A while ago. I realized that I got quite a few solar panels laying around
that are not being used for anything
Due to the nature of solar panels, it is obviously not possible though
To directly connect them to projects and thus use them as power sources
They're constantly altering output voltage and current due to different solar radiations is the reason for that
But feel free to watch my basics video about solar panels to better understand their working behaviour
The solution to this problem would be a circuit that uses a solar panel to charge up a Lipo or Lithium-Ion battery
Which we can then use as a power source for our projects
But that such a circuit actually exists was my initial question
While searching for it on eBay and the answer is yes such a circuit does apparently exist
The only problem is that after I did a couple of tests with its I realized that it is not perfect for my application
So in this video, I'm going to ask the age-old question once again
Does it make sense to DIY such a solar battery charger, or should we just buy the commercial solution instead?
Let's find out
[2011 Lookalike by Bartlebeats playing]
This video is sponsored by JLCPCB whose service I utilize to order 10 PCB's for this project
For only $2. They were produced in 24 hours and shipped to Germany in only three days
So feel free to give JLCPCB a shot by uploading your gerber files today
First off, let's take a closer look at the Buy option
At first sight, the board quality, as well as all the solder joints seem pretty decent
The only thing I subjectively disliked were the JST connectors
But thankfully the board offers breakout points for the solar and battery connections
The main IC of the board is the CN3065
constant current/constant voltage linear charger for single cell lithium ion and lithium polymer batteries
Say that three times fast
After inspecting its pin description, as well as its detailed description in the data sheets
It seemed like a suitable choice for small solar panels
that deliver 4.4 volts to 6 volts
Thankfully I got such solar panels laying around
Like this 110 by 60 millimeter one that can do 6 volts and up to 1 watts. So
So for testing purposes, I directly connected the solar panel to the PCB
through two thin wires and
As you can see by shining light on the panel, the solar charger does react to it,
which was a good sign
next it was time to find a test battery which ultimately turned out to be a
1200 milliamp-hour Lipo battery
But right before I was done soldering it to the PCB
I realized that the solar charger IC here
only features an over-voltage protection during charging
But misses and over-discharge protection and more importantly a short-circuit protection
This is especially important since the battery voltage is simply bridged over to the SYS OUT JST connector
Which means we could easily short the battery here if we make a mistake
So to protect the Lipo for now,
I added an additional protection circuit to it
before hooking it up to the solar charger
At this point I wanted to test the circuit setup outside but surprise
it is December and the sun did not feel like working with me
As an alternative I had to create my own sun with an LED panel
After attaching the solar panel to its with duct tape and cranking up the brightness
The charge LED turned on and the battery started charging move around to 27.5 milliamps
After then measuring the input current at the solar panel side
as well as its voltage and the battery charge voltage
I came to the conclusion that I was dealing with an efficiency of around 70% at the time
Since this circuit works in a linear fashion and does not utilize
MPPT for the solar cell, that is definitely acceptable and
Because the circuit had no problem reactivating the charging
while the solar radiation strength was oscillating a bit
I have to say that the circuit is definitely usable for small projects
But then again it got not sufficient protection features for the battery
No boost converter to create stable 5V for your projects
and no MPPT feature for the solar panel
So naturally, those were the goals for my DIY solution
Which is why I visited the website of Consonance Electronics next
Which are in fact the manufacturers of the CN3065 IC
There I found out that they got a whole section with solar panel powered charger IC's
After comparing all of those, I decided on the CN3791
Which works with a single cell as well
But uses a switching technique which should guarantee a higher efficiency at higher input voltages
Works with up to 28 volts solar panels charges with up to four amps and features MPPT functionality
Along side its other features which were mentioned in the data sheets. It seemed like a perfect fit and
Luckily, the IC was also easily available
So I used the typical applications circuit of the data sheet as a reference
to create my own schematic with the Easy EDA software
The datasheet also provided guidelines for the capacitor selection
as well as the inductor, MOSFETs and diode selection
Which was certainly helpful while creating the schematic
the only thing which I was uncertain about at this point were the resistors R8 and R9
Which are used for setting the MPPT voltage but more about that later
Once the solar charge IC schematic part was complete
I simply opened up my old Lipo charge protect boost project and added the protect/
boost components to my new Solar Charger schematic and
Through this simple addition. I added an overcharge
over-discharge and over current protection
as well as a 5 volt boosted output
Which means that all my goals for my DIY solution were fulfilled
Next I clicked the convert to PCB button
and started arranging all the components in a logical and functional order
After then creating all the traces and turning the top and bottom layer into a ground top layer
The boards did not look half bad
So I exported the gerber files of it
and then ordered the boards for pretty cheap
Just like I mentioned it earlier from JLCPCB
Alongside, I also ordered all the mandatory components for the board and
after waiting for a few days
I was not only greeted with a ton of components but also the PCBs
which made a good first impression under the microscope
That means it was time to solder the teeny-tiny components to the PCB
For which I mainly utilized a soldering iron with a very fine tip
as well as a bit of additional flux and solder
The whole process took me around 2 hours
and to my own surprise did not turn out that badly this time
At least in my opinion
Anyway, after soldering all the SMD components
I added the through-hole components and just like that
My DIY solution was complete, but did it work?
To find that out. I firstly hooked up my lipo battery
this time though without the protection circuits and
After flipping the power switch the output of the board
delivered 5.13 volts. Perfect.
Through the help of the USB constant loads and my lab bench power supply,
I then calculated the efficiency of the boost converter
which turned out to be at a maximum of 81 percent
and features a maximum current draw of eight hundred and seventy milliamps
Afterwards, I also tested all the protection features which worked just fine
Now for the solar panel tests,
I did not feel like working with such a tiny panel
instead I went with this 10 watt one,
which features a maximum voltage of 21 volts,
a maximum current of 0.61 amps and an MPPT voltage of 18 volts
This voltage is important for our DIY board
Since the MPPT pin of the IC
needs to be at a voltage of 1.205 volts
when those 18 volts are applied to the resistor network
So after a bit of calculating I got the required resistor values
and thus soldered them in place on the PCB
Once that was done. I hooked up my solar panel
to the boards and positioned it very professionally
next to my LED panel
After cranking up the brightness to the maximum.
The circuits came to life and charged up the battery successfully and
Yes, you can charge it while outputting a constant 5 volts for your projects
but after measuring the input power
/output power, it seemed like the efficiency of the circuits was still a bit lacking to say the least
As a result, I went outside in order to find some proper sunlight
while monitoring a couple of important values
But no matter how much I tried I never got any decent results
due to inconsistent solar radiation
But what I can say from the outdoor test,
is that the MPPT tracking seems to function correctly
even though the set voltage seems to be a bit higher and
Luckily, I was able to perform a successful indoor efficiency test
later on which turned out to be around 62 percent
which was acceptable considering the terrible experiment conditions
So all in all I'm pretty happy with my DIY solution
Since it adds all the important missing functions of the commercial products
While still being versatile due to its wide voltage input range
The only negative things are they spent time for soldering and the money aspects?
But for me that is definitely worth it,
which is why DIY is this time the winner
But what do you think?
Let me know in the comment section below
As always, thanks for watching. Don't forget to Like, share, subscribe
Stay creative, and I will see you next time!
