[Noisebridge-discuss] driving multiple LEDs with minimal batteries

Corey McGuire coreyfro at coreyfro.com
Tue Jan 4 14:49:38 PST 2011


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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