
736
There seems no disadvantage to using a single cell LiFePO4 ... these have a lower voltage than Li-ion and a very flat discharge of 3.2V ( max charge 3.6)... realistically you will get well over 10years of use from one.
You must know the energy consumption per day in WHrs , then decide panel size from that .... A "1W panel" supplies on average 3.5WHrs/day ... on the very cloudiest day say 0.5W/day ... a small LiFePO4 (32650) holds 16WHrs ...
I have a large solar array and have never seen output below 10% (close to noon) on even the cloudiest day.

Again .... a 3.5V panel connected direct to LiFePO4, connected direct to device nothing else needed.... The battery will discharge very slightly at night into the panel , but too small a current to bother about.

Don't worry about life of LiFePO4 ... they are in practice very long lived I have many old ones ....

Panel should be cleaned regularly ... if not possible could loose perhaps 50% in 10Years , but this loss is very variable dependent on conditions .
 
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