This is a battery pack to a heated jacket. The led lights (circled in red) should light up when charging and when the button is pushed to change levels. The pack is still heating but the lights don’t work. I don’t see anything obviously wrong with the pcb but I don’t know much about pcbs so thought maybe one the brains around here might give a possibility.
It is 18650 battery and produces light so it’s sorta a light 
Well, those are LEDs, so you’re in the right place!
If you’re certain all the ‘real’ functions work, can you get some bright light and magnification on the LEDs to see if they look burned? Or perhaps tiny DMM probes to test for Vf across them?
Assuming it works otherwise, maybe since this is on clothing, (and with all this “global warming” going on, I personally would LOVE a “heated jacket”!!! AND pants!!) there’s some “stealth” or “polite society” mode where the lights stay off on purpose…
Since “RTFM” is such an anachronism any more, you’d probably have to call the manufacturer for a clue. That’s where I’d start, anyway; if it’s bad, they can arrange an RMA for you.
Mmm, heated pants! (But that’s only funny to Brits and various colonists…)
Anyway, in all seriousness, can I suggest you try to figure out what charge level you’re getting on the batteries? (Might involve de-soldering, unfortunately.) My concern is that if one thing is going wrong on the board, the charging circuit might be going bad as well, which could lead to cells over 4.2 volts. Which would be a huge problem. (Quadruple that if it’s in your pants.)
But, it’s supposed to be a heater, so if the cells are overcharged, it would just heat up faster! J)
can’t really tell, this side of the board has no obvious problems, that we can tell from a pic. but i suspect other side of the board has components too.
Yes the other side does as well. I guess since I’m going take the batteries off I can get a good picture then.
I actually have considered how horribly wrong things could go if those batteries ever got out of balance. Bright side is,its not a sealed case so its only some extra heat instead of blowing up.
I’lll mess around with my DMM and see if I can find were the current is stopping.
Edit
I just realized there is a date on the PCB…hmm I wonder the batteries are that old too . Maybe its not the board at all.
well, i have little doubts the board has protection for each cell, nothing will go horribly wrong, (assuming protection works) and even if ballance will be off by a lot, it will just use les of its capacity, and shut off sooner.
however should protection fail, cell may explode.
1st member in the brotherhood of the exploding pants.
Did you use a DMM to test if the LED’s are getting and voltage when you press the test button (when they should light up)?
Not sure where in ohio you are and dont really think there’s much to try and diagnose over the internet but it your somewhat local and want to drop it off I can take a look at it and probably get it going again, it might take a total rebuild tho.
My dog decided the my DMM leads would make a good chew toy so no testing for a few days.
Maybe I’ll breakdown and finally order a good DMM from Amazon. I’ve just been using a cheapy and not sure its best for this anyway.