I notice Core have several ESP32 boards with LiPo connections - but only the DFR1075 FireBeetle 2 ESP32 C6 IoT Development Board promotes connecting a solar panel directly.
I would prefer an ESP32-S3 and more flash/RAM, so looking at other boards - particularly the UM Feathers3 - and thinking how to recharge the LiPo from solar panel.
Am I correct in assuming I just need a decent quality 5V buck/boost or step up/down voltage regulator ?
Might the charging circuit on the ESP have trouble if the 5V in is not a constant current ?
Can I have a solar panel connected to 5V pin, and connect a powerbank to the USB-C when an emergency charge is required ?
Anything else I need to look out for ?
The 5V solar panel I am currently using outputs approx 4.3V when there is daylight - but current is close to zero for much of that time, with peak about 2.2A. I assume it has a regulator built-in
Problems are
- The 4.3V that it is outputting is noticeably short of the 5V expected by the ESP32’s 5V pin
- unfortunately I have recorded voltage peaks up to 6.3A
- actual current generated is very low most of the time, even when at 4.3V
Would it be worthwhile to add a 5V buck/boost to further regulate the output - or would that be too many inefficiencies ?
I note Core have a variety of POLOLU 5V Step-Up/Step-Down Voltage Regulators, such as S8V9F5 and S13V15F5, plus boards from DFRobot, Sparkfun and Adafruit with prices from $6.05 to $27.50. Which would you recommend ?
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Hey @Donald23173,
Having a look at the datasheet for the solar management chip on the FireBeetle 2 ESP32 C6 IoT Development Board it looks like you should be safe to directly input your solar into the VIN and GND pins.
The recommended input voltage is 4.5-6V as reported by the manufacturer of the board but the datasheet for the chip reports an absolute maximum rating of 6.5V.
While I wouldn’t recommend supplying the board with a constant voltage over 6V it seems safe to me to exceed this on a very rare occasion. This is the statement from the datasheet about this kind of thing:
"Stresses beyond those listed under ‘Absolute Maximum Ratings’ may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions above those indicated in the operational sections of the specifications is not implied. Exposure to Absolute Maximum Rating Conditions for extended periods may affect
device reliability."
I would interpret that to mean that you can push up to 6.5V, but they can’t promise it won’t have issues even if you keep this limitation in mind.
If you want to be safe however a 5V regulator could work for this. There are a fair few options for these ones that vary mainly depending on the input voltage they can handle and the maximum current output. If you are getting an average output of about 4V from the solar panel with a few peaks around 6V then I would suggest the following regulator.
As long as you are not expecting more than 1A of current from the solar panel, then this looks like this will give you the best value for your money (with expected efficiency of about 85%)
Hope this helps! 
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Thanks Samuel, but I did start by saying that this is the only ESP board that I am aware of (at least in Core’s catalog) that does advertise connecting a solar panel direct; and that I am interested in using a solar panel with one of the other boards (in particular the UM FeatherS3).
Thank you for the S8V9F5 recommendation.
Since this will be used to charge the LiPo 1A seems more than enough. I assume the voltage regulator and ESP32 charging circuitry will only draw as much current as they can handle … so it’s more about running the voltage regulator well within its spec so it doesn’t get too hot.
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Hi Donald,
Its also worth mentioning that a regulator wont give the best efficiency from a solar panel, a dedicated solar IC should be used with Maximum-Power-Point-Tracking (MPPT).
On the Sparkfun page there is a good explanation of why MPPT is best if you intend to use a solar panel - Sunny Buddy Solar Charger V13 Hookup Guide - SparkFun Learn
It depends on the charging IC used, on the UM FeatherS3 the charging IC used is the
TP4065, all it wants is a constant power - which can be maximised through the MPPT IC,
I didn’t get far enough through the datasheet to find out if/how the chip would behave when there is not enough power to charge the battery.
Maybe? See above, though the 5V port will be more than sufficient to charge a battery, its just if the LiPo charge IC would enter some fail-safe state, where removing the battery/second input releases it, allowing the battery to be charged.
I believe the Sparkfun and Sunflower boards handle this quite well.
Liam
Add a capacitor (470µF+) on regulator output to buffer dips.
Check whether your ESP32 board has a protection diode on the 5V line — or add one yourself to prevent reverse feeding.
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Sorry Liam, I’m getting a little confused here with all the options and combinations…
I thought MPPT is for solar panels which are not already regulated.
When I was bench testing, the solar panel was facing the sunset through a grimy window. I noticed that the voltage jumped to 4.3V as soon as it was daylight despite there being miniscule voltage until late afternoon. I assumed my solar panel must have a voltage regulator built-in. There were only a couple of occasions when the voltage went higher (max 6.3V recorded) which I attributed to inconsistencies in the measuring.
Now that the solar panel is installed at my greenhouse and facing north, I am still seeing the voltage jump to 4.3V at dawn … but now electricity is generated most of the day, with the voltage regularly going higher. Is this typical of a solar panel without built-in voltage regulation ?
I can see that power profile may not be what many of these other ESP32 boards are expecting to recharge their connected LiPo batteries. I have bought a Polou 5V Step-up/step-down voltage regulator S8V9F5 and will try inserting that between my solar panel and INA3221 and DFRobot solar power manager.
The regulator on the solar panel is to prevent excessive voltage being output. This would be important in the case where the panel could provide a voltage above what the device it is powering can handle. A MPPT will usually have its own protection against excessive input voltage, or will have a maximum well above what the solar panel can output. So it is correct that you would not usually find both MPPT and a regulator, but if you did then that indicates that either the regulator is not needed or the MPPT is not suitable for the panel, and in either case it is an inefficient arrangement.
Hi Donald
Just curious.
What variety of solar panel is this. I have seen panels advertised as having a 5V regulated output, then there are those with a nominal 6V and open circuit voltage of 7V plus.
Another one is the type used for charging a LiPo cell in those proximity sensor type LED lamps like the one on my car port. This has a panel which outputs 4.2V which coincidentally is the maximum charging voltage for such a cell.
Your panel would not be one of these would it. The 4.3V is highly suspicious. If so it might not work very well. I think these boards which look after charging from solar/USB etc might be expecting 5V and I have no idea how they behave at 4.3V (or 4.2).
As far as regulation goes looking at your graph I would chuck it. Does not regulate very well. It looks like it is supposed to regulate to 4.2 (or 4.3) V as evidenced by the flat sections of the graph but the excursions above 6V are a bit of a worry. I do not think this would be normal and looks like any regulator is failing.
Cheers Bob
It’s one I got cheap off aliexpress several months back before i learnt that their sellers are not to be trusted.
I was looking for one that would output 5V to go into my DFRobot Solar Power Manager 5V board, and there is bugger-all choice at that voltage range 
I just looked, but the seller no longer has it online. From memory it was like
https://www.aliexpress.com/item/1005006834933447.html in that the product title says 20W but the body of the description mentions 4W and the seller sees no problem with that deception
Anyway, it let me get started, and I am rather surprised that the <0.2A output has actually kept my 6600mAh LiPo fully charged.
I was assuming that the output was regulated; but now that it is getting proper sunlight I don’t think it is. That was why I asked Is this typical of a solar panel without built-in voltage regulation ?
Still wondering whether it would be worth inserting the S8V9F5 voltage regulator.
Hi Donald
Looking at your graphs a lot of the outputs (both graphs) are very flat at about 4.2V which suggests a regulator. The spikes look like the regulator is not too good. The voltage (4.2 or 4.3) looks like a panel similar to the sensor light I mentioned above. As this is the charging voltage for a LiPo cell I would guess that it is meant to be connected directly to such a cell for charging.
Anyway to me this voltage is very suspicious. I would personally consign this to the nearest rubbish bin and cut losses.
Cheers Bob
