Solar power advice re the 1w solar panel and solar charger

Hi Guys,

I am hoping some clearly soul might be able to assist me

I interested in building a little Solar powered/charging esp8266 running off a 3.7v lipo. I am fine working from the charger to the esp8266 but scratching head on the solar to charger side.

I have the following 2000mAh lipo…

This little solar panel seemed cheap and cheerful enough…

A few reviews said it paired nicely with this solar charger …

The issue I see is the panel can push up to 10v which means if I plugged it into that charger it could push 10v into that charger which according to specs likes up to 6v solar in (even its bigger sunflower brother also only accepts up to 6v solar in). This surely isn’t good right?

Just wondering…

  1. Will it work anyway (ie can the controller live with 10v OC)
  2. do I need to put something between the panel and charger to limit the voltage (if yes, is there a recommended product?)
  3. is there a better solution if this isn’t a great combo? (eg better panel regulated or charger with wider input)

Cheers in advance,

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Some years ago I purchased these items to do solar panel testing for charging a powerbank.

Solar Lipo Charger (3.7V) SKU: DFR0264
Sunflower: Solar Power Manager 5V SKU: DFR0559
Solar Panel – 1W 80 x 100 mm (Seeed Studio)
Solar Panel – 2W 80 x 180 mm (Seeed Studio)

From my notes the solar panel voltages were within specs for the chargers. I never measured anything near what is mentioned on the web page (8.2V). With nothing connected the highest I could get the 2W panel to was 6.78V which decreased to 6.3V after 10 minutes in the sun. The angle to the sun is very important, a small amount of shade on the panel significantly reduced its output. When attached to the charger the voltage did not go over 5.5V.

I tested both panels with both chargers in bright sunlight, summer day.
The rated current out was never reached, even under optimum angle to the sun.
(1W 140mA, 2W 240mA, 80% of rated amount)

The Sunflower with only a solar panel and powerbank attached was very inefficient.
150mA charging current for 231mA input from solar panel.

The Sunflower worked better with a LiPo attached. It is designed to power a USB device with the solar panel topping up the LiPo during the day. Even so the USB current drain needs to be less than the charge current from the solar panel or the LiPo would never charge.

I had better success with a 20W solar panel mounted on my house roof connected to a switch mode power supply producing 5V. This currently powers a radio and is used to charge LiPo’s & powerbanks as needed. The O/C voltage of the solar panel is 17V. The maximum charging current from the supply is about 1.3A.



Hi Steve,

That solar LiPo charger is capable of charging your LiPo battery, albeit slowly, but it does require the battery to be disconnected from your project in order to charge.

If you did want to be able to charge the battery and run the project at the same time you’ll need a solar power manager like Jim has mentioned.
The Sunflower: Solar Power Manager 5V and the Semi Flexible Monocrystalline Solar Panel (6V 1A) are recommended as a good pairing by the manufacturer.

Solar panels are often quoted assuming laboratory testing conditions (an unrealistic amount of sunlight) this is great for allowing an apples-to-apples comparison of solar panels, but does mean you need to factor in extra solar power because the real world output is always lower than the amount on the sticker.


In other discussions about solar panels, someone stated the standard test is a specified lamp and intensity at a certain distance from the panel. This allows manufactures to give a specification which is comparable to other manufactures. It does not represent real world situations.

Regarding the Sunflower, my tests have shown whatever device is being powered needs to consume less than the output of the solar panel, otherwise the battery will not charge. Switching off or removing the device to charge the battery is the best option.

Tests of the DFRobot Solar Lipo Charger (3.7V) showed it needs about 5V from the panel to full charge a battery. There is a diode in series with the input, the voltage drop on this means the panel output needs to be optimum. The charger easily falls out of charge when the panel falls below 5V.

As I said I had better results with a higher voltage higher wattage panel and charger that could accept the higher voltage.



I’d go with a 20W and Sunflower setup for a few reasons.

In the absence of any data on your application and the power usage of the ESP8266, you have to use some ‘rule of thumb’. Searches will estimate that an ESS8266 board will average 70mA(plus) at 5V = 350mW, 350mW for 24 hours is 8.4Wh. Your battery is rated at 2Ah at 3.7 V (=7.4 Wh), so you are a bit behind the eight ball to start.
If you start out with a fully charged battery and get 6-8 hours of ‘good sun’ in the day a 20W panel should give you sufficient to charge the battery and run the ESP, but not if you get a few cloudy/wet days like there have been this year.
From experience, a 12V 12AH Gell cell dies after 3 days with a minimal sunlight this winter in a setup I was running to monitor my beehive and a lithium will perform better than that since it can deliver much more of it’s capacity than a Gell cell.

Also regarding solar panels, they produce maximum current at 0 Volts (i.e. Zero power out) and maximum voltage at 0 amps (again zero power). Solar chargers try to maximise the power available by controlling the (apparent) voltage the panel is working at to get a reasonable(useful) current. This is why you’s see MPPT (Maximum Power Point Tracking) in more sophisticated controllers. The Seeed Studio device is not that ‘clever’, and the complexity is not warranted here, but don’t expect the full 20W from your panel.

Jim, efficiency is more about the power not just the current, there would be fair difference between the input and output voltage(s).

Give it a go and report back so we can all learn.



Hi Dave and Steve
A little point that is often not considered is that every time you convert voltages there is a bit of a loss. Some (but not all unfortunately) manufacturers of these conversion devices publish the efficiency figures and a common number is 85% although some devices claim 95%. Chargers the same, if you want 1W of charging power you have to input somewhat more than 1W of input power.

Steve: So you can you can see you are a bit further behind the 8 ball than Daves estimate.

I am not going to research it again but I think that Sunflower controller, charger, conversion device do publish the individual process conversion factors.
Cheers Bob


Thanks everyone for your feedback. By sounds of it I will start with a bigger panel (10w), the Sunflower solar charger and see how we go. I also have a few different batteries so might test this as well :slight_smile: Once I have it up and running I’ll report back

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That’s the way, give it a go with what you’ve got, try some modifications, experiment. :grinning:

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Hi Steve,

Sounds like you’ve got a plan for the initial experiments worked out, please let us know how you get on :slight_smile:
Thanks @James46717 @dave50358 and @Robert93820 for sharing your advice, I think this project will be a real winner after its refined through some test runs.

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