I have two solar panels charging a 18650 battery using a TP4056.
The orange LED is lit when the solar panel voltage is 4.1 (not full sunlight but fairly bright sky). The spec for the charger module is minimum 4.5V. Is the battery actually being charged?
I have an ESP8266 which is drawing a little more power than the solar panel can provide, meaning that after a few days the project stops.
Would the addition of another solar panel help?
Any ideas would be appreciated.
If your panels are only reaching 4.1V and the spec says 4.5 then the module is unlikely to actually charge the battery, even if the orange LED is on—it may just indicate “power present” rather than “charging.”
Adding another panel in series would increase the voltage, which could bring it above 4.5V and allow charging. Adding a panel in parallel would increase current but not voltage, so if the voltage is the limiting factor, that alone won’t help.
Since your ESP8266 draws more than the panels provide, even with proper charging, the battery may still eventually discharge. You’d need either more panel capacity (higher voltage and/or more current) or to reduce your circuit’s power draw to maintain the charge.
Yes but it will never fully charge.
You need to provide 5V to the TP4056 for optimum performance.
What is the power output and voltage of the panels?
What is the mAh rating of the battery?
Does the ESP go to sleep?
Thanks so much for the quick response.
The panels are 2 X Mini Solar Panel Cell 5V 230mA 1.15W.
The battery is rated at 3500mAh
The esp doesn’t use any sleep mode, I want to continuously monitor wind speed.
From your advice it seems clear that the solar panels aren’t up to the job.
I guess that 2 of these panels in series would provide too high a voltage.
Thanks
Yes.
Also, in general it's not a good idea to use the battery at the same time it is being charged. It may never completely charge and/or may be continually charged which is not a good idea
Have a look at this:
Is that open circuit or connected.
That voltage seems normal when the battery is not fully charged yet.
(remember that a solar panel is a current source, not just a voltage source).
When the battery voltage increases with charge, so will solar panel voltage.
A 5volt panel normally reaches about 6volt in bright light, maybe a bit more in sunlight.
A third panel in parallel will increase current and thus also lift voltage.
Leo..
Maybe you should sleep the WiFi part during the night.
Leo..
I use Adafruit Universal USB / DC / Solar Lithium Ion/Polymer charger - a good feature is that Solar is prioritized over battery power when available, to keep the battery from cycling.
4.1V when connected
You need at least 4.2V to fully charge the battery.
4.2 out of the module and thus a bit more for the module itself, I guess hence the 4.5 of the spec.
If the LED is lit, then the panels are supplying current. You can confirm this by disconnecting the panels from the charger and measuring their open circuit voltage, which will be higher than the voltage when they are connected. The panels will supply as much current as they can at the minimum voltage needed to charge the battery at its present charge state. However, if they can provide more current than the charger is set to use, then the panel voltage will go up in bright light.
It would be good to know what range of current is drawn by your load circuit. And is the charge current resistor on the TP4056 the default 1.2K (for 1A charge current)?
Maybe someone here can clear up something for me. I'm wondering how much headroom (dropout voltage) the buck converter needs to produce 3V. What value in the datasheet tells me that?
Thanks, ESP8266 should be taking about 35mA most of the time and the voltage divider is taking about 0.3mA.
Don’t know what the TP4056 charge current resistor is at the moment.
I think you mean 28µA
3.7V / (100K + 32K) = 28µA
Don’t know what the TP4056 charge current resistor is at the moment.
Does not matter since you don't need it if you use one of the MPPT chargers that were suggested.
ESP8266 should be taking about 35mA most of the time
Probably much higher (maybe 170mA) if you are continually sending data.
Hi,
Sorry but I think what most people assumes is that a PV is like a battery, it isn't, a PV has to be treated as a CURRENT source.
TP4056 is not designed to operate of a current source with varying potential.
Nowhere does it say PV regulator.
The devices indicated by @J-M-L and @jim-p are designed to reliably do the job.
Tom.... 
I’ll start looking into a suitable MPPT Solar Charger.
I originally came across this article Power ESP32/ESP8266 with Solar Panels and Battery | Random Nerd Tutorials
and since I had the components in stock it seems worth pursuing.
Thanks again
That's all good if you panels can output 5-6V like in that article.
No, but it will. The TP4056 needs 4V per the datasheet, or 30mV above the battery voltage, whichever is higher, to charge the battery. If the panel's open circuit voltage at the current illumination level is at least that high, then it will supply as much current as it can at the needed voltage. So I think the original circuit should work ok so long as the panels can supply enough current. If they can't, MMPT isn't going to make much difference. You need a third panel. At least.
Also, if your battery is really 3500mAh, then the charging resistor can be set to provide the full 1A maximum for the TP4056, which would set the termination current at 100mA. Unless you are constantly transmitting, your load current should be low enough often enough to allow proper termination. However, if your two solar panels can supply 460mA in bright light, but they can't keep up with your load current over time, then you might want to really pin down how much current you are actually drawing, and look for ways to reduce that.
All the ESP8266 boards I measured pulled about 100mA, and short burst of 400mA during WiFi trafic. You can only reduce that by turning off the WiFi radio.
Leo..
