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HELP!! recharging batteries in portable LED gift
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azntwboy
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Joined: 27 Dec 2006
Posts: 7

PostPosted: Wed Dec 27, 2006 4:45 pm    Post subject: HELP!! recharging batteries in portable LED gift Reply with quote

hi, i want to make an acrylic flower that glows.
my original plan was to just have a wall adapter to power the LEDs, but now i think it would be cool if i can use rechargable batteries, so when the flower is unplugged, it will still light up for a short time.

i see a lot of charger controllers for batteries, but they're too complex for me to understand and i dont know anything about charging batteries. im only an architecture student. im afraid to use them because i dont know what to connect the pins to or how it works.
i also dont have much space to put the components in, so a simple circuit would be better.

i was wondering will it work if i use a 6v wall adapter to directly charge a depleted 7.2v battery? suppose the batteries are drained to 2v. will the 6v adapter bring the batteries up to 6v? or will it not work at all? will the charging stop automatically or keep going and overcharge the batteries?


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Last edited by azntwboy on Tue Feb 01, 2011 9:10 pm; edited 1 time in total
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Nginuity
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Joined: 13 Nov 2006
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Location: Dallas, Texas

PostPosted: Wed Dec 27, 2006 10:20 pm    Post subject: Reply with quote

I would recommend you add some kind of charge regulator in between the charging resistor, and the batteries to prevent overcharge. It's good that you are slowing the charge, but it could still overcharge if the unit isnt on. A comparator circuit might be the easist way to do this.

As far as actual charge voltage, NiCd batteries charge the best on a constant current, rather than voltage, until they reach a predestined voltage (which is why I suggest the comparator). Your supply voltage should be sufficient, but, I would add a circuit for a constant current charger, instead of just using the wall plug, which is constant voltage.
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azntwboy
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PostPosted: Wed Dec 27, 2006 10:59 pm    Post subject: Reply with quote

why doesnt it stop charging by itself when it batteries reach 6v?
wont the wall and the battery cancel each other out?
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Last edited by azntwboy on Tue Feb 01, 2011 9:10 pm; edited 1 time in total
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Nginuity
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PostPosted: Thu Dec 28, 2006 12:33 pm    Post subject: Reply with quote

azntwboy wrote:
why doesnt it stop charging by itself when it batteries reach 6v?
wont the wall and the battery cancel each other out?


It does slow, to a degree, but I wouldn't count on that being an effective method by any means. If your constant current charging the circuit, then the level of voltage charging and the level of voltage in the battery will never be equal.
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anotherhackmember
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PostPosted: Thu Dec 28, 2006 12:56 pm    Post subject: Reply with quote

azntwboy:
Currently as is without an active charger turn-off, I suggest to reduce the charging current down from 100mA. Here is why. The wall adapter could be unregulated itself, hence will deliver more than 6v to around 8v unloaded and < 8v when loaded. How far the adapter goes beyond 6v depends on the ratio between the load and the power of the adapter. Given the batteries will be charging all day everyday, you don't want the batteries to over heat and melt down. Most always-on charges intentionally charge at a low .1C to .5C rating to keep the device cool. .1C for 120ma would be 12mA and .5C would be 60mA. Small batteries get small .xC values.
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LiesOfXIII
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PostPosted: Thu Dec 28, 2006 5:10 pm    Post subject: Reply with quote

thats a great idea. Think I'll do something similar but just have it usb, seems like alot less work Razz
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Nginuity
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PostPosted: Thu Dec 28, 2006 5:49 pm    Post subject: Reply with quote

Now that I have had some time to look at it, I see a somewhat flaw in your drawing....sorry I didn't notice it sooner.

Are your batteries in series or parallel?

If they are in series, your voltage doubles. Why not put them parallel and double the amount of time your LED can stay lit? If you do it in parallel, you also wont need the voltage regulator from the battery (right now all of that extra energy is getting dissipated by resistance heat). Once you do that, you could use a very small resistor for current protection to the LED's. The greater amount of time for the LED's to stay lit is because you now have twice the current capacity that you did from them being in series. With them in series, you only have the current capacity of one.

And, from that, you can move the regulator down to the charger side if you need it there.

How's that sound?
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azntwboy
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PostPosted: Thu Dec 28, 2006 6:17 pm    Post subject: Reply with quote

ill change the resistor to 500 ohm for the battery to reduce the charge current to 12mA.
so, is it still safe? what will be the final voltage/charge of the batteries? what happens afterwards? the current will burn off as heat?
will the 12mA x 6V = 72mW cause damage to the batteries, or will it be fine?
i really appreciate all the help. thanks!

oh, that makes sense. but if i put the batteries in parallel, how do they react to the 6v wall, or if i move the regulator down, how will the batteries react to the 3.3v charge?
im still very confused about how the batteries will behave in the whole charging sequence.
sorry.
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Last edited by azntwboy on Tue Feb 01, 2011 9:10 pm; edited 2 times in total
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Nginuity
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PostPosted: Thu Dec 28, 2006 6:30 pm    Post subject: Reply with quote

I really would advise against charging at a voltage higher than the batteries are rated for. It's a good way to blow holes in the cells Smile
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azntwboy
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PostPosted: Thu Dec 28, 2006 11:14 pm    Post subject: Reply with quote

i found this diagram

on this website
http://www.owlnet.rice.edu/~elec201/Book/batteries.html#SECTION001330000000000000000

it claims that it will automatically shut off charging when the battery
equalizes with the charge voltage with the use of the zenner diode
shutting off the base. i dont know. im not following it completely.
this might work if i also put a relay between the battery, wall, and
LEDs so that only either the battery or the wall provides power to the
LEDs.


i dont know. it still doesnt make much sense to me how it works, and
how i can use it correctly. maybe ill just leave the batteries out completely
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Nginuity
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PostPosted: Fri Dec 29, 2006 7:53 am    Post subject: Reply with quote

I see what it is doing. The Zener diode is sort of "measuring" the voltage of the charge to 13 volts. The LED won't light up until there is 13 volts going to it, because of the diode (which is how Zener's work by nature anyway)

The way it cuts off is by the BPJ transistor (TIP 31C) that is in the diagram. As the battery capacity gets closer and closer to full, the base (pin 2) varies its resistance to the emitter. The result is a gate that allows lots at first, and ends up not alowing any more when the battery is full.

The LED should tell you when it is fully charged.

This looks like it was made for a 12 volt application, you may need a smaller Zener diode to make this work for you, and you are probably also going to need to play with the resistor values to suit your batteries amp hour rating. The resistors are doing 3 jobs in this circuit: Protecting the diode, protecting the LED (Although it wont have to until the battery is charged), and regulating the charge amperage.
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azntwboy
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PostPosted: Tue Jan 30, 2007 4:44 pm    Post subject: Reply with quote

i finished the website, and a video too. there are more pics up now.
please to go http://www.thomashuang.net
thanks!

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Last edited by azntwboy on Tue Feb 01, 2011 9:10 pm; edited 1 time in total
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Alan
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PostPosted: Tue Jan 30, 2007 5:05 pm    Post subject: Reply with quote

Looks great! How many hours do you think it took?
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azntwboy
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PostPosted: Tue Jan 30, 2007 5:35 pm    Post subject: Reply with quote

over 60 hours. the next ones, if i do make more, will probably be 30-40
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Nginuity
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PostPosted: Tue Jan 30, 2007 6:18 pm    Post subject: Reply with quote

Looks really good! I hope we were able to help you some kind of way.

The petals...what kind of machine was it that cut them? It looks like really thin Plexi-Glass that has been finely sanded, or something along those lines? Very nice.
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