Is there a risk of 18650 li-ion battery explosion in a low current application?

Question

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:
(values of some resistors/trimmers are not entered yet but I will use as high values as possible)

Answer 1

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.

Answer 2

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.