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Today my wife gave me a little box of solar-powered Christmas tree lights. She bought them last year 2019 but had forgotten she had them so it was only today (a year later) that I took them out of the box. These lights consist of a small solar panel wired to a rechargeable solar battery and an alkaline battery plus wiring which connects the circuitry to a string of 100 little LED lights. There are no moving parts that I can see.

I took everything out of the cardboard box and placed the components onto a white melamine table in our living room and screwed the connector for the wire from the LED lights string into the back of the solar panel box. Then I turned the solar panel itself up to the light within the room - which wasn't particularly bright as I was indoors. None of the LEDs glowed at all, so I assumed that I would need to place the solar panel outside in bright sunlight for some time to gain enough 'light energy' to power the LEDs. But then .. I accidentally knocked the solar panel box face down onto the white melamine table and the LEDs lit up. Every time I picked the box off the table and turned it to face the room, the LEDs went off, and every time I turned the solar panel to be back face down on the melamine table - the LEDs lit up again. I have left the solar panel box lying face down and the LEDs have now been lit for 3 hours and are fairly bright.
So my question is this: the solar panel is face down on the table so no room light or natural light or solar light can strike its face, and I think that even if I was to black tape around the box (i.e. stick it to the table) that wouldn't make any difference ... so from where is the solar panel deriving the energy it needs to light the LEDs?

I have removed the alkaline battery and the LEDs still light; if I remove the solar battery then they go off because that surely breaks the circuit between the solar panel and the LEDs. If the solar battery had had charge after 1 year in the box, then surely the LEDs would light up when I turn the solar panel away from the table wouldn't they? What am I missing here, please?

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    $\begingroup$ could there be a connection problem? i.e. face down something makes contact whereas face up the contact breaks? Try it with the alkaline battery, does it do the same? Then it means there is charge left in the rechargeable. $\endgroup$ – anna v Dec 7 '20 at 16:11
  • $\begingroup$ @Richard Hammond did you check out the connections ? Wasn't there any failure ? Did you try it on rough surface or a dull surface or any other surface too ? $\endgroup$ – A Student 4ever Dec 7 '20 at 16:12
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    $\begingroup$ Is there a photocell that turns the lights off when there is ambient light and turns them on when it gets dark? $\endgroup$ – Brick Dec 8 '20 at 17:06
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Here's what I think:

  1. It doesn't take a lot of charge to light LEDs for a few hours. Your "Solar Battery" probably still has enough charge from the factory to power the lights for a while. I imagine if you keep the solar panel out of the sunlight, eventually you will run down the solar battery and it will stop working.
  2. Your solar panel likely also has an ambient light sensor on its front. That way, the lights will never turn on when it's light outside (when you probably don't want them anyways) and they will automatically switch on when it gets dark. When you dropped the panel on the ground, you covered up the ambient light sensor, and the lights switched on. (The idea being that if the solar panel is placed outside, the lights only turn on at night)
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    $\begingroup$ Thank you for all your comments thus far, I'm going to leave the solar panel face down on the table for ... well...as long as I can before the wife makes me move it just to see if I can run the battery down. I would have thought that out of the box after a year there would have been no charge in it... but its worth a try. @Bunji its too cheap (!) to have a sensor on the front I think. Thanks for all the help thus far, I will be back in a week if the LEDs are still on! $\endgroup$ – Richard Hammond Dec 7 '20 at 16:17
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    $\begingroup$ @RichardHammond, it can use the solar panel itself as the ambient light sensor. When the solar panel is receiving light, it charges the battery. When it isn't receiving light, it uses the battery to turn on the lights. The cost of the electronics to do that is a maybe $0.05. Most customers will want the lights on when its dark and not when it's daytime, so not including this feature would reduce sales considerably. $\endgroup$ – The Photon Dec 7 '20 at 17:45
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    $\begingroup$ All the dollar store outdoor lights have this sensor. ("dollar store" = a store where cheap stuff is sold in North America). $\endgroup$ – nasu Dec 7 '20 at 19:15
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    $\begingroup$ @nasu: another evidence for inflation. When I was a child they were five and dime stores because much of the merchandise was priced at 5 or 10 cents $\endgroup$ – Ross Millikan Dec 8 '20 at 3:05
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    $\begingroup$ W.r.t. point 2: Some outdoor lights I have use the solar panel itself as the light sensor. The batteries are shipped charged and will easily hold charge for years $\endgroup$ – Chris H Dec 8 '20 at 9:01

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