[Noisebridge-discuss] driving multiple LEDs with minimal batteries
Corey McGuire
coreyfro at coreyfro.com
Tue Jan 4 23:02:42 UTC 2011
Want to meet Thursday before our respective meetings and discuss how to
handle it?
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.
On Tue, Jan 4, 2011 at 2:59 PM, Michael Shiloh
<michaelshiloh1010 at gmail.com>wrote:
> This info needs to be wikified. led and battery questions are easily in
> the top 5 FAQs.
>
> i was about to do it but wonder about the structure.
>
> do we have a technical information category? i couldn't figure out how
> to get a list of categories.
>
> should there be a link to technical information from the front page?
>
> where would you expect to find this sort of information?
>
> On 01/04/2011 02:54 PM, Corey McGuire wrote:
> > Typically with NiCad and NiMH batteries, Sub-C cells are the best bang
> > for buck and have the best energy density. This is because they are
> > used in just about every industrial/hobbie rechargeable device and
> > battery companies focus on this packaging. These are the cells roombas
> use.
> > http://www.batteryspace.com/subcsizeseriesbatteries.aspx
> >
> >
> > On Tue, Jan 4, 2011 at 2:49 PM, Corey McGuire <coreyfro at coreyfro.com
> > <mailto:coreyfro at coreyfro.com>> wrote:
> >
> > The trouble with rechargeable batteries is how you handle over
> > charging and over discharging. You want the simplest solution
> possible.
> >
> > A drawback of LiPO batteries is over discharging. Other battery
> > technologies can also be over discharged, but the advantages of the
> > others is, their voltages drop to the point where the LED's would be
> > very dim before the cells reached a critical charge level. LiPO's
> > typically hold their voltage to the bitter end...at least to the
> > levels that we humans can detect with our senses. Motor's will
> > happily whir, LED's will burn bright, and you won't know it's too
> late.
> >
> > To prevent over discharge with LiPO's, you need to have a voltage
> > cut off circuit of some kind.
> >
> > Using Alkaline cells (AAA, AA, C, D, etc) means people can opt to
> > use NiMH or NiCad batteries. Then battery charging is their
> > problem, and not yours.
> >
> > If you want to solve the recharging problem, your self, you can
> > include NiMH or NiCad batteries ( http://www.batteryspace.com/ ) and
> > just provide a wall wart that gives 1.5v per cell wired in series (2
> > cells, 3v, etc.) at 50mah-100mah of current, and you won't have to
> > worry about over charging.
> >
> > The same can be done for the A123, LiFePo4 cells I linked, only they
> > require 3.6v per cell at a low current.
> >
> >
> > On Tue, Jan 4, 2011 at 2:26 PM, Dr. Jesus <j at hug.gs
> > <mailto:j at hug.gs>> wrote:
> >
> > On Tue, Jan 4, 2011 at 12:06 PM, meredith scheff
> > <satiredun at gmail.com <mailto:satiredun at gmail.com>> wrote:
> > > I'd like to do a soft circuit scarf or three, but I'm always
> > running up
> > > against the problem of power. I usually use fairly low power
> LEDs
> > > (<2v) driven by a 9v battery or one of sparkfun's LiPos.
> > > I've heard tell of somehow being able to power more, but I'm
> > still learning
> > > this EE stuff. Could some kind person point me in the right
> > direction?
> >
> > You want to wire them up in parallel:
> >
> > (+) -|>|- (-)
> > (+) -|>|- (-)
> > (+) -|>|- (-)
> >
> > Not series:
> >
> > (+) -|>|- -|>|- -|>|- (-)
> >
> > If you have too many LEDs on the same battery it won't work
> because
> > they will draw too much power. How many is too many depends on
> the
> > LEDs. If you hook them up directly to the battery, they may
> > draw more
> > current than they're rated for, which is bad for the LEDs and may
> > cause the lipo battery to catch fire.
> >
> > The cheap and easy way to make sure they don't draw too much
> > power is
> > to put a resistor in series with the LED to limit the current.
> >
> > (+) -/\/\/\-|>|- (-)
> > (+) -/\/\/\-|>|- (-)
> > (+) -/\/\/\-|>|- (-)
> >
> > The resistor value in ohms is (battery volts)-(LED voltage drop)
> /
> > (the LED current you want in amps). If you want 20 milliamps
> > through
> > a single 2 volt LED and you're using a LiPo battery:
> >
> > (4 volts - 2 volts) / 0.02 amps = 100 ohms
> >
> > The LiPo battery voltage is only 4 volts when it's fully charged.
> > When it begins discharging, it drops to about 3.7 for most of its
> > discharge curve and then to 2.7 right at the very end. Even
> though
> > the "right" number is 3.7 volts for most of the time the battery
> is
> > discharging, use 4 volts in your calculations to avoid using too
> > little resistance and putting too much current through the LED.
> >
> > If you have too many LEDs in the circuit, the battery will try to
> > supply too much current. If the battery is unregulated it might
> get
> > hot and catch fire.
> >
> > The resistor "throws away" the extra energy going to the LED in
> the
> > form of heat, but a resistor is really cheap and you can put lots
> of
> > them in your circuit easily. To make the battery last longer,
> you
> > need to build or buy a constant-current regulator or a switching
> > regulator, which is harder and a little more expensive.
> > _______________________________________________
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> > Noisebridge-discuss at lists.noisebridge.net
> > <mailto:Noisebridge-discuss at lists.noisebridge.net>
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> >
> >
> >
> >
> >
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>
> --
> Michael Shiloh
> KA6RCQ
> www.teachmetomake.com
> teachmetomake.wordpress.com
> _______________________________________________
> Noisebridge-discuss mailing list
> Noisebridge-discuss at lists.noisebridge.net
> https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss
>
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