Can 12v 100mA solar panel run the Arduino and one question about Li-Ion battery

Hi All,

I am new to Arduino and have recently started on my first serious project :slight_smile: I am making an automatic plant watering system which will water the plants at a fixed time every day say 10 am and then put the Arduino in sleep mode for 24 hours. I have a Freeduino with Atmega168 and have purchased a 6V DC water pump as well. Now since this is going to be a portable project i.e. no access to wall wart or USB power I'm working on the power supply part. I'm considering two approaches and have questions in both.

Approach 1- Solar power

I purchased two 6V DC 100 mAmps each solar panels and connected them in serial to get 12 V DC 100mAmps. However my Freeduino does not run on this power supply. When I contacted the manufacturer of my Freeduino they said that the power regulator on any Arduino/Freeduino requires at least 7V and at least 300mAmps current. Is this correct information? If yes will adding a 6V 100mAmp + a new 3V 200 mAmps solar panel be able to power my Freeduino?

Approach 2- Lithium-Ion battery
In this approach I have got info of a 7.4 V DC 700mAmp Lithium battery on this site. The description says this battery has an inbuilt charging and overprotection circuit and that it can be used while it is being charged. Can someone help me with a circuit diagram of how this can be done? Since from the product image at least I see only one pair of connections, so how can it be charged and used at the same time?

Hoping to get some advice from experienced folks and get started with the project.

Arduino/Freeduino requires at least 7V and at least 300mAmps current

No, that is not correct.
7V on the Vin is the most desirable, but it can be less and still run.
40 - 50 Ma is all the arduino requires.
If you are driving other loads of course they will require more.

What is the specs on that water pump. It is probably the biggest power hog.

At this point I have not yet connected the motor pump. Its only the Freeduino.

The solar Panel might deliver 100mA at full sun, but the specs are not very stable.
You could combine the solarpanels with the lipo battery to have sufficient power for arduino and waterpump. The panels will act as a charger for the lipo and you can get the power for the arduino with a parallel connection. Vin to lipo+ and GND to lipo- .
How will you switch on the waterpump? I'd use a small 5V relais..

Being that you will probably not always have enough power to run the pump, you can easily monitor the voltage, and run the pump until the voltage gets below a cutoff point, then turn the pump off, and wait for the voltage to get above a cut on point to turn it back on. On sunny days you can run the pump longer than on cloudy days, but you should be able to get as much water out, just taking longer .
You can calebrate and determine how much water flow you get per minute, and let the program keep track of the days requirement ( how many minutes of pumping).

Thanks for the answers, I will try them out. I am planning to buy a multimeter to check how much current the solar panels are producing v/s how much current my Freeduino is consuming at present.

Another solution might be using a supercap.
In the 1-5 Farad range.

Better than a rechargable battery if you dont need it to run during nighttime.

Using code with sleepstates could keep the Arduino running 24/7.

Yes, you could replace/augment the battery with supercaps. I have some. The down side to that tho is that supercaps cost lots more than a rechargable battery, and to run the pump you would need larger than 5 farad. You can get some that close to 2000 farid, but lots of money.
For the up side of super caps. You can expect them to last many years longer than rechargable batteries (if you don't apply the wrong charge). And for short time loads, they can provide more amps than rechargable batteries. (2 seconds say).
I don't think we know yet what amps the pump will draw.

The panels will act as a charger for the lipo

Do NOT do this without the proper lipo charging circuitry in place. Plugging solar panels directly into a lipo is a recipe for disaster. Your project could quite literally go up in flames if you just dump unregulated current into a lipo battery.


The panels will act as a charger for the lipo

Do NOT do this without the proper lipo charging circuitry in place. Plugging solar panels directly into a lipo is a recipe for disaster. Your project could quite literally go up in flames if you just dump unregulated current into a lipo battery.

I was referring to the Lipo battery he linked us. It has an inbuild Charge-circuit.

Sorry, if I wasnt clear enough and expected everyone had followed the link...

Regarding the lithium battery on this link

"This battery includes an inbuilt charger and protection circuit which allows you to use this battery without worrying about over discharge, over charge or short circuit. For charging just connect to any 9V & max 1A power source. This battery can be use inline just like your mobile phone battery, when connected to charger you can still use your circuit or robot and charge battery simultaneously."

Can someone please guide how the circuit for inline usage and recharging look like? Since in the photo it seems to have only one connector!

You’ll probably want to remove the plug and strip the wires.

Any 1N400x diode should meet your needs.


Thanks Chagrin for the circuit diagram! :slight_smile: I will think about this option.

Finally I got my multimeter yesterday! Did some quick measurements of my solar panels and Freeduino. Following are the results
Solar Panels:

  1. My house faces East and at morning 10 AM with solar panel directly facing the sun I’m getting Short circuit volatage of 6.4 V and open circuit current of just 30mA from each of my solar panels! The advertised values are 6 V and 100mA each!
  2. At around 2:30 PM when the sun is facing west and not directly seen from my balcony the current was only 3mA!
  3. I ran the Freeduino from an old 9 V battery and the current consumption was about 12.5 to 13 mA. The led on pin 13 was set to blink every second. Note my Freeduino does not have on board FTDI chip. Perhaps this is what’s giving so low current consumption.
  4. As against advertised current of 100mA i’m getting only 30mA in direct sunlight from solar panel. Is this normal behaviour of solar panels?
  5. I did manage to run Freeduino with one solar panel at around 2 PM through the DC barrel jack. The power on led and the pin 13 led kept flickering continuously. And the LEDs were not as bright as with the 9V battery. Is this a problem? How to solve this?

As for solar panels. Their maximum power rating is hard to accomplish in real life. We don't know where you are located, but you can get close to the rating at high noon, when the sun is directly above you, and not a cloud in the sky. Other than that, it will be diminished.
Depending on when you need the power (morning, evening etc), you can position the panels. For max daly power, aim at the sun at high noon.

Not to be picky but

I’m getting Short circuit volatage of 6.4 V and open circuit current of just 30mA

I think you got these backwards. If you have a short circuit, you will have no voltage, and if you have open circuit you will have no current. But we got the jest of it.

If you want to run through the DC barrel jack, I suggest you hook two panels in series, giving 10-13 volt range. It may run at 5 volts, but will not be reliable, and modules it drives may not respond well.

In order to make a project like this work off Solar, its critical that you know the maximum daily total energy consumption.
So you need to determine how much power the Arduino uses and how much power the water pump will use per day.
Based on that its possible to determine whether the Solar panels are sufficient or not.
The water pump will most likley be the biggest energy consumer.

With any kind of Solar powered project, sizing the Solar Panels is the last thing you do after determining the projects total daily
energy consumption.

Since your Freeduino does not include the FTDI chip, you can reduce the average power consumption a great deal by using the sleep states.

I suggest you use a 6V SLA battery and have the solar panels charge that. If the regulator on the Freeduino is of the low dropout type and you do not run anything else from the 5V pin, then 6V will be plenty. if the regulator is not an LDO one, than use an external LDO regulator to reduce the 6V to 5V.

Thanks jack, mauried and dc42!
Thanks Jack for your advice and for pointing out my newbie goofup of open circuit current! :slight_smile: I will remember it now!

Dc42, i will be exploring the option of sleep modes and hope to reduce current consumption.

But I have an immediate problem though, since my arduino is flickering with my solar panel, i'm thinking of adding an electorlytic capacitor to add some buffering so that at least passing clouds and minor current drops can be handled successfully. If i put a 10V 1000uF cap how many seconds will it last? I'm not planning to add a super capacitor. Can you recommend which rating capacitor I should use?

If you draw 13mA from a 10000uf capacitor, then for every second that passes, the voltage will drop by (13e-3 * 1)/10000e-6 = 1.3V.

Maybe 4 or 5 AA, rechargable batteries (in series) would work better.

I would recommend you to have a look at the Stalker waterproof kit:

For your requirments the solar panel that comes with the kit can easily power the Arduino compatible Stalker board.

I would then use a separate solar panel and battery for your pump.

Jan Willem Smeenk