I opened a new bag of the ten for 5 bucks DX keychain lights to replace the one on my keyring. Nine of the ten were just as bad as the old one currently on my key ring. Since I got the bag at least a year ago I figured the batteries had gone bad. Nope. There was no difference in battery voltage between the good one and the bad one. Then I cleaned the led leads with an eraser thinking they had oxidized and didn’t have good contact. Same result. Any ideas on what’s going on here?
Maybe the one on your keychain is still working well...
The old one on the key chain barely lights up any more. There was one in the bag of ten that worked right. The rest were mostly unusable.
Hmm... Have you tried connecting it to the battery without the switch?
Do you have a link to those light? Just curious.
Aaahhh, have seen them before 
Thanks for the link.
Well, plastic “switch”, cell and emitter. If something is wrong - clean contacts 
If still nothing - dead cell. Dunno you have dead LEDs :~
Even if the voltage is the same, you should try the good one’s cells in the bad one . If it’s still dim, then apparently you got high Vf leds. Bad luck.
That’s what it looks like. That one led works well with the other batteries but not the other way around. Guess I’ll just toss them. 
Guess if it isnt to difficukt you could swap out the leds there cheap?
Cool, thanks for idea!
Im ordering bunch of green, red, uv and ir 5mm leds and bunch of those 5mm button lights and will replace leds and use them in the field with red/green/uv :)!
Thanks again for the obvious tip!
http://www.ebay.com/itm/270965902053?ssPageName=STRK:MEWNX:IT&\_trksid=p3984.m1497.l2649
Bought one of these to test the coin batteries. Different voltage to a multimeter since they test under load.
I get one of those at least once a year at my dentist....