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Can I hook a 3v solar panel up to charge a 3v lithium coin battery to keep a 3v chip led going? Answered

The project is to light up the inside of a shadow box.  

Currently I have a 3v 20ma chip led that is plugged into a 3v lithium coin battery ( CR2032) with a switch in between and that all works out fine.  My math figures that should get me maybe 5 days max of light for one chip led.  Which led me to thinking about a solar panel instead for the top of the frame for something that might last longer. 

I'm looking at the PowerFilm MP3-25 Flexible Solar Panel 3V @ 25mA right now, but I think if I just do that, at best it only lights up with the lights are on and bright enough in the house if I use that in place of the battery. I'm kinda new to all this, and I'm trying to figure out is there a way I can hook the solar cell up to the coin battery holder to charge the lithium battery so it can use solar power or the battery or so it is always charging the battery?   My guess is that's a bad idea without something to cut off the charging once the battery is full.  Also, I don't know if that would fry the chip led because that would then be 6v coming through rather than 3.   I also think I read I would need something with slightly higher volts to charge a 3 volt battery.

Also, to back track a little, the reason I'm using a coin battery rather than AAs or a 9v and a resistor is because I can fit the coin battery nice a flush with the back of the picture frame and it still hangs nicely, and leaves enough clearance for the power switch to stick out just enough that all you have to do is press the frame to the wall to turn it on and off.  Which gets us to if a solar charger could charge a coin battery, I'd still want to pull it off without extra circuitry on the back for the clearance reason (it's maybe 3/16" clearance at best).  Without adding a lot in parts, would it maybe be possible alternatively to make a flip switch so one position is using solar and the other is using battery?  If it was just adding the solar panel to the top and flip switch on the side, that would totally work.  I just can't find any info for something this small yet.

Thanks for any info anybody has.

Did some more research and I guess you can't recharge just any old C2032 battery.  But there are some that are rechargeable.  Amazon has these.  http://www.amazon.com/Battery-Li-Ion-Rechargeable-Button-LR2032/dp/B0058E94O6/ref=reg_hu-rd_add_1_dp

So the question is still can I just hook a solar cell up to this battery terminal and have it basically just trickle charge?   Here's the coin battery holders I'm using.  http://www.modeltrainsoftware.com/3v.html
It looks like the way they come before they are wired up is they have a third prong coming out that they clipped off for this version.  At least from what I've seen sold on other sites that isn't per-wired.   No idea what that prong does or if that would be needed for charging.

Also, I'm trying to follow this instructable but it seems to skim over a lot of things, or at least the details.  Sounds like maybe I need a rectifying diode and dpdt switch is what I'm looking for?   I'm trying to read up on the dpdt switch but I don't really fully understand what it does in this case.   The diode I get is for making sure the solar panel doesn't suck energy back out of the battery when dark.   Is this the right track?

**** Edit
Kept searching.  Are Super or Ultra Capacitors an option for something like this over the battery?  Just use that with a solar cell?



3 years ago

You need to understand how the center slide or bat switch works.

The diode should be a low forward voltage drop like a 1N5817


which is less then o.45 Volts in conduction.

You need a diode to prevent the battery from discharging through the solar cell at night or if a cloud blocks the Sun because when the solar cell voltage falls below the battery the current wants to run back through the resistor like solar cell.


You know this can be done using a simpler single pole double throw

SPDT switch.

Then if the voltage penalty of using a diode and need an extra cell is too much.

Then you can make an equivalent diode with TEN times less forward voltage loss of only o.040 volts ( see the last pic )


3 years ago

CR2032? No. You need ML2032. Sorry but that CR2032 will rupture.

A 3 volt solar pnl won't produce enough voltage to charge a 3 volt battery - Your better bet would be a super capacitor BUT a lot depends on how long you need the supply for. Generally they are/were used to keep volatile memory active when the PC was switched off - minimum current drain.

You will need a bigger solar pnl. look up your battery specification sheet it should tell you the charge current and voltage required.

So I'm still reading up but it sounds like I could use three solar cells and wire themin paralell to get the needed voltage and then use a resistor. I'm not sure if the resistor would be before or after the battery though in that case. A bigger pannel won't fit on the edge of the frame. Sounds like I need at least 4v to charge a 3v battery, but I don't know about the ma. I think the battery was only like 25ma maybe, but the rechargable were closer to 250 and the charger I saw was maybe 2500. Not sure and can't check on my phone easilly at the moment. I've read up some more on the dpdt switch but I still don't quite understand what he's doing with it in that instructable. Is he reversing the poles for some reaon? Or is just one end of the switch battery and the other solar without reversing the poles? Also I found a 10f 2.5v super capacitor which could be wired in parallel for 5f 5v. But I have no concept of how long that would last with a 3v LED running all the time. So can anybody give me some clairifcation in how those things might work best?

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