Why do my LED lights require a different voltage when they are hardwired?

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.

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?

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?

• The internal resistance of the battery is probably providing current limiting. Are these cell batteries? Nov 17, 2020 at 15:07
• You should also measure the current in the battery case and in the fixed supply case. Nov 17, 2020 at 15:11
• 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. Nov 17, 2020 at 15:26
• Is there a resistor in the battery pack? Nov 17, 2020 at 15:34
• 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. Nov 17, 2020 at 17:35

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