Battery life and 555

Thread Starter

Martino Chiro

Joined May 1, 2015
102
In this article:
http://www.allaboutcircuits.com/textbook/experiments/chpt-8/cmos-555-long-duration-minimum-parts-red-led-flasher/
is described a circuit powered by two 1,5V batteries. The author specify that the use of a 100uF cap in parallel to the batteries "extends the life of the batteries, since it will store the voltage during the 97% of time the circuit is off, and provide the current during the 3% it is on".
But i have not understand how, given that every time the cap discharges, as many times the battery must provide the same energy.
Could someone let me understand the principle that let the batteries last longer with the cap C2 ?

kindly regards
 

crutschow

Joined Mar 14, 2008
23,141
It's not really the cap as such. The long battery life comes from the low 3% duty-cycle of the circuit.
The capacitor is just to help provide the pulse of current needed when the LED fires as the battery gets run down and its resistance goes up.
 

Thread Starter

Martino Chiro

Joined May 1, 2015
102
It's not really the cap as such. The long battery life comes from the low 3% duty-cycle of the circuit.
The capacitor is just to help provide the pulse of current needed when the LED fires as the battery gets run down and its resistance goes up.
Thank You Crutschow, but what means "... as the battery gets run down and its resistance goes up." ?
 

Wendy

Joined Mar 24, 2008
21,840
The gel inside the battery that drives the chemical reaction that produces electricity becomes more solid, and less conductive. A discharged battery has the same voltage but can not provide the current any longer. The capacitor picks up the slack on current.

I am the author of the articles BTW. I had LEDs flashing on top of my TV for over a year as I tested several circuits out.
 

Thread Starter

Martino Chiro

Joined May 1, 2015
102
The gel inside the battery that drives the chemical reaction that produces electricity becomes more solid, and less conductive. A discharged battery has the same voltage but can not provide the current any longer. The capacitor picks up the slack on current.

I am the author of the articles BTW. I had LEDs flashing on top of my TV for over a year as I tested several circuits out.
Thank You Wendy,
my understanding is:
- the cap provide the brief peak current during the 3% the duty-cycle is ON.
If it had been The Battery to work in this way, the battery lasts less.
- most of the time the battery provide a litle current to slowly charge the cap. In this way the battery last longer.
It is correct ?
 

Wendy

Joined Mar 24, 2008
21,840
Correct.

The cap squeezes more useful life out of the battery, the life of the battery is the same either way.
 

Thread Starter

Martino Chiro

Joined May 1, 2015
102
When you write "the life of the battery is the same either way." means that the LED on top of your TV
would last the same time, also without the 100hF cap ?
 

Wendy

Joined Mar 24, 2008
21,840
No, the state of discharge for the battery is the same. The cap allows an almost dead battery to keep working in an acceptable manner. Without the cap you would probably throw the battery away.
 
Top