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I have a string of Christmas lights (mini LEDs wired in parallel.)

My LEDs were too dim, so I took it apart. I found they are powered by a single 1.2V NiCd same shape/size as 1.5V AA battery.

The NiCd is recharged by a small solar panel. Inside the controller box, there is a small chip and a few components which allow the LEDs to flicker or remain constant when you push a button.

I am pretty sure the LEDs are wired in parallel, because there are so many of them, all powered by this one 1.2V battery. Another possible factor is that the LEDs become dimmer the further away from the source they are. At the end of the string, there is practically no light, but there are no actual breaks or LED failures. I couldn't detect a resistor before each LED in the string of LEDs.

Trying to repair, I replaced the NiCd by a 1.5AA battery (not rechargeable) and the string of LEDs work fine (mostly.)

Because the LEDs were a little dim for my liking, I thought I would try to use a different source of power: a bench top power supply (BTPS), set to 1.5 volts. I connected this exactly in place of the battery; the power then went through the chip and other components before it reaches the LEDs. When I did this, the LEDs lit up for a moment only, and then there was no light. Turning the power on and off at the BTPS gave me the same flicker each time. Removing the BTPS and replacing with a 1.5AA battery, the LEDs worked fine once again.

I thought this phenomenon must be an issue to do with internal resistance of the NiCd battery / AA battery, and a relative lack of internal resistance in the BTPS. Other questions like this mention internal resistance. I also read this, which insists that the parallel LEDs need a current limiter, which my BTPS I think will override.

But then I tried different voltages to try to compensate for a lack of resistor, but no success. I didn't have the means on hand to insert a resistor in series with the BTPS. Would this have made it work properly?

Why does the battery work.differently from the power supply?

Tim
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    Did you connect the lab power supply in place of the battery or directly to LEDs? The circuitry at the battery compartment must include a boost converter as otherwise a single 1.5V battery can't light up LEDs, not white ones at least. – Justme Dec 22 '20 at 06:53
  • that's an over-voltage protection on the controller chip. kind of surprising, but good to see. – dandavis Dec 23 '20 at 00:11
  • @Justme. I connected the lab power supply exactly where the battery was. (Not directly to LEDs). – Tim Dec 23 '20 at 10:18
  • The LEDs are really small. I also can't understand how the 1.2V NiCd drives a whole string of them – Tim Dec 23 '20 at 10:20
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    I already mentioned, the chip you mentioned must be a boost converter of some sort. A 1.2V would not even light up a single LED by itself. – Justme Dec 23 '20 at 10:42

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