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Question about battery charging
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Question about battery charging

by james_lan on Sun Sep 12, 2010 11:09 pm

I've been asked by a friend to look into the possibility of building some solar powered LED lights, for use possibly at night. I think I can handle everything else, except the storage of energy. Light sensor, LED power/control, DC voltage conversion etc are all things I've done before, or think I can handle easily.

My first thought is to use some form of rechargeable battery, possibly an AA NiMH (or more to simplify voltage conversion), to manage the storage of power. However, I've over the years seen a lot of 'this is dangerous' with regards to charging NiCd/NiMH, and know it would require watching the voltage, and for a voltage drop and/or increase in temperature. Frankly, the internet hasn't seemed to much increase my knowledge of this (or I'm not searching for the right things). I was wondering if perhaps someone had already made an Arduino battery charger for AA or similar, or if there was a reason (all the warnings about it being dangerous) that no one had. If they hadn't I was hoping someone could point me in a good direction to start, as I'm completely self-taught on electrical issues. (excluding one semester of physics in college which dealt a tiny bit with circuits.)

Another possibility is if there were some sort of battery pack designed for this. I've heard of some ICs which might handle almost all of this but don't have part numbers.

(My apologies if this should go into the Microcontroller forum, considering I'll probably use an Atmega 328 for it. (Familiarity and such))

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Re: Question about battery charging

by franklin97355 on Sun Sep 12, 2010 11:34 pm

Take a look at Lady Ada's tutorial on solar battery charging.

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Re: Question about battery charging

by james_lan on Mon Sep 13, 2010 1:20 am

franklin97355 wrote:Take a look at Lady Ada's tutorial on solar battery charging.


Thank you for your suggestion, Is this what you are referring to: http://ladyada.net/make/solarlipo/index.html

Because it's somewhat like the second option, but seems to be a little big (I know with the idea of using solar power, the panel maybe big) plus, that doesn't seem to provide the option to use the battery as a power source. (Unless I'm missing something, I don't think it would react well to a voltage converter taking the voltage up to 5V to power the arduino.)

Your suggestion also led me to this: http://www.adafruit.com/blog/2010/05/10 ... -off-grid/ (I was limiting to ladyada.net for a while, oops, if I missed the tutorial you are talking about, can you point me to it?) Which seems both more like what I'm thinking of and easier to deal with. (step down voltage seems to be much easier to deal with in my limited experience.) That's not far from my thinking about NiMHs, and will actually make it smaller.

I have seen a solar charger/AA rechargable/LED light, but in that case it wasn't anything involving microcontroller control of the LEDs, so I know it works at least without the microcontroller, in a package about the area of an Arduino (granted a fair bit deeper). (Unfortunately, that thing is long gone, as weather can do a number on electronics.) I think that used basically trickle charging, something which might be possible. So I think I may have a solution, either of a trickle charge of NiMH or of the 3rd suggestion above.

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Re: Question about battery charging

by Entropy on Mon Sep 13, 2010 2:00 pm

Charging NiCd/NiMH batteries isn't dangerous in any way unless you're trying to fast charge them. If you don't properly charge them, their lifetime in charge/discharge cycles will be reduced. If you try to fastcharge them improperly, they can burst.

For a solar powered LED light (such as the solar garden lights you can find from many sources), there is no charge control circuitry because of the very low current delivery capability of the cells. (Note - this only applies to a "single 5mm LED" style light. Higher brightness lights need bigger batteries and larger charge currents.)

Similarly, lead-acid batteries can be charged easily with very little charge control circuitry if they are charged slowly. A car battery sized lead-acid can safely be charged from a 5W panel without charge regulation. (You might have to periodically add distilled water to the battery every few months at worst case.)

How does your friend's requirements differ from the various solar garden lights on the market? It may prove easiest to purchase some of these and cannibalize them for parts, as quite a few of the solar powered lighting Instructables do. :)
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