Hi guys, just joined but been lurking for a while here and I love this forum. About a year ago I got my first LED flashlight which is a Ultrafire C8 with an XM-LT6 emitter and I was floored by how bright it is for the money I paid. Came with 2 18650’s and a single slot charger. One of the batteries is giving me trouble. It has plenty of voltage (4.17v) and works in the flashlight BUT, only if I hold the light vertically. As soon as I drop to horizontal or lower it goes out until I aim it upwards again. The batteries are Energy brand eBay specials like these which I know (now) are junk. The other battery works great and I’ve since bought others and they were fine as well. This battery is unprotected so it can’t be related to that. Any thoughts on what could be the cause here? Just to add a little humor; I took the white wrap off the cell and clearly stamped is 3,800mah vs. the 5,000 they advertise. Both numbers aren’t even close but that’s beside the point 
Welcome , goldstripe .
Sounds like a contact issue .
yeah that’s the first thing I thought about but the contacts are fine on the light (other batteries work fine). there is one small ‘booger’ on the negative end of the battery but it’s not even where the spring makes contact with it. could there be something off inside the battery and I’m just SOL?
The cell is going to perform exactly the same no matter what orientation. The contents are not gravity sensitive. So as others have pointed out, it has to be the length of that particular battery. Perhaps the button top got partially compressed, so that when oriented in just the right way the contact is lost.
I wouldn’t bother with modifying the battery (like putting a drop of solder on the top). And frankly, cheap batteries like this aren’t worth the trouble. I’d invest in some better 18650 batteries.
Try a little neodymium magnet on the end and see if that helps.
it sounds like contact issues .. does it change modes if you bounce the light ,Adding length is a good idea ..
no matter what there's no sol because losing one of these cells is like losing a bad shoelace ...there's only an upside
Add a magnet ,use a longer battery or stretch a spring .......or buy another light ..hows that for a good idea ? :P
thanks for the suggestions guys. I will not beat my head into the wall for a POS battery but will try to clean up the negative contact and check the length with my digital calipers. May have a small magnet I could try as well. thanks again
Funny you should mention that. I just bought a BTU Shocker and some extra keepower 18650’s to have around. the Shocker is insane
Shocking, right :P
LMAO! Indeed Sirius9.
Back to original question. I measured both of my Energy cells and both are within thousandths of a millimeter of each other at around 66.15mm. I think I disagree that orientation doesn’t matter because if I shake the cell I can hear whatever is inside go back and forth and I bet when it goes forward (down) toward the positive end of the cell it’s losing juice somehow. So it’s going to be recycled.
Guess I’ll just have to make a bonfire and toss it in 
You can get these at several places. I know that Lighthound.com carries them. Also Harbor Freight carries them. They are good to keep around just in case.
http://www.lighthound.com/search.asp?keyword=neodymium&search=GO
http://www.harborfreight.com/catalogsearch/result?q=neodymium+magnet
thanks for that HF link. Those will come in handy for all sorts of things.
This same thing sort of happened to a certain light I have. I used to be able to shake the light quick and it would lose connection a reconnect, switching to the next mode. It no longer happens, despite using the same batteries…
With lights that utilize the inside of the tube for electrical contacts, a metal spacer touching it could present a problem. Does anyone know where one can source spacers that have a plastic/resin outer rim to prevent this?