# How to use a 10W 6V solar panel to charge a 3S LiPo battery (11.1V)

I'm an experienced robotics engineer, but beginner when it comes to power distribution and especially solar power. For a project I need to charge a 3S LiPo battery with a 10W 6V solar panel. I've done some research on this topic, but a) could not find any suitable commercially available circuits that I can just buy, b) did not find any tutorials how to DIY such a boost charger myself.

The problem sounds simple to me:
One one side, there is a 6V solar panel that delivers variable power and on the other side, there is the battery that needs to be charged to full capacity at 12.6V.

So somehow, I need a circuit to

1. boost the voltage from 6V to 12.6V (or higher/lower?),
2. implement an MPPT (or simpler) algorithm as charge controller to extract most energy from the solar panel,
3. include a battery management system to ensure protection from overcharging and balanced charging.

For my specific case, I am interested in simplicity of the circuit, rather than getting most optimized results. I would like to buy as many components as possible off-the-shelf. And if I have to implement these circuit myself, can I use an Arduino to do so?

I would be grateful for any guidance, suggestions, or links to resources that could help me build this system effectively.

You can use a boost converter OR use 2 panels in series OR use a 12v panel.

Does your 3S battery have a balance connector on it?
Like this?
Is safety, reliability and battery life a concern or just circuit simplicity?

PWM chargers are simple but not as efficient as MPPT.
Solar panels are current devices. My 135w panels produce 7 amps at pretty much any output voltage. At it's Maximum Power Point in bright sun, it produces 135 watts. At a battery voltage of 12 volts, it produces only 84 watts. MPPT chargers keep the output power at 135 watts, no matter the battery voltage. Your choice.

A 6v panel should have a Maximum Power Point in bright sun of 8 volts.
A 12v panel should have a Maximum Power Point in bright sun of 17 volts.

Thanks a lot for your prompt responses. I appreciate your inputs, @SurferTim and @jim-p.

My solar panel is actually not one large 10W 6V solar panel, but rather 10 independent 1W 6V solar panels with all panels orientated differently. Unfortunately, the non-alignment of the panels is a project requirement. Currently, I connected the panels in parallel to form one large 10W panel. I could think of connecting pairs of two in series to get a 12V solar cell instead and then connect that solar cell directly to my 3S BMS, e.g. Amazon.com, but connecting the non-aligned panels would limit my current output of each pair to the less illuminated panels, right?

@jim-p: Yes, my LiPo has a balance connector. I already tried connecting the outputs of three 1S LiPo MPPT boards, TZT MPPT Solar Charge Controller 1A 4.2V 3.7V 18650 LiPo Li-ion Lithium Battery Charger Module SD05CRMA Solar Panel Battery Charging - Battery Protection Board - TUOZHANTENG HK co.,LTD, in series to charge each cell individually, but that didn't work, as the input and output share the same ground and I thus short-circuited the battery.

Reliability and battery life are no concern at all ( the circuit needs to work ones ).
Safety: It would be great if I don't overcharge the LiPo so that they catch fire, but I can also monitor them manually.
Efficiency would be nice to maximize, but I am very willing to trade-in some efficiency for circuit simplicity.

Add two more of those solar panel in series. PV panels don't work well with boost converters, as they are current sources rather than voltage sources.

implement an MPPT

Not practical for a small project like this.

include a battery management system

You need a balancing charger with series connected LiPo batteries. Various balancing schemes can be found on line (example below), and presumably ICs are now available.

Yes, this is the BMS I currently have: Amazon.com That looks very similar to the one you suggest, @SurferTim, right?

@jremington: You suggest that I put three or two 6V solar panels in series to get high enough voltage to charge the 3S LiPo? The batteries will pull down the voltage of the solar cells to the current voltage level of the LiPo (e.g. 11V) and thus the 2 or three solar cells with operate at a reduced voltage, i.e. 11/2 or 11/3, (which might actually be close to their MPP). Did I understand that correctly?

Yes, that is what the entire PV industry does. The series PV cells have to produce significantly higher open circuit voltage than required by the battery charging system.

The panel wattage primarily determines the time to recharge (assuming full sun).

That looks about right. I didn't go through all the specs.
If the overcharge circuit works as advertised, connect the solar panels to the input.
I would check to see if the panels will backflow current in the dark. I believe my panels have a diode.

If you want working MPPT solution, put them all in series. Otherwise you need multiple MPPTs.
It's easier to find charge controller for 60V to 3s .
This could work, read the specs.
Victron BlueSolar MPPT 75/10

You mean you will stand there all day with an ammeter and wait till the current drops below say 50mA then disconnect the batteries from the solar panels?

If that is what you want to do, then just buy a variable output boost converter, connect it to the batteries, set the output voltage so that the output current is at the maximum battery charge rate and wait for the current to drop. Can't get any simpler.

That way you can do manually MPPT as well, adjusting current all day long keeping eyes on voltmeter...

Would a variable boost converter for example be this one, https://www.amazon.com/HiLetgo-Adjustable-DC3-0-30V-DC5-35V-Converter/dp/B07BNHR4HW/ref=sr_1_3 ? I could adjust the desired output voltage with a screw driver.

Let's assume my battery is at 10.6V: If I set the boost converter to 12.6V, it actually draws too much power from the solar panels, leading to a crash of the panels voltage and power dropping. Would the boost converter draw less current if the output voltage is set only marginally higher than the current battery voltage, e.g. 10.8V?

If it is about adjusting the voltage, see my prior command to @jim-p, would there be a boost converter whose output voltage can be programmed through an arduino rather than by adjusting a variable resistor with a screw driver?

There are. But that \$40 charge controller I linked do the job better without hacking or writing a line of code

Yes that boost converter would work

You don't monitor the voltage you monitor the current. If the max charge rate for your battery is say 1A, then I would set boost output current to maybe 800mA just to be safe.

You will continually need to adjust the current until the battery is charged to 4.2V.

Good point, thx @kmin.

With my solar panels, we are taking about max. 10W (that's their rating). The charger seems to be quite overpowered for that. Unfortunately, I am also weight constraint in my application and search for a light-weight solution.

The system already has microcontrollers integrated, so writing some code doesn't matter (if I can spare some weight).

Then look for something lighter. Just make one series string, 1ch MPPTs with buck converter boards for charging are commonly available. Multichanel boost converters I have never seen.

Thanks a lot for your responses, @jim-p.

Indeed, at my battery and panel size, we are talking about absolute max. charging currents of 1A.

As far as I know these little boost converters (from the link above) only allow for regulating the output voltage based on a potentiometer. I could replace this potentiometer (that defines the reference voltage at the boost converter which in turn regulates its output voltage) with a reference voltage set through an analog pin of my arduino.

I don't understand how this limits my output current though; simply by the reducing the difference between battery voltage and (target) boost converter output voltage?
Or would I need to find a small boost converter (I already found larger ones) that has a potentiometer for current limiting (and replace the current limiting potentiometer)?