[Noisebridge-discuss] driving multiple LEDs with minimal batteries

Corey McGuire coreyfro at coreyfro.com
Tue Jan 4 16:04:45 PST 2011


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.

On Tue, Jan 4, 2011 at 3:26 PM, Dr. Jesus <j at hug.gs> wrote:

> The sparkfun batteries have over/under protection.  Since the scarf is
> a thermal insulator I'm assuming the current protection can't be
> trusted.
>
> 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.
> >> _______________________________________________
> >> Noisebridge-discuss mailing list
> >> Noisebridge-discuss at lists.noisebridge.net
> >> https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss
> >
> >
>
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