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> Solar Powered Battery Recharger W/ Regulator -help, Homemade battery recharger
swimfan24k
Posted: January 19, 2012 02:53 am
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Hey, I was hoping I could get some help with a project that I hope to finish in the next couple of months... I have searched the depths of the internet, and have not been able to find any help on this at all!

Simply, I am looking to build a recharging circuit for 3x1.2V NiMh batteries (3.6V total) which are fueled by a single 5V solar cell (700mA output max).

The one function on this recharger that I want, and am not sure if this is even possible, is to install a regulator which would begin charging the batteries once they got to a certain low voltage (i.e. 2V), and then would stop charging the batteries once they got to a certain high voltage (i.e. 4V). Is there a name for this type of IC? or could anyone give me some guidance as to how this could be done?

Would be greatly appreciated!!

S
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kl27x
Posted: January 19, 2012 03:05 am
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If your batteries can tolerate 4.2V, there's a ton of Li ion battery charging IC's available that'll do the job, specically made to charge from 5V (USB), with adjustable or fixed current limit. One 5 pin IC, one cap, and just a couple connections.

BTW, is there any reason for the charger to NOT charge the batteries until the voltage reaches 2V? I'd think you'd want the charger to charge whenever the sun is available.

With a Li ion, you want the battery to stop DISCHARGING whenever it reaches 2.5V or so. This can easily be done with a voltage detector and a MOSFET, if the charge IC doesn't have that feature built in.

Alternatively, you could keep your NiMh, use a li ion charging IC and use a voltage detector to cut power when V goes above 4V. Or find an IC for NiMh, I suppose. I'm sure they're around. Or just put a Schottky diode on the output of a li ion IC to drop the voltage by around 0,3V.
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swimfan24k
Posted: January 19, 2012 03:11 am
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The batteries should be able to tolerate 4.2V.
And the only reason I wanted the charger to stop charging at a high (4V) and then start charging when the batteries dropped to a low (2V), is to maintain the cycle lifetime of the batteries, and make sure they last as long as possible..

would i still be able to use the voltage detector and MOSFET with this set up?
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johansen
Posted: January 19, 2012 03:59 am
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if you want to kill it with silicon...

http://www.national.com/pf/SM/SM72442.html#Overview



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kl27x
Posted: January 19, 2012 03:59 am
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No. You'd need some kind of smart control. Otherwise, once it starts charging, it'll automatically stop.

So you need to add some circuitry to get it to do that. You can use a comparator to set the charge voltage and add some hysteresis to it, so that it doesn't stop again until 4V is reached. And doesn't start charging again until 2V is reached. But that'll drain a little bit of power all the time.

If battery life is critical, the best thing to do is to swap in a li ion battery. Draining a battery that relies on solar power all the way down to 2V before allowing it to charge again seems rather inefficient.
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johansen
Posted: January 19, 2012 04:07 am
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If the battery is fully charged, you've got about 3 watts to dump into a shunt circuit, if all you do is just hook up the panel directly to the battery. (Use a schottky diode in series to prevent back flow at night.) 3 watts is low enough that a shunt regulator to keep the battery below 4.2 volts (i'd go with 4.1 for long life) is probably the best way to go.

There's no reason to let the battery discharge more than 30% before you charge it, the weather will do that for you on a random basis.


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tekwiz
Posted: January 19, 2012 11:12 pm
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Batteries last the longest if kept charged & discharge levels are kept small. The only thing you need add to turn your solar cell to a charger is a reverse blocking diode, if the cell doesn't already have one, & a 4.0V 5W Zener diode in parallel with the cell. Power will constantly flow to the batteries until 4.0V is reached, after which the Zener will absorb the excess power & convert it to heat.
The problem with regulator chips is that they all require some excess voltage "overhead" with which to operate. A regulator that's not a low dropout type requires 3 or 4 volts of overhead, meaning you'd need an 8V solar cell. LDO regulators need half a volt or so.
A simple Zener shunt regulator is much easier & virtually foolproof.


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To him, mere toys of the moment, to be overturned at the flick of a finger.

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