23 Jan 2006, 00:32
As far as I can recall LED watches are similar to LCD when the are not displaying the time. IE, a tiny current is being drained, like 0.01ma, or 10 microamps or even less. LCDs drain the same whilst showing the time, hence their advantage.
With early LED's the drain shoots up to 20-50ma when its ON, and even more if its a non-mulitplexed display.
The problem with charging is you need to put power into a rechargeable battery which is not 100% efficient process, and you need full on sunlight, not a desk lamp. My experience with PV solar cells is that the drop off in charge current is huge as soon as the sun is not on it. Cloudy days and fluorescent lights don?t even generate 10% of its full capacity. Its got to be direct sunlight !
As for drain :-
Loosely speaking, there are 8,760 hrs in a year, and most largish button cells have a mah rating of approx 100mah ? 150mah ( mah = milli-amp-hours), or in other words a button cell could supply 0.1amps for one hour and then be dead.
So to last a year, you basically take the mah rating, divide it by the number of hours in a year to work out how much current drain it can tolerate in order to last a year.
So lets take an average mah rating of 125mah, divided by 8760 and you get 14 microamps or 0.014ma of continuous drain to last approx a year. So a LED or LCD watch can easily last 1 yr if its only drawing an average of 0.010 ma.
When the LED display is on the current drain shoots up to, say, 20-50ma, but only for a few seconds, and maybe 1 minute in a whole day. (Then you?ve got alarms and backlights to think about with LCDs)
It is possible to work out how long a battery is likely to last by combinations of minutes used per year and the ?on? current drain, and factoring in the tiny drain when it off , and knowing the mah rating.
EG, if your watch draws 50ma when display is on, and it?s a 125mah battery, and you held the button pressed in until it went dead, you might get 2 to 2.5 hrs display before it went flat !!
