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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 08-08-2011, 01:56 AM
BigJason BigJason is offline
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Default Solar NiMH to USB Charger Help

Okay,

I don't know exactly how to go about saying this. I've been doing some camping and hiking lately and especially when I go out for a few days at a time, I always struggle with some of my electronics staying charged. I really do not use them when I'm out there, but it would kind of suck if something bad actually happened and I needed communication or a GPS to get the hell out of there.

So I was thinking about building a simple-ish way to make a USB charger that runs off of solar power. I'm not electrical engineer so maybe I'm off. Here's my initial thoughts.

6V 2W Solar panel -> MAX712 NiMH Charger circuit -> 2x 1.5V NiMH -> MC4063A Buck boost -> 5V USB Out

Now I don't know if MC4063A is the right chip for this job. I know the MintyBoost can use a Max756 or a LT1302. Does anyone have any ideas in that regard.

Anyone have any better ideas?

Thanks.
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Old 08-08-2011, 02:13 AM
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praondevou praondevou is offline
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Quote:
Originally Posted by BigJason View Post
Anyone have any better ideas?
Yes, even if it's fun, I wouldn't bother building one. There are several of these chargers you can buy off-the-shelf, for example at e-bay.

They have a built-in battery which is being charged when you not use it.
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Old 08-08-2011, 02:54 AM
TheLaw TheLaw is offline
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Maybe get a Mintyboost?
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Old 08-08-2011, 09:21 PM
BigJason BigJason is offline
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What if I did this:

6V solar panel -> 5x 1.2V NiMH (6V) -> 5V LDO -> USB...Excluding a actual charging chip and removing the buck boost. It would be simpler.

I have a question about charging via solar. I know that a lot of the solar landscaping lights just use a trickle charge and no advanced charging circuitry. If I have a 6V battery pack, at what voltage do I have to charge it at? Below 6V? Or do I have to be slightly above?

Also, is there a "primitive" battery protection circuit that could be used to prevent the pack from being overcharged? Something that wouldn't require a fancy chip...maybe just some passives and a PNP or something?

Any ideas? Thank you.
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Old 08-08-2011, 10:04 PM
BigJason BigJason is offline
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So I read that a NiMH cell should be given ~1.5V when being charged (rule of thumb). What would happen if I instead gave it only 1.2V? Would it just not be fully charged? Or would it fail to charge?
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Old 08-08-2011, 10:19 PM
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iONic iONic is offline
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Quote:
Originally Posted by BigJason View Post
What if I did this:

6V solar panel -> 5x 1.2V NiMH (6V) -> 5V LDO -> USB...Excluding a actual charging chip and removing the buck boost. It would be simpler.
To maximize your charging ability a solar panel closer to 12V would be better as you will not have perfect sun at all times.

Quote:
Originally Posted by BigJason View Post
I have a question about charging via solar. I know that a lot of the solar landscaping lights just use a trickle charge and no advanced charging circuitry. If I have a 6V battery pack, at what voltage do I have to charge it at? Below 6V? Or do I have to be slightly above?
You can not charge a battery with a voltage equal or less than it's rates voltage. You will need more than 6V.

The solar landscaping lights do not charge at high currents and use NiCad batteries. NiCad's are tolerant of overcharging conditions.
If you a six-pack of NiMH 1.2V batteries you would not want to charge them at a voltage much higher than 1.42V - 1.44V per cell (8.52V - 8.64V)


Quote:
Originally Posted by BigJason View Post
Also, is there a "primitive" battery protection circuit that could be used to prevent the pack from being overcharged? Something that wouldn't require a fancy chip...maybe just some passives and a PNP or something?

A diode between the solar panel and the batteries is required so that the batteries do not discharge when the solar voltage is lower than the battery voltage.

Some people have used a combination of current sense and a transistor to switch the charging current from a higher one to a lower one. This in combination with a regulated voltage will protect from overcharging.

If you can imagine the size of the solar panel used for most solar lighting devices and realize that the use but one or two battery cells. Then you will need a panel that is 3 - 6 times the size to charge the six-pack depending on the load requirements.

Any ideas? Thank you.
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Old 08-09-2011, 12:10 AM
BigJason BigJason is offline
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Thanks for the response. I appreciate it.

Judging from what you said, I think 12V might be too high, because IF the panel did actually produce 12v on a sunny day, a six pack of AAs would be getting ~2.0V each, which seems a bit high in my opinion.

However, if I used a 10V panel, the maximum voltage per cell would be around 1.67V, which is not too high. If the voltage dropped to 9V, you still get 1.5V. If it drops to 8.5V, each cell is getting 1.42V. None of these are too bad. I do not know to what extent solar panels fluctuate. (i.e. Could the panel drop to say 5V?) I would assume not..but I don't know.

So I think I would try a 10V panel. I saw some decent ones on eBay which I may give a look.
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Old 08-09-2011, 12:23 AM
wayneh wayneh is offline
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Don't worry about the higher open-circuit voltage unless you plan on using no regulation. You can always add additional blocking diodes to drop ~0.7V each. But anyway the peak voltage will drop a lot once the panel starts supplying current. Best efficiency is when the panel is at ~80-90% of the open circuit voltage. You'll likely be below that unless you get a pretty large panel. Too low a peak voltage, and you won't get any charging at all without full sun.

It's a tough balance. Having a circuit to protect your batteries (even a constant voltage LM317 circuit) allows you to use a bigger panel, under more conditions, without fear.
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Old 08-09-2011, 12:51 AM
BigJason BigJason is offline
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Well I wasn't planning on using any regulation because that would mean I would need a higher voltage panel...however...now that you said it...

I'm trying to make this something that is semi-portable, that I might be able to put in my hiking backpack or atleast bring along somewhere. If it's too big, it's practically useless because who wants to tote a giant panel into the woods so they can charge their tiny little cellphone.

Anyway, if I did use an LDO 9V regulator (1.5V/cell), I might be able to do something.

But then again, with most LDOs having drop out voltages of around 0.5V, the minimum voltage I could have from the panel to still achieve 9V would be 9.5V (duh). I guess the question is: How much can the output of the panel fluctuate? Can it drop over 2.5V if it is cloudy? (12-2.5V= 9.5V).

There is another option. I could remove one cell from the 6x AA, to make 5x AA, which would mean I could ensure that the cells are always getting charged. 12V panel should give plenty of headroom for 5x AAs.

HOWEVER, a new problem is formed because now there is only 5x 1.2V AAs which would nominally produce 6V. But when the cells drop to 1.1V, you only get 5.5V. And when they drop to 1V, you get 5V out. These batteries would need a regulated 5V output, which would be an LDO 5V regulator. Again though, If the batteries drop below 1.1V/cell, the LDO will no longer regulate at 5V, which is a problem.

So do you use 6 cells which might have a possibility of not getting charged from 12V, but WILL have enough voltage for the LDO to always produce a constant 5V.

OR

Do you use 5 cells which will always get charged by the panel, but has a possibility of not having enough voltage to produce 5V?


Is there solution? I think getting a higher voltage panel would be ideal. But that means getting bigger or sacrificing output current.
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Old 08-09-2011, 01:20 AM
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iONic iONic is offline
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A 12V panel generally has an open circuit voltage of about 18V and could average 14V. This is where you would start with respect to regulating the voltage.

But let me suggest that unless you are planning on keeping the cell phone on all day just a backup battery is more than enough to call for emergency assistance. Heck, from a space comparison you could take three spare batt's instead of the six-pack and charger.
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