[Noisebridge-discuss] driving multiple LEDs with minimal batteries

Michael Shiloh michaelshiloh1010 at gmail.com
Tue Jan 4 22:59:28 UTC 2011

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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Michael Shiloh

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