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I have a cheap set of LED string lights. There are 20 lights, all wired in parallel connected to a small battery bank. There are 2, 3V batteries wired in series, providing 6V of power to the string.

This is a little confusing, since I know LEDs run on 2-3V, but I verified my physics with a voltage meter, and two 3V batteries in series is really 6V. If I try putting the batteries in parallel, they produce 3V and the lights do not come on.

LED String Lights

For a small DIY project, I would like to hardwire these lights. That is, I would like to connect them to the 120V system in my house. I found an old 12v DC converter and I have a buck step down converter. Wiring the two units together, I can easily create a DC power supply, supplying 6v.

But should I?

Buck Converter

It turns out the answer is NO, they will burn out very quickly. In fact, I can power the lights just fine with 2.3V when they are hardwired.

Why can I power the lights with 2.3V of power when they are hardwired, but I require 6V when using a battery pack?

A follow-up (safety) question. Just because I can do this, should I? Is there a safety concern with the LEDs when hardwired using a DC and step down converter?

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    \$\begingroup\$ The internal resistance of the battery is probably providing current limiting. Are these cell batteries? \$\endgroup\$
    – user1850479
    Commented Nov 17, 2020 at 15:07
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    \$\begingroup\$ You should also measure the current in the battery case and in the fixed supply case. \$\endgroup\$
    – pjc50
    Commented Nov 17, 2020 at 15:11
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    \$\begingroup\$ Are you measured batteries voltage without load? Check it under load. You hardwired PS just can provide bigger current when batteries. It is all about current, not voltage. \$\endgroup\$
    – user263983
    Commented Nov 17, 2020 at 15:26
  • \$\begingroup\$ Is there a resistor in the battery pack? \$\endgroup\$
    – Justme
    Commented Nov 17, 2020 at 15:34
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    \$\begingroup\$ Coin cell batteries have a high internal series resistance. If you measure the battery voltage when the LEDs are on, then you'll see that the voltage is no longer 6V. \$\endgroup\$
    – Aaron
    Commented Nov 17, 2020 at 17:35

2 Answers 2

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Coin cell batteries have a high internal series resistance. If you measure the battery voltage when the LEDs are on, then you'll see that the voltage is no longer 6V.

Direct from energizer:

For example, the starting IR of a 2032 battery is near 10 ohms

IR = internal resistance

https://data.energizer.com/pdfs/lithiumcoin_appman.pdf



As for your safety question:

Is there a safety concern with the LEDs when hardwired using a DC and step down converter?

That depends on what you are using for your offline power supply. If it provides some sort of isolation, then it's most likely safe. There is no inherent reason you can not do this. It is in fact how almost any LED lighting system works (that plugs into the mains supply).

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So why can I power the lights with 2.3v of power when they are hardwired, but I require 6v when using a battery pack?

Because the battery pack hides a big series resistor.

Some of these LED stings come with a clear plastic case where you can see the resistor.

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