[ot] [sdiy] Battery questions

[Roy J. Tellason]

On Monday 17 January 2005 02:21 pm, Tim Parkhurst wrote:

> Some real basic LED info (my apologies if you already know this). The
> battery life will depend on how much current you’re drawing from it.

Yep.

> To figure out how much current you’ll need to light your LED, you need to 
> find out a couple of things: 1) the forward voltage drop across the LED and 
> 2) the maximum current the LED will take. Both of these can usually be found
> in a spec sheet for the LED. There are a couple of ‘general’ assumptions
> you can make to get you in the ballpark: The forward voltage drop is
> usually around 1.7V.

Most of the ones I check seem to be somewhat higher,  over 1.8V usually.  
Unless they're IR devices.

> Some LEDs drop as much as 2.2V, so again, check the spec sheet if at all
> possible.

Or clip-lead a resistor in series with it,  apply some power,  and measure!

> Second assumption – most LEDs will take a MAXimum of 20mA (0.020A) current.

I would argue with this.  Most that I've looked at spec sheets on say _50mA_ 
as an absolute maximum rating.  I typically _run_ them at 20mA,  sometimes a 
bit higher.

> With that in mind, you can figure out what resistor you’ll need to run the
> LED.

Yep.  Or take the clip-lead-and-junkbox-full-of resistors-and-measure-things 
approach...

> R = Vbatt-Vled / Iled (this is just Ohm’s law R = V / I, but remember we
> have to subtract the voltage drop across the LED)

Which is gonna vary from one part to the next,  why I measure them.

> For a 3V source (two batteries in series), and running 10mA (usually gives
> good brightness),

Not with those "radio shack assortment" parts I got some of (years ago, and I 
still haven't used 'em all up!) -- typically those do better brightness-wise 
at 20-25mA.

> we get
>
> R = 3 – 1.7 / 0.010
>
> R = 1.3 / 0.010
>
> R = 130

> So a 130 Ohm resistor and a three volt supply will give you 10mA through
> the LED. You could probably get away with even less current. It depends on
> the LED. Some high efficiency LEDs will have a lower voltage drop and will
> give very good light levels with at very low currents.

It's quite possible that newer stuff will give more / better brightness at 
lower levels than what I'm used to.  And it's amazing how bright some of them 
are getting any more these days.

> After you know how much current you need to run through the LED, THEN you
> can check the mAH rating of the batteries. This is where I’m a little
> unclear – so perhaps a battery guru can step in.

I have a chart here someplace...   (Looking)   Ok,  this one chart shows AA 
cells as having 2000mAH capacity.  I'm inclined to think that's overly 
optimistic,  but maybe it'd work.  The chart also gives "typical drain" of 
50mA,  so for just driving an LED it should last considerably longer.

> Basically, mAH is a rating of how much current the battery can supply over
> time. I don’t know how to apply this at this point, but this can probably be
> found out with a little Googling (funny how that word has become a verb).

Best place for that info would be on the battery maker's site.  One thing to 
bear in mind with battery AH ratings is that they're specified for a certain 
time period.  Like for example lead-acid gel cells are spec'd for "a 20-hour 
rate",  so a 10AH unit will deliver 500mA for 20 hours.  Pull 1A out of it 
and it won't last quite 10 hours,  the more you go in that direction the 
worse it gets.  Smaller drains last proportionately longer...

I used to manage a retail battery store,  and there were people who had a hard 
time grasping this.  One guy wanted to buy a 7AH unit to drive a small motor 
he used to start model airplanes.  I didn't recommend this.  He came back 
later and that battery was fairly well cooked.  I told him to bring the box 
in with the motor and I'd measure.  It pulled around 3-4A with no load!  
Definitely too much for that unit to handle for long...