I’ve been investigating solar power options for some external monitoring devices, so I invested in three (3) “5V 1W” solar panels from eBay, the amazing price of $1.69 AUD each (around $1.20 USD or £1 UK), not really knowing what to expect.
When the panels arrived, the first thing I noticed as that the solar collector on each of them, didn’t quite look the same.
Also, one of them had distinct bulges of what appears to be excess resin, down the center line of the panel
Nevertheless, I connected them to my trusty analog multi-meter, so that I could easily test both the voltage and current output from the panels (but not voltage and current at the same time)
In the bright Australian sun, all the panels produced just over 6V when offload, and produced in excess of 250mA when the meter was switched to its current setting.
This was a promising start, but of course the panels would not be producing 250mA at 6V, as that would be 1.5W not 1W as claimed in the eBay listing.
To get an idea of the actual amount of power (W) that these panels are capable of producing, you need to apply an optimum value of load, so that output voltage x output current = maximum wattage.
In commercial solar controllers, this load is provided my a “Maximum power point tracking” controller https://en.wikipedia.org/wiki/Maximum_power_point_tracking which optimises the amount of current being drawn from a panel to produce the maximum power,
But as I don’t have a MPPT controller capable of operating on a 5V 1W panel, the next best thing is to apply various load resistors to find when the output voltage drops to its specified nominal output of 5V.
Using this method, I found the optimum resistance that still produced 5V from the panel was somewhere around 30 ohms. (33 ohms gave just above 5V and 27 ohms gave just below 5V)
Hence, I’ll assume that in ideal conditions that a 27 ohm resistor in parallel with the panel would give 5V output, and hence a current of 185mA and total power of 0.92W
This is a much better result than I expected for a panel in this price range, so I’m happy with this purchase and may buy a few more of the same type of panel.
Of course this is really just half the story in terms of powering a microcontroller with transmitter etc, so the next stage in this project will be to determine the best way to charge a 3.7V LiPo cell using this panel, so I will post again when I’ve sound a solution to this problem.
Thanks, can’t wait for the followup
I did some research and it may be possible to use a sub $1 LiPo charger module, which uses a TP4056 to manage charging a LiPo from the solar panel.
I found this schematic
Basically, the TP4056 has a charge current control pin (PROG) to which a resistor to ground is connected. The higher the resistance the less current.
Normally the modules use something like 1.5k which gives a charge current of 1000mA.
So potentially this pin can be used to control the charge current to optimise the solar panel output voltage to 5V
This would not give a true “tracking” charge controller, but it may hopefully be better than just fixing the charge current at some low value that the panel can normal sustain when in full sunlight.
Probably the first step to determining the possible range of charge currents are to remove the existing R-PROG from one of those modules and it a 500k pot
The datasheet for the TP4056 has a table showing charge current vs R-PROG, and doing some rough mathematics the current in mA seems to be 1200000/ R , i.e so that if R is 1200 ohms (1.2k) then the charge current is 1200000 / 1200 = 1000 mA
This seems to hold true for most of the range, but on high resistances this coefficient seems to increase to 1300000 at 10k, so I have to assume that it probably increases even further at 100k e.g. perhaps to 2000.
So potentially 100k would yield around 20mA.
But I will need to do some practical tests to confirm this.
Then I’ll need probably need to somehow model the schematic with the 2 transistors in LTSpice.
So there is still a lot of research to do
BTW. It is possible to buy Solar lipo charge controllers. But AFIK there aren’t any cheap modules that do this.
Adafruit claim to have a module but its $20, and also it just appears to be a Lipo charger not really a solar tracking controller.
There is a IC that does this, (I think it may be my Linear Technology), but its $10 just for the chip, which makes it far to expensive for me.
So I’m hoping that a sub $1 TP4056 module and some 10 cent transistors may do a good enough job