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I am working on making a costume that will require the use of fairy lights also known as string lights. It requires 16 separate strings and each string has its own small battery pack which contains 2 cr2032 3v batteries and a on/off switch. So a total of 32 cr2032 3v batteries

These are the lights I purchased.

So for testing purposes I tested this with 4 fairy lights and 1 9V battery. Note I have tried wiring 2 strings to 2 AAs and the brightness was not there. Not to mention having to carry around 32 AAs would be a bit tedious. Attached is an image of how I wired it up in series and parallel. wiring

  1. Series wiring - LEDs would not light up

  2. Parallel wiring - LEDs would light up however the battery slowly started getting hotter and hotter.

My question is, what would be the easiest way to wire all of these into 1 switch safely. Please keep in mind that this will be part of a costume so I'd like to keep weight down to a minimum. Note that carrying 4 9V batteries is alot ligher than 32AAs. I'm just not sure how I would wire them. And if resistors are needed, what size and where would they be inserted?

New Image wiring:

New Image

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    \$\begingroup\$ Note that both button cells and 9v batteries have poor energy density and high cost, so they should be used only when form factor makes them necessary. AAs have such higher energy density that you likely can use smaller mass, add a voltage converter and pay it's losses, and still come out on top. \$\endgroup\$
    – K H
    Commented Jan 7, 2019 at 21:02
  • \$\begingroup\$ As a compromise, you may wish to consider "fake" AAs and 9vs. There are AAAs, AAs and 9V batteries available now that actually have a 3.7V li-ion cell and a voltage converter inside them, giving much higher performance than an actual alkaline 9v. These batteries include a microusb input for charging them. \$\endgroup\$
    – K H
    Commented Jan 7, 2019 at 21:05
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    \$\begingroup\$ a 9V batttery contains six 1.5V AAAA cells .... way less power than AA or even AAA cells \$\endgroup\$
    – jsotola
    Commented Jan 8, 2019 at 1:10

3 Answers 3

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In the second one, the switch is shorting the battery through the path highlighted in blue, so it gets hot.

enter image description here

Correct wiring: the fairy lights are in parallel, but the battery and switch have to be in series.

enter image description here

If the fairy lights work on 3-4V they're probably white LEDs all wired in parallel, you will need resistors (blue):

enter image description here

It is preferable to use one resistor per string, that will make the brightness more even and the resistor will heat less. No idea what value you need, to know that you'd need to know how much current the lights use. If they used CR2032 batteries, these have quite high internal resistance, so you can start with 100 ohms.

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The 2nd is correct, all LEDs in parallel. Two CR2032 make 6V. Use a 4AA battery pack to drive the LEDs. Put the switch in series between the battery+ and the LED+.

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  • \$\begingroup\$ Thanks for the info about wiring them in parallel. In regards of the batteries, I am using 16 strings, which with your reply I would need 64 AA batteries which is a bit much to carry. What other options do I have? Also noted, while wired in parallel with the 9v battery and 4 strings the battery started getting hotter which is kind of a concern. \$\endgroup\$
    – Freqz
    Commented Jan 7, 2019 at 19:30
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    \$\begingroup\$ I think the battery got hot because you either shorted it out with the switch (which your 2nd drawing shows), or were drawing too much current from the battery.The lights only want 6V, not 9V. One 4xAA pack should be fine, or one 4xC on one 4xD if you want longer on time. \$\endgroup\$
    – CrossRoads
    Commented Jan 7, 2019 at 19:34
  • \$\begingroup\$ Just one battery pack for all the light strips in parallel. \$\endgroup\$
    – CrossRoads
    Commented Jan 7, 2019 at 19:36
  • \$\begingroup\$ You are right about the switch wiring. I just changed it to per your suggestion and its good. \$\endgroup\$
    – Freqz
    Commented Jan 7, 2019 at 19:40
  • \$\begingroup\$ So what you are saying in regards of the battery, if I were to use 4xD sized batteries I will be able to power all 16 strings? If so would it be any difference if I were to use 8xAA since I already have a holder? \$\endgroup\$
    – Freqz
    Commented Jan 7, 2019 at 19:41
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Two things to consider. Led fairy lights typically have all the leds in parallel.

1, cr2032 have high internal resistance. As the current draw gets higher, the voltage drops. If your fairy lights do not have a resistor, it may rely on that internal resistance to keep the voltage and current in check. AA batteries do not have this high resistance so that may not be good for your leds. Whatever voltage you get, use ohms law for the resistor you need. (Source Voltage - measured voltage) / Current = Resistance. See below for current.

2, how long do you want the batteries to last? And how much current does each string pull? If you measure the current, say the string pulls 80mA. Then you have 16 strings, that's 1280 mA. With standard good energizer Alkaline AA with a capacity of 2800mAh, that's roughly 2.25 Hours of run time. Let's round to 2 hours of good brightness. If the current is only 40mA then that's 4 hours. Etc.

So first you need to know the voltage across the led string with the cr2032. Maybe it gets pulled down because of the current. And you need to know the current draw. Both are easy to find with a multimeter.

Having known both, I recommend either 3x AA for 4.5V or a usb power bank at 5V. Use a 1n4001 diode to bring the voltage down 0.7V for safety. Then add calculate the formula. For a 5V usb power bank, minus 0.7V for the diode, (4.3V - 3.5V measured voltage) / (0.08 A current per string * 16 Strings) ~= 1 Ohm resistor.

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