LED / batteries update (from a "Gary" out there somewhere)
The following info. arrived through a reasonably long chain of events :
A typical AA "alkaline" battery is good for about 2.5 amp hrs (which is quite a bit). So if an LED were run at a nominal current of 20ma (current typically used in an ordinary LED when you don't care about battery life) the battery would be good for about 125 hours, or 5.2 days. At 17 hours per day, it would be 7.35 days, or about 1 week.
Now in real life, they probably wire the 3 batteries in series, and simply use a resistor to set the current. So wiring them in series doesn't do anything extra for battery life, it just lets the circuitry run off 4.5v instead of 1.5 (the LED drops 1-2v, so you need at least 2-3V just to light it up reliably) and reduces the change in LED brightness as the battery voltage drops slightly with use.
So with a 2.5 amp/hr rating, you can guess they probably don't run 20ma, they probably run in the vicinity of 5 ma or less if you are getting 3 weeks out of the batteries at 17 hours per day. This represents about 350 hours.
They do have high efficiency LEDs that generate more light at lower
currents, and it is likely that is what they are using to get reasonable brightness at currents in the 2-5ma range.
While a battery life of 3 weeks or less may seem short, keep in mind that a typical filament type light bulb in a 2 "AA" cell penlight uses 250ma. This would be only 10 hours except it is twice the discharge rating for a "AA" cell, so you won't get 10 hours of use at this discharge rate. A typical "AA" ni-cad cell is only good for 500ma/hr, or about 1/5 as much, but its discharge curve is flatter (the voltage stays close to 1.2v). The zinc-carbon cells are rated discharged when they reach .9v, and they start at 1.5 so they change more as they discharge. For more bucks you can get "AA" ni-cad's good for 1 amp/hr, or NiMh good for 1.5 amp/hr.
Note that "alkaline" batteries don't really have more capacity, they are just designed for higher current shorter duty discharge (such as toys with electric motors). The reason is that as a conventional cell discharges at high currents it "polarizes" the electrodes, which reduces its capacity. At slow discharge currents, the electrolyte takes care of this okay, but at high rates it reduces the capacity, or useful life of the battery. Alkaline cells have "depolarizers" (which if I remember right are magnesium dioxide powder) in the electrolyte which prevent loss of capacity at high discharge rate by increasing the depolarizing rate.
<This was all from someone named Gary.>
Jon. |
