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I have a bag of unlabeled LEDs and intend to use them in a project. I connected one to a 9V PP3 to test it out, and was quite surprised to see that it didn't blow, given the fact I hadn't used a resistor.

I figured that they must be rated for 9 or 12 volts. I then decided to hook one up to a 3.75V Li-ion battery, and it promptly died. Why is this? Do PP3 batteries have some sort of voltage protection, or is it a current thing?

Davide Andrea
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Harry Spencer
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    Maybe visit https://electronics.stackexchange.com/questions/151627/why-does-an-led-have-a-maximum-voltage Do you want to *test* the LEDs, or do you want to *fry* them? – U. Windl Oct 24 '22 at 09:11
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    Please stop playing around and shorting li-ion cells into LEDs before you hurt yourself or start a fire. – winny Oct 24 '22 at 16:18
  • Are you leaving out something vital? Why would you expect any 3.75V battery to do the job of a a 9V PP3? – Robbie Goodwin Oct 25 '22 at 18:43
  • @RobbieGoodwin I don't expect it to. I was merely testing the LEDs with it because I was perplexed as to why the 9V PP3 didn't fry them – Harry Spencer Oct 26 '22 at 22:53
  • Sorry, Harry. How did that Question not depend solely on the expectation that 3.75V and 9V batteries would do the same job? Yes, you suggested further logic and how does that change the premise? – Robbie Goodwin Nov 19 '22 at 17:41

3 Answers3

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The 9V battery can't source much current into a load or short circuit. Approximately 500mA depending on many things.

Lithium-ion batteries, depending on many things, could potentially source tens of amps into a load or short circuit.

They have different internal resistance.

Davide Andrea
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Justme
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A fresh alkaline 9V battery can easily fry a lot of smaller LEDs. It can deliver a couple of amperes into a short circuit. A zinc-carbon 9V battery has a considerably higher internal resistance (hence their unsuitability for certain applications) so the LED may well survive (although possibly with damage).

Not-so-fresh cells have higher internal resistance, so even if they measure a similar open-circuit voltage, that does not provide a good indication of what they will do when connected across the hapless LED.

Most Li-ion cells can deliver quite a bit of current (low internal resistance) so it's not surprising the LED did not survive.

At the other end of the spectrum, a CR2025 or CR2032 cell has enough internal resistance (and a lower ~3V voltage) so that most LEDs will survive without damage, so one sometimes sees vendors at electronics markets demonstrating LEDs by connecting them directly across such a cell.

Spehro Pefhany
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    Those cheap keychain lights also often consist of just CR2032 cells and LEDs. I had one without even a switch - squeezing it in the right place simply made the LED legs touch the cell. – user253751 Oct 24 '22 at 17:17
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given the fact I hadn't used a resistor

You did use a resistor. The relatively high internal resistance of the 9V battery limited the current. Charged Li-ion cells of similar volume have internal resistance lower by a couple orders of magnitude compared to a 9V battery.

Kuba hasn't forgotten Monica
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