Yeah, but they are also spendy. A $3 NiMH has twice the Wh of Sparkfun's cheapest (and only?) protected LiPO at a third the cost, and the LiPO still needs a special charger. The only advantage to the lipo is it's flat.<br>
<br><div class="gmail_quote">On Tue, Jan 4, 2011 at 3:26 PM, Dr. Jesus <span dir="ltr"><<a href="mailto:j@hug.gs">j@hug.gs</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;">
The sparkfun batteries have over/under protection. Since the scarf is<br>
a thermal insulator I'm assuming the current protection can't be<br>
trusted.<br>
<div><div></div><div class="h5"><br>
On Tue, Jan 4, 2011 at 2:49 PM, Corey McGuire <<a href="mailto:coreyfro@coreyfro.com">coreyfro@coreyfro.com</a>> wrote:<br>
> The trouble with rechargeable batteries is how you handle over charging and<br>
> 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 others<br>
> is, their voltages drop to the point where the LED's would be very dim<br>
> before the cells reached a critical charge level. LiPO's typically hold<br>
> their voltage to the bitter end...at least to the levels that we humans can<br>
> detect with our senses. Motor's will happily whir, LED's will burn bright,<br>
> and you won't know it's too late.<br>
><br>
> To prevent over discharge with LiPO's, you need to have a voltage cut off<br>
> circuit of some kind.<br>
><br>
> Using Alkaline cells (AAA, AA, C, D, etc) means people can opt to use NiMH<br>
> or NiCad batteries. Then battery charging is their problem, and not yours.<br>
><br>
> If you want to solve the recharging problem, your self, you can include NiMH<br>
> or NiCad batteries ( <a href="http://www.batteryspace.com/" target="_blank">http://www.batteryspace.com/</a> ) and just provide a wall<br>
> wart that gives 1.5v per cell wired in series (2 cells, 3v, etc.) at<br>
> 50mah-100mah of current, and you won't have to worry about over charging.<br>
><br>
> The same can be done for the A123, LiFePo4 cells I linked, only they require<br>
> 3.6v per cell at a low current.<br>
><br>
> On Tue, Jan 4, 2011 at 2:26 PM, Dr. Jesus <<a href="mailto:j@hug.gs">j@hug.gs</a>> wrote:<br>
>><br>
>> On Tue, Jan 4, 2011 at 12:06 PM, meredith scheff <<a href="mailto:satiredun@gmail.com">satiredun@gmail.com</a>><br>
>> wrote:<br>
>> > I'd like to do a soft circuit scarf or three, but I'm always 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 still<br>
>> > learning<br>
>> > this EE stuff. Could some kind person point me in the right 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 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 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 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>
>> _______________________________________________<br>
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>> <a href="mailto:Noisebridge-discuss@lists.noisebridge.net">Noisebridge-discuss@lists.noisebridge.net</a><br>
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><br>
><br>
</div></div></blockquote></div><br>