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How can I make a solar charger for extended battery/capacitor life? Answered


I want to build a solar charger that could keep a rechargeable, say, CR2032 battery always trickle charged without damaging anything. I want to make this circuit as simple as possible.

I want to just hook up a solar panel, with an operating voltage of 3V that delivers 1mA, to a rechargeable CR2032 that was in-use. Assuming the solar cell was delivering power for 12 hours in the day, would what could go wrong? I'm sure there could be some danger, but I don't know enough.

Do I need a charge regulator? Can I charge the cell and also consume power from it? If it's forgotten about for a year, will the battery explode?

Can this be done with a capacitor? Can I keep trickle charging a capacitor that is attached to say a heat sensor which uses very little power?

Discussions

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Downunder35m

2 years ago

A standard CR2032 is not rechargable, so only try if yours really is designed to be recharged.
Since it is a 3V cell you need more than 3V to charge it, can't be too certain but I guesstimate around 3.3-3.4V.
A solar cell with 1mA is pretty much useless as it won't provide enough juice if not in full and direct sun.
Even then I doubt 1mA will do anything.
The only thing I could think off that might work on such a small scale would be a "flashing joule thief":
The solar cell is connected to a tiny joule thief (check my Instructable about it ;) ).
The output of the joule thief charges a small capacitor that discharges into a LED, peferable a green one with around 4V operating voltage.
A small diode (so the button cell won't discharge into the LED) connects the button cell parallel to the LED.
With each flash the battery gets a small amount of charge.
Here you can find an example for very simple flashing joule thief, combine that with the info from my ible and it might just be enough - but only for rechargable cells!!

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JamesS691Downunder35m

Answer 2 years ago

To your first point, it isn't but they make rechargeable ones like these.

Second, you're certainly right but the voltage dips off to about 2.5V relatively quickly, as shown here, so a 3V panel can keep it at a high enough operating voltage. Also in that source you will see that the standard load is 0.12mA so a 1mA solar cell would do just fine, right?

Lastly, your joule thief is just a coil that up converts the current, no? If not, can you explain how/why it works. Are there any cheap, commercially available joule theives?

I am only just learning about the field, so please correct any of my misunderstandings.

Thank you for your reply!

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Downunder35mJamesS691

Answer 2 years ago

A joule thief works with very little input voltage and provides a much higher output voltage.
Almost like a step up converter.
As a flasher you can misuse it to get charging pulses...
You can build one from scrap parts very easy.
But it might be easier to get a prover charger that runs a USB or 12V connection and to connect a suitable solar panel - plus maybe the right battery as well ;)

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steveastrouk

2 years ago

You can safely continuously charge NiMh or Nicad at a very low rate, like C/50, but the only CR (or LR) 2032 I know of are lithium, and you can cause major issues continuously charging them.

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JamesS691steveastrouk

Answer 2 years ago

Thank you! Are these the only batteries that can be safely continuously charged? I haven't found any that reach 3V, is that just a limitation of this cell?

Also, is the C in C/50 overall capaity in mAh?

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steveastroukJamesS691

Answer 2 years ago

Well there ARE lead acid !

C is, as you surmise, the capacity of the cell in mAh.