﻿Today I will show you a project with several aspects. Even if you do not need to charge
batteries with voltages of 35 volts and above using a solar panel, you will find learnings for
your projects. Or you enjoy that not only you encounter some difficulties in your projects.
Grüezi YouTubers. Here is the guy with the Swiss accent. With a new episode and fresh ideas around
sensors and microcontrollers. Remember: If you subscribe, you will always sit in the first row.
My wife's sister asked me if I could build her a solar charger for her bicycle batteries.
Without thinking and without having a concept in mind, I said: Yes, I can. This should not be too
complicated. Right? Back then, I did not know that this project would take more than a year.
First, I made a list of tasks: 1. I had to find the appropriate
plug for the battery 2. I had to hack the battery
interface. Because it did not charge when I only applied a voltage to the two thick pins
3. I had to find a solar panel strong enough to charge the 635Wh battery in less than a day
4. I had to boost the voltage of the solar panel to the level needed to charge the 36V battery
The plug is from HiGo, and after a lot of searching, I found their distributor
in Belgium. They sent me the “minimum” number of 5 connectors, including short cables. I do
not want to tell you how much this costed, including shipping. After all, it is for
my wife's sister and, therefore, essential. Point one was solved in less than a month.
After doing some measurements, I found that the battery started charging when I applied 5 volts
to this small control pin and that this pin is connected to the green wire. And the maximum
voltage of the original charger is 42 volts. Cool! Point two is solved. This was fast.
For the solar panel, I looked at these flexible and portable panels. They are ok, but their
price/performance is not good. So I decided on a standard panel with an aluminum frame.
The panel will be stationary, and its weight is an advantage, as we will later see. So I went for
a 100W peak panel of eBay. It should be able to charge a not fully flat battery in less than one
day, even if there is not always sun. 635Wh/100W = 6-7 hours of solid sun. Point 3 is solved, too.
Then I came to point 4. How would you solve this problem? First, I thought: Easy, I use a cheap
boost converter and boost the roughly 20 volts to the 42 volts needed by the battery. As you see,
this works fine on the bench. I even added this lovely volt and ampere meter. After adding a 5V
regulator to create the voltage required for the control pin, I was ready for a test in the sun.
I connected this converter to the input because its maximum input voltage is only 24 volts,
not the 42V of the converter's output. Unfortunately, it did not work at all if
connected to the solar panel. Do you know why? Because the battery has a low internal resistance,
it acts like a short. And what does the boost converter do if its output is nearly shorted?
It tries to draw the current from the solar panel, of course. And the solar panel?
Unfortunately, solar panels have such a curve. If the current is reasonable, they keep their
voltage. But if it is a bit higher, the voltage drops rapidly till the booster switches off.
Then, the panel's voltage increases, and the whole thing starts over. So, what to do? I exchanged the
boost converter with one where I could limit the output current. Excellent! This works if we limit
the current to a level below this bend. But what happens if the solar radiation is a bit smaller?
Then, the curve is reduced, and we are in the wrong spot again till we adjust the potentiometer
to a new level. What we need is an MPP tracker. Because the project already was quite expensive, I
tried to go cheaply. And because I read some good reviews, I ordered this Solar charge controller.
It sometimes is sold as MPPT but usually as a PWM controller. At least it has “MCU control”.
I hoped it would produce “optimal” power at the output for my boost converter. When it arrived, I
discovered that it does not work without a battery to charge. Only connecting the boost converter
did not work. So I added the LiFePo4 battery I use for my transmitters. Now it worked. The controller
charged the battery, and the boost converter charged the bicycle battery. Exactly as planned!
I even was able to get good power. Do you see the next problem? Yes, you are right. This battery is
much smaller than the bicycle battery. When there is not enough sun, the small battery discharges
fast and switches off. Not exactly what I need. Also, this controller is not very good. It seems
it only “chops” the current from the panel to the battery. This is why they call it a “PMW”
or “pulse width modulated” controller. The effect: The panel's voltage is way below its maximum power
point, and its power is only around 50W. I am moving in the wrong direction! Only 50% of the
expected power, and because of the needed battery, the price of this setup is relatively high.
So I need a better solution. But how does it look like? And where to get it?
Before starting KiCAD to create my own design, I again searched AliExpress. After some time,
I found this device: An MPPT controller with a boost, not a buck converter.
These are pretty rare. After all my setbacks, I was very skeptical. Nevertheless, I had a promise
out, and my wife asked from time to time about the project's status. Of course, she did not want
to know the details, just a date when it would be ready. So I hit the “order” button. Because of the
new “15 days delivery time”, the controller came fast. And, because spring is here, also some sun
is available for testing. But one problem has to be solved: I need 5V for the small pin. As before,
I wanted to use the buck converter connected to the solar panel. But other than before,
the input and output ground of the boost converter is not connected. On the PCB of the 5V converter,
however, ground on the input and ground on the output is connected. So my small converter would
shorten the two grounds and probably draw a high current. Not exactly what I want.
As mentioned before, the maximum input voltage of this 5V converter is only 24v.
So I cannot solve the problem by connecting it to the output. So, what to do? Again, not easy.
I needed some insulation between the grounds. I remember buying very strange power supplies:
5V to 5V DC. I no longer remember why I bought them, but fortunately, Aliexpress keeps all my
orders in its database, and I could verify that I should have a few of them somewhere in my lab.
After searching a bit, I found them in one of my many boxes. I connect its input to the 5v of the
converter and the output to the control pin for the battery. Now I am sure the battery is
protected, and the converter is not shortened. After adding some heat-shrink tubes,
everything is ready for a test. I connect the solar panel to the converter, and really,
it shows around 20 volts input voltage. The display does not flicker in reality, BTW. It
is because of an interaction with the camera. I can adjust the output voltage to the needed
42 volts. We are on the right track! But what happens now? The output voltage is 0V.
Only if I press the buttons, it is 42 volts again. But only for a few seconds.
Not what I expected. It seems I got a defective device, and all my hopes are gone!
This is the hard life of an engineer. And the even harder life of a husband who must tell his
wife that she has to tell her sister that the man she married is not as helpful as expected.
But before I give up and file a dispute on AliExpress, I connect the bicycle battery.
Look what happened! The voltage is around 40 volts, as expected from a half-empty battery.
And even better, currents are flowing. So the battery is charged. Not too much,
but it is early in the morning. So maybe my reputation is saved. And at least in low
sunlight, the converter behaves much better than all my former designs.
So let’s wait till noon. Now we have nearly full sun. Look at that.
The voltage is almost 20 volts, and more than 5A flows into the charger. Nearly 100 watts. And it
is only April. Cool! Like that, the battery will be charged till the evening. Exactly as promised.
And really, after a few hours, the battery switched off charging. Is it really full?
Let’s check with the bike. Yes, it reached nearly 100%. The missing few percents are not
the responsibility of my charger; the battery does it also when charged with the original charger.
After all the setbacks, I am happy with this converter. It does what I expected. It varies
the input voltage all the time to find the sweet spot for maximum power, as you see here.
With minimal sun, it regularly starts at around 15 volts and searches for the best point. The output
voltage is precisely what the battery needs, but there is hardly any current flowing because
the panel cannot deliver. This is proof that it is also stable at the lowest radiation levels.
But I must add two things to the to-do list: The controller must be protected against rain.
And more important, against thieves. Frequent viewers of this channel know precisely why.
But this is work for the partner of the sister of my wife. He is a mechanic and
much better suited for such problems. And he probably also wants to prove his usability…
So finally, this story has a happy ending.
My wife is happy, and I can continue with my next project.
This was all for today. As always, you find all the relevant links in the description.
I hope this video was useful or at least interesting for you. If true,
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