Want to meet Thursday before our respective meetings and discuss how to handle it?<br><br>I'm an RC car/heli/plane nerd, so I have spent way too much time learning about batteries from my arm chair. I am not, however, an EE or a Chemist.<br>
<br><div class="gmail_quote">On Tue, Jan 4, 2011 at 2:59 PM, Michael Shiloh <span dir="ltr"><<a href="mailto:michaelshiloh1010@gmail.com">michaelshiloh1010@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
This info needs to be wikified. led and battery questions are easily in<br>
the top 5 FAQs.<br>
<br>
i was about to do it but wonder about the structure.<br>
<br>
do we have a technical information category? i couldn't figure out how<br>
to get a list of categories.<br>
<br>
should there be a link to technical information from the front page?<br>
<br>
where would you expect to find this sort of information?<br>
<div class="im"><br>
On 01/04/2011 02:54 PM, Corey McGuire wrote:<br>
> Typically with NiCad and NiMH batteries, Sub-C cells are the best bang<br>
> for buck and have the best energy density. This is because they are<br>
> used in just about every industrial/hobbie rechargeable device and<br>
> battery companies focus on this packaging. These are the cells roombas use.<br>
> <a href="http://www.batteryspace.com/subcsizeseriesbatteries.aspx" target="_blank">http://www.batteryspace.com/subcsizeseriesbatteries.aspx</a><br>
><br>
><br>
> On Tue, Jan 4, 2011 at 2:49 PM, Corey McGuire <<a href="mailto:coreyfro@coreyfro.com">coreyfro@coreyfro.com</a><br>
</div><div class="im">> <mailto:<a href="mailto:coreyfro@coreyfro.com">coreyfro@coreyfro.com</a>>> wrote:<br>
><br>
> The trouble with rechargeable batteries is how you handle over<br>
> charging and over discharging. You want the simplest solution possible.<br>
><br>
> A drawback of LiPO batteries is over discharging. Other battery<br>
> technologies can also be over discharged, but the advantages of the<br>
> others is, their voltages drop to the point where the LED's would be<br>
> very dim before the cells reached a critical charge level. LiPO's<br>
> typically hold their voltage to the bitter end...at least to the<br>
> levels that we humans can detect with our senses. Motor's will<br>
> happily whir, LED's will burn bright, and you won't know it's too late.<br>
><br>
> To prevent over discharge with LiPO's, you need to have a voltage<br>
> cut off circuit of some kind.<br>
><br>
> Using Alkaline cells (AAA, AA, C, D, etc) means people can opt to<br>
> use NiMH or NiCad batteries. Then battery charging is their<br>
> problem, and not yours.<br>
><br>
> If you want to solve the recharging problem, your self, you can<br>
> include NiMH or NiCad batteries ( <a href="http://www.batteryspace.com/" target="_blank">http://www.batteryspace.com/</a> ) and<br>
> just provide a wall wart that gives 1.5v per cell wired in series (2<br>
> cells, 3v, etc.) at 50mah-100mah of current, and you won't have to<br>
> worry about over charging.<br>
><br>
> The same can be done for the A123, LiFePo4 cells I linked, only they<br>
> require 3.6v per cell at a low current.<br>
><br>
><br>
> On Tue, Jan 4, 2011 at 2:26 PM, Dr. Jesus <<a href="mailto:j@hug.gs">j@hug.gs</a><br>
</div><div class="im">> <mailto:<a href="mailto:j@hug.gs">j@hug.gs</a>>> wrote:<br>
><br>
> On Tue, Jan 4, 2011 at 12:06 PM, meredith scheff<br>
</div><div><div></div><div class="h5">> <<a href="mailto:satiredun@gmail.com">satiredun@gmail.com</a> <mailto:<a href="mailto:satiredun@gmail.com">satiredun@gmail.com</a>>> wrote:<br>
> > I'd like to do a soft circuit scarf or three, but I'm always<br>
> running up<br>
> > against the problem of power. I usually use fairly low power LEDs<br>
> > (<2v) driven by a 9v battery or one of sparkfun's LiPos.<br>
> > I've heard tell of somehow being able to power more, but I'm<br>
> still learning<br>
> > this EE stuff. Could some kind person point me in the right<br>
> direction?<br>
><br>
> You want to wire them up in parallel:<br>
><br>
> (+) -|>|- (-)<br>
> (+) -|>|- (-)<br>
> (+) -|>|- (-)<br>
><br>
> Not series:<br>
><br>
> (+) -|>|- -|>|- -|>|- (-)<br>
><br>
> If you have too many LEDs on the same battery it won't work because<br>
> they will draw too much power. How many is too many depends on the<br>
> LEDs. If you hook them up directly to the battery, they may<br>
> draw more<br>
> current than they're rated for, which is bad for the LEDs and may<br>
> cause the lipo battery to catch fire.<br>
><br>
> The cheap and easy way to make sure they don't draw too much<br>
> power is<br>
> to put a resistor in series with the LED to limit the current.<br>
><br>
> (+) -/\/\/\-|>|- (-)<br>
> (+) -/\/\/\-|>|- (-)<br>
> (+) -/\/\/\-|>|- (-)<br>
><br>
> The resistor value in ohms is (battery volts)-(LED voltage drop) /<br>
> (the LED current you want in amps). If you want 20 milliamps<br>
> through<br>
> a single 2 volt LED and you're using a LiPo battery:<br>
><br>
> (4 volts - 2 volts) / 0.02 amps = 100 ohms<br>
><br>
> The LiPo battery voltage is only 4 volts when it's fully charged.<br>
> When it begins discharging, it drops to about 3.7 for most of its<br>
> discharge curve and then to 2.7 right at the very end. Even though<br>
> the "right" number is 3.7 volts for most of the time the battery is<br>
> discharging, use 4 volts in your calculations to avoid using too<br>
> little resistance and putting too much current through the LED.<br>
><br>
> If you have too many LEDs in the circuit, the battery will try to<br>
> supply too much current. If the battery is unregulated it might get<br>
> hot and catch fire.<br>
><br>
> The resistor "throws away" the extra energy going to the LED in the<br>
> form of heat, but a resistor is really cheap and you can put lots of<br>
> them in your circuit easily. To make the battery last longer, you<br>
> need to build or buy a constant-current regulator or a switching<br>
> regulator, which is harder and a little more expensive.<br>
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</div><font color="#888888">--<br>
Michael Shiloh<br>
KA6RCQ<br>
<a href="http://www.teachmetomake.com" target="_blank">www.teachmetomake.com</a><br>
<a href="http://teachmetomake.wordpress.com" target="_blank">teachmetomake.wordpress.com</a><br>
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