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I have made a rechargeable torch using 4 LEDs in parralel and an 18650 battery. The battery is charged by using a TP4056 module. Due to the fact that I got 10 for 1 pound on ebay it does not have over discharge protection. What is the minimum voltage I can take the battery to. As the 4 LEDs wired in parallel are connected to a 100 ohm resistor will the LEDS switch off when the voltage is too low ?

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    \$\begingroup\$ "18650" is a size, not a battery chemistry. \$\endgroup\$ Commented Aug 5, 2018 at 12:47
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    \$\begingroup\$ But all 18650 battery's are Li-ion \$\endgroup\$
    – F.Ahmed
    Commented Aug 5, 2018 at 13:01
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    \$\begingroup\$ Unless they're LiFePO4 or SLA. \$\endgroup\$ Commented Aug 5, 2018 at 13:05
  • \$\begingroup\$ Normally, you refer to the datasheet to find out the minimum voltage. Since you don't have a datasheet, use 3.5V as the minimum. Also, there are a lot of unbelievably cheap crappy lithium ion batteries out there. If you got 10 for a pound, chances are good that the batteries are complete junk. \$\endgroup\$
    – user57037
    Commented Aug 5, 2018 at 15:28

2 Answers 2

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As already mentioned by 'Transistor' your battery will indeed be completely drained by the LED's, which is not a good thing. Since you took the battery out of a power bank that you got from Ebay, you don't have any information about it. So in advance, please excuse my 'simplistic' solution to your problem:

You could buy a ready made protection module from Ebay. Something like this or this for instance. They look like this:

enter image description here

So it fits the shape of your battery. As you can read in the specs on Ebay (which might or might not be applicable for your specific battery) the cut-off voltage of those modules is somewhere around 2.45V - 2.55V. That works for a battery that is rated around 3.7V.

EDIT: To be able to charge your batteries correctly, you'll need to know their rated voltage. Please be careful as you charge them, as charging them in the wrong way may damage them, set them on fire or make them explode...

Please be safe, those batteries can be dangerous! Hope this helps.

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  • \$\begingroup\$ You said the cut off was 2.5v, isn't that the cut off for an LED ? \$\endgroup\$
    – F.Ahmed
    Commented Aug 5, 2018 at 13:51
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    \$\begingroup\$ The 2.45-2.55V volt I referred to was only about the battery. The cut-off of an LED depends on the type and color of LED that you're using. But they don't call that cut-off when you talk about LED's. I think what you mean is the forward voltage, or Vf. But that's a whole different topic... \$\endgroup\$
    – MartinF
    Commented Aug 5, 2018 at 14:22
  • \$\begingroup\$ My LEDs are the white unbranded ones from China available as 500 for £3 \$\endgroup\$
    – F.Ahmed
    Commented Aug 5, 2018 at 14:35
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    \$\begingroup\$ If the cut-off voltage of your battery and the Vf of your LED is the same, then that's a total coincidence. They don't have to be the same, but it's not bad if they are. It doesn't really matter. If you shared the information about your LED's because you have a question about them or about how Vf works, I'd suggest asking a fresh, new question on the website about it. \$\endgroup\$
    – MartinF
    Commented Aug 5, 2018 at 17:12
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    \$\begingroup\$ Do leds still take energy when they are not lit due to under voltage ? \$\endgroup\$
    – F.Ahmed
    Commented Oct 9, 2018 at 18:49
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I have made a rechargeable torch using 4 LEDs in parralel and an 18650 battery. The battery is charged by using a TP4056 module.

OK.

Due to the fact that I got 10 for 1 pound on ebay it does not have over discharge protection.

I don't know why you mentioned the price or eBay. Those are irrelevant details.

What is the minimum voltage I can take the battery to.

This should be specified in the battery datasheet.

As the 4 LEDs wired in parallel are connected to a 100 ohm resistor will the LEDS switch off when the voltage is too low?

No. There is no switching element in your circuit. Current through the LEDs will fall but will continue to flow and discharge the battery.

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  • \$\begingroup\$ I took the battery out of a power bank and there is no writing on it so how am I supposed to find a data sheet for it ? \$\endgroup\$
    – F.Ahmed
    Commented Aug 5, 2018 at 13:05
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    \$\begingroup\$ @F.Ahmed, I don't know how you are supposed to find a data sheet. Do you know how anyone else is supposed to find a datasheet based on the information you have presented? I would say the minimum voltage should be somewhere around 3.5V. But without a datasheet, who knows. \$\endgroup\$
    – user57037
    Commented Aug 5, 2018 at 15:31

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