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Battery Christmas Lights

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trinculo54

New Member
Okay I'm trying to think of the best way to wire about 20 - 30 lights small christmas lights to stay on for at least 4-5 hours. I was thinking of using LEDs. I am currently experimenting with other lights and they don't stay on long enough, can some one suggest a possible solution; would LEDs extend the battery life. Any help would be great, thanks!
 

hotwaterwizard

Active Member
Leds would do the trick. Each one is 1.5v and you would need to hook them in series and then paralel depending on your voltage.
What is your voltage?
I can give you a diagram if I have the info.
 

trinculo54

New Member
I'm trying to keep the bulk of it down, so maybe a 9v? Or two AA, do you have any suggestions? A diagram would be great if you have one!! Thanks!
 

trinculo54

New Member
Super cool, so I could use a series/parallel cicuit and work it that way, any idea how long 25 LEDs would last with one 9v battery? How about 2? Thanks
 

hotwaterwizard

Active Member
Look at the rating of the battery in miliamps per hour and you can figure it from that.
 

audioguru

Well-Known Member
Most Helpful Member
Hi Trinculo,
A battery manufacturer's site is here:
https://www.energizer.com/
Their Energizer alkaline 9V rectangular battery will power (20mA through each LED) 6 strings of 4 red LEDs in series for about 1 hour, 2 paralleled batteries for about 3 hours. Their very expensive alkaline batteries will last only a few minutes more.
As the battery runs down, the LEDs will get dimmer and dimmer. The battery life that I calculated is when the LEDs are very dim.
Consider using very bright LEDs that are operating at reduced current to extend battery life, but don't select LEDs that are bright only because they have a very narrow-angle light beam.
 

trinculo54

New Member
Okay, let's see if I can do the math, if I assume each red LED draws 1.5 volts I would multiply the 6 strings times 1.5 to get 9 volts, I would then take each string and multiply that by the number of LEDs on each string 24 and then multiply that by the power for each LED thus I would draw 9 volts at 480mA. is that right?

With that logic I want to use white LEDs which draw 20mA at 3.3v so with one 9v I could get 2 strings with 12 LEDs each?!?! thus dropping 6.6volts with the same current of 480mA? That sounds wrong can anyone help :?: Thanks
 

trinculo54

New Member
Would I be better off using 3 AA and then tyring my luck with running the LEDs in parallel? I see from the charts that I can more then triple my life with AA or even use C's. Thus I would have to run one string with let's say 25 LED's in parallel. Thus using 3 volts at 500mA. The life of a AA at 2850mA (Energizer website) per hour I can expect 4-5 hours which is my target. Any thoughts or correction to my math. Thanks again
 

audioguru

Well-Known Member
Most Helpful Member
Trinculo,
Nope. The red LEDs are actually about 1.8V, not 1.5V. If you put 4 in series then their total voltage is 7.2V. Add a current-limiting resistor in series that is calculated for 20 mA through it with the difference voltage of 1.8V across it makes the resistor's value 90 ohms, but use a standard 100 ohms. Let us call that a string of 4 LEDs (and a resistor).
Make 6 of those strings and connect them to the battery. The total current is 120mA.

Use 2 white 3.3V LEDs in series, and also in series with a current-limiting resistor. Call that 1 string. If you want 20mA through each LED then 12 strings will draw 240mA.

A rectangular 9V battery contains 6 tiny cells that don't have much capacity. AA cells are much bigger so have much more capacity. 3 AA cells in series produce 4.5V, which is enough for strings of 2 red LEDs but only 1 white LED. Of course, all LED strings need a current-limiting resistor. 3 AA cells in series will power 12 strings of 2 red LEDs for about 3.2 hours. The same amount of time for 12 white LEDs.

Watch out for the battery spec's. The AA cell is rated with a "capacity" of 2850mA/hr, but to an end voltage of only 0.8V (extremely low) and with a low operating current of only 25mA. See the detailed graphs for your own end voltage (1.2V) and operating current (120mA or 240mA) and you will see that the actual capacity will be much less.
 
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