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I have several used 18650 batteries from my old laptop battery. They all look OK, I have charged them and discharged. The capacity is around 1700 mAh, nominal capacity was 2500 mAh when new.

I'm thinking about how to use them.

There is wall clock in kids bedroom. I'm about to add some LEDs to it so the clock is slightly illuminated during night. Just a little bit so it is not too bright to disturb sleeping but it is just bright enough to barely see clock hands. The current consumption will be around 1 mA.

But I am pretty worried about safety, which is priority #1 for me. The last thing I want is to put a "incendiary bomb" in kids bedroom. Is there a risk that 18650 battery can explode or catch fire in this situation? Or is it better to use 4 NiMH batteries instead? (and dispose my old 18650 batteries).

This is schematic of my circuit:
enter image description here (values of some resistors/trimmers are not entered yet but I will use as high values as possible)

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  • \$\begingroup\$ Do you have any source (book, article, etc.) that correlates low amps and exploding 18650? Short-circuit a battery, sure it can lead to fire. Penetrating lithium with a knife, sure it can lead to explosion. But low amps? \$\endgroup\$ – thece Jun 13 at 7:48
  • \$\begingroup\$ @thece No, I do not have any such source. All I have is knowledge that li-ion batteries can catch fire in some (specific) situations. Maybe the risk is not the low current, but simply just the presence of such battery in kids bedroom. Maybe I'm just a extreme worried/careful parent... \$\endgroup\$ – Chupacabras Jun 13 at 8:28
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    \$\begingroup\$ Do you let your kid(s) handle or use a smart phone? They also have lithium batteries in them. Are you worried about the phone exploding whilst it is being charged? \$\endgroup\$ – Michael Karas Jun 13 at 8:41
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    \$\begingroup\$ Maybe you're worrying too much but it's a pleasant change from all the questions asked by people building their own Li-ion chargers without having a clue what they're doing. \$\endgroup\$ – Finbarr Jun 13 at 8:42
  • \$\begingroup\$ @MichaelKaras Are you saying that the OP's circuit and design are as safe as a commercially manufactured smart phone? You seem to imply that but I don't think you have justified the assumption. \$\endgroup\$ – Elliot Alderson Jun 13 at 12:11
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No one here will give you a safety guarantee!

There simply are no guarantees that can be given.

However what you describe is far from the typical scenario where lithium based cells cause safety issues. Safety issues generally occur:

  • when fast charging

  • when charging doesn't stop when it should have

  • the cells are mistreated

  • there is too much mechanical stress on the cells (Samsung Galaxy Note 7 debacle)

If you use a safe charging solution (a dedicated chip which stops charging when needed) and a battery protection solution (like the DW01 you propose) then the risk of any incident happening should be fairly minimal.

Personally I would not consider the risk large enough to worry about.

Oh and a small tip: If you do decide to go for the NiMh cells instead, be sure to use "Low Self Discharge" cells for that as most "normal" high capacity NiMh cells have a very high self discharge. Some of them leak themselves empty within a month.

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  • \$\begingroup\$ I would charge that battery separately, not in this circuit. I would simply take out the battery and charge it in regular charger. When I charge it (by 1A current) then the battery is only very slightly warm. I think I made everything in the circuit to prevent short circuit, over discharge. At this point I am not worried about my circuit, just the battery :( It's a shame you won't give me any safety guarantee ;) \$\endgroup\$ – Chupacabras Jun 13 at 8:41
  • \$\begingroup\$ @Chupacabras Remember that safety guidelines apply not only to intended use but also to foreseeable misuse. What might happen if the device gets knocked off a table on to a hard floor? If a glass of water is spilled over the device? \$\endgroup\$ – Elliot Alderson Jun 13 at 12:12
  • \$\begingroup\$ With "Regular Charger" I hope you mean Regular Li-Ion Charger. \$\endgroup\$ – A.R.C. Jun 17 at 6:16
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It depends on battery chemistry also. IMR and INR are very safe, while ICR are dangerous. Since you already have a protection device on the input side, I would think that even commercially available lights aren't any better made. Except they are probably made from Alu tube case that absorbs the explosion.

What is more concerning about your application is the current regulator, which is linear and it will heat a lot, the efficiency of your light will be very low.

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  • \$\begingroup\$ which is linear and it will heat a lot, the efficiency of your light will be very low OP mentioned that the current consumption is around 1 mA so the power dissipated should be very low even despite the low effciency. \$\endgroup\$ – Bimpelrekkie Jun 13 at 8:15
  • \$\begingroup\$ I doubt I could get better efficiency with SMPS. Li-ion battery voltage is between 4.2V-3.0V (mostly around 3.7V). LEDs I have have voltage drop around 2.7V @ 0.3mA in one LED. So there is 1V for current regulation in resistors. My total current will be around 0.8mA (so maybe 0.15mA per LED). And it will be only during night hours, maybe 8 hours a day. \$\endgroup\$ – Chupacabras Jun 13 at 8:16
  • \$\begingroup\$ @Chupacabras Then it doesn't worth spending on switching converters. If you decided to use Li ion battery, then I would say that there are also batteries with built-in protection circuit, no need to solder + Samsung INR are very safe, you can see on some videos that they don't explode even short circuited. But the most probably hazard is when you charge the damaged batteries. \$\endgroup\$ – Marko Buršič Jun 13 at 8:31

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