Charging LiPo Battery with Solar Panel

Using Arduino General Electronics
#1

I want to use my Arduino with two solar panels and want to have two LiPos in between in order to still be able to use it when a cloud passes by.

* So, This is what google taught me on charging a LiPo battery:

Do not charge to more than 4.2 Volt as it will damage the cell (causing fire and explosions) You can charge it to 4.3 Volts, but it will get hot and you will want a heat sink.

Do not discharge under 3Volts. If it hits 2.5Volts it is definitively dead.

Some forums claim that if the Voltage drops below 3.7 Volt, the battery is essentially empty.

Commercial chargers charge the battery up to 4.2 volt with constant current, then continue charging it for a little time while reducing the current until it drops to near zero volts.

Is this correct so far? Can I use a battery with under 3.7 Volts, assuming my application is ok with only receiving that low voltage?

*

I plan on charging two LiPo cells. For each of them I have a protective circuit which interrupts the connection to the battery if the voltage exceeds 4.28 Volts or drops below 3V.

My LiPo Cells are rated for 350mAh at 3.7 Volts My Solar Panels have a maximum output of 7.9Volts at 165ma

Method One: Connect the LiPos in series. (so. 7.4V @350mAh) and connect the Solar Panels in Parallel. (7.9V @330mAh). I figure I should be able to charge the cells to about 3.5V each, giving me a 7 Volt battery with which I can run my Arduino for ~30 minutes or so. Is this reasonable? (again, if the current drops to below 3 volts per cell, they are automatically disconnected.) Problem: Google tells me that a LiPo is 'empty' at 3.7 Volts ---> I think this is bs, but then again... I have no idea.

Method Two: Connect both in series. LiPos in series can handle a maximum charge of 8.4V according to battery specs. The connection is terminated at 8.56V. I will be supplying up to 15.8 Volt but only at 165mA. However my assumption is that the actual voltage which I supply will usually be far less than that, because the will usually not be both fully exposed to the sunlight, plus I do not really believe in those maximum values anyway. Problem: -Low Amperage -The LiPos can be charged with a current slightly higher than the specs -Chance of wasting energy -Can be outside of recommended input voltage of Arduino

(either way I the Arduino, solar panels & batteries will connect in parallel, correct?)

*

Alternatives - use a lillypad or something which runs on 3.3v, and work with the 3.7 Volts.

*

Anyway, I am a bit out of my waters here, and feel like I should not be doing this trial and error style as I dont want to harm myself or my belongings. I would like to hear whether what I am doing is safe, in principle (I want to integrate this in a backpack or possibly clothing... ). How you would go about connecting it, and whether the "failsafe circuitry" which I am using (disconnecting below 3v and above 4.28v) is a sufficient protection for the LiPos, or whether I need additional safeguards. (I will be using diodes, to make sure the solar panels are not giving me negative voltage).

If you know of a good tutorial, I would love a link. The best a found was by LadyAda, however that was too basic and did not have any of the information I need to build this without buying her product.

Thanks in advance for any tips you might have

p.

#2

Hi, What I have read says you need to charge LiPOs separately (often if not all the time) because the cells will become imbalanced.

Look at the chargers used for Model boats and cards etc. The battery packs have a connectors that goes to each cell, just for charging. Discharging is by two large cables at the battery ends.

i have one of these, for example: http://www.hobbyking.com/hobbyking/store/__5548__IMAX_B6_Charger_Discharger_1_6_Cells_GENUINE_.html

Find it on Ebay and etc...

#3

hm. I have very low current and voltage for charging the cell. Also I want a super lightweight solution, which requires as little energy as possible (I dont want to invest into powering a display etc.). This sort of eliminates commercial solutions (this here comes very close to what I want though ... http://www.adafruit.com/products/259).

I am not interested in efficiancy of the battery use - my primary concern is safety. As long as I can get my setup to 'sort of' work without blowing myself up or setting anything on fire I am satisfied. Is this feasable with the setup I am suggesting?

How bad is the inbalance issue? I am aware of why you want identical charges on your cells, but are LiPos more volatile in this regards than other types?

#4

but are LiPos more volatile in this regards than other types?

Yes if individual 3.7 volt cells are wired in series to make a higher total voltage battery, and then when charging said battery of cells, if one cell gradually has more terminal voltage over time then the other cells in the string, there is a risk of overcharging that cell unless there is individual cell protectors built into each cell. Balanced charges were designed to prevent such a problem.

Lefty

#5

ok, I am using individual cell protection circuits, so that should not be a problem.

Is there anything else which can go especially wrong?

#6

hi,

I’m an absolute newbie in electronics so pls take my post with a fair bit of caution.

To start you’ll probably want to have a diode between your solar panels and batteries to avoid the batteries leaking into the solar panel! A diode will eat between 0.5 to 0.7V so your method 1 seems compromised as V solar panel after going thru the diode is going to be < V batteries.

Your method 2, just reading your own comments seems crazy - you’re feeding the batteries way too much voltage.

As such the only method I see working is method 2 with a voltage regulator. A simple voltage divider (see wikipedia) will not work as the voltage output will depend on the sun (I believe) so that you’ll need an actual chip to do the regulator. In this scenario you have plenty of voltage to play with, and 7 to 8V feeding the arduino is perfect.

#7

Most charging circuits try to output a constant current until the battery is charged, but this doesn't work with a solar panel because the power output fluctuates unpredictably. Unless the panel is very large and well-lit the charger will likely try to draw too much current, causing the panel voltage to drop and the charger to shut off before it's even begun.

You need a charger that senses the panel voltage and draws only as much current as the panel can provide at the required battery voltage. Adafruit has a suitable charger, along with comprehensive documentation: http://ladyada.net/products/usbdcsolarlipo/. In fact this is the only solar-powered LiPo battery charger I have come across. If anybody knows of any others, I'd be glad to know.

#8

Google tells me that a LiPo is 'empty' at 3.7 Volts ---> I think this is bs, but then again... I have no idea

This is not BS. In fact, discharging a LiPo below this minimum 3.7v rating will basically kill the battery. There's no danger of fire or explosion or any kind of hazard in that respect, but discharging a LiPo below 3.7v will in fact ruin that cell and require it be replaced (and if it's an individual cell in a sealed pack, the entire pack needs to be replaced).

If the cell has built in protection, this will include shutting down the cell at 3.7v to prevent it from being damaged. This will appear to the outside world as if the cell is 'empty' though, and will no longer provide power until it is recharged.

#9

This is not BS. In fact, discharging a LiPo below this minimum 3.7v rating will basically kill the battery.

3.7v is the average rating for the cell. At just below 3.7v you still have about 65% charge left in the battery pack. You don't want it to discharge below 3.0v because that is where the damage can occur.

#10

Hi everyone, I am looking for something like this:

http://www.ebay.com/itm/Arduino-Solar-Charger-Panels-Shield-LiPo-rider-Solar-Adaptor-4-8V-6-5V-/282387531797?hash=item41bf9bc815:g:E34AAOSwax5Yv7ho

But stronger, I want to be able to hook it up with multiple solar panels, charge relatively big battery and be able to take that power with big output.

I saw different types of this boards but usually max output I can see is 1A.

This could be an answer:

But its still not available.

So lets say I want to have big lithium battery 10000mah, 3.7V and I want to be able to charge it via USB, I think its called quick charge 3.0. OR charge slowly from array of solar panels.

This board has to work as ups - so its either charging from solar/usb or not, but if you connect output to it, it will drain power from battery. Obviously if its connected to USB, it should bypass battery and drain from source…

THX in advance.

PS. I found multiple devices (mptt)

https://www.ebay.com/sch/i.html?_odkw=mptt&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xmppt.TRS0&_nkw=mppt&_sacat=0

Which has good outputs, and stable USB output but there is lack for UPS feature, possibility to charge battery from different source than SOLAR.

#11

Try this: https://www.adafruit.com/product/390

it uses MMPT like charging and by changing a resistor can go up to about 1.8 amps charging. Only drawback is it uses a 6 volt input. But you could use a switch mode buck converter to efficiently drop the solar voltage, e.g.:

https://www.pololu.com/category/131/step-down-voltage-regulators

remember to put the voltage sense of the MCP73871 on the solar panel output (via resistor divider) rather than the output of the buck converter.

Great for UPS type scenario. Read how at : https://learn.adafruit.com/usb-dc-and-solar-lipoly-charger/using-the-charger?view=all#downloads

Don't try to use two lipos in series as they soon get out of balance without a balancing charger and won't work for you. If you want more power connect several LiPos in parallel with this charger.

#12

hi there, thanks for all your info.

Im planning to use a small solar cell that will provide me just few (2 or 3) uAmps at 5,5V due it will not be in a lightly place and, i need to charge a lipo battery (3,7V/100mAh).

Battery is powering a device that just consumes around 8 uAmps, and the idea behind is just to extend the battery life of it as much as possible. It can be seen that solar output current does not even cover the device consumption, but at least could be around 25%.

So, to extend the device life 20-25% is the key for me. My question is if with such low solar output current, can I even start to charge the battery?

All around internet i just can find optimal charges(0,7C,1C,...) but i can not find any information of how to deal with very low currents, hope anyone could help me

thanks in advanced!

#13

lucas01: It can be seen that solar output current does not even cover the device consumption...

lucas01: My question is if with such low solar output current, can I even start to charge the battery?

It seems to me that you've answered your own question there. For the sake of completeness I'll point out that you'll struggle to find a charger that will function at such low current and still have any current left to charge the battery with.

#14

An issue you might face is that lithium batteries like to be charged at constant current. Doing this from a solar panel is very difficult unless you have a clear and cloudless sky.

Here in the UK I get around this by charging lead acid from the solar panels as they are much more flexible in this respect. Then I charge the lithium batteries from the lead acid ones with appropriate charger.

Works for me here. Not as efficient, but safer and less dead batteries.

#15