[Noisebridge-discuss] driving multiple LEDs with minimal batteries

Sean Cusack sean.p.cusack at gmail.com
Tue Jan 4 14:55:10 PST 2011


For the new Tron suit (which has 188 LEDs BTW), I ended up using a LiPo
battery pack despite the risk of my crotch catching on fire because I needed
serious power density. Although it was spendy, I ended up buying a LiPo pack
which some Serious Professional has already integrated the cut off stuff
into. I used this pack:
http://www.batteryspace.com/Polymer-Li-ion-Box-Battery-12V-13.2V-74Wh-4A-rate-w/-Regulator-and-Low.aspx

Its pretty big, but the whole LED setup draws about 1.4A @ 12V, so I needed
something pretty substantial.

Sean

On Tue, Jan 4, 2011 at 2:49 PM, Corey McGuire <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> wrote:
>
>> On Tue, Jan 4, 2011 at 12:06 PM, meredith scheff <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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>>
>
>
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