I recently bought a thrunite tn12 (2014) and two thrunite 3400 Mah batteries for it. I just saw nitecore has a 3400 Mah battery as well and started to wonder, which is better? Any help on this subject guys? Much appreciated.
Avoid any of them with the word "fire" in the name. Seriously.
Whichever ones you use, make sure they are internally protected. Not only does that protect you from them rupturing or catching fire, but the overdischarge protection will make the cell last longer. I use Orbtronics which are Panasonic cells with protection.
This is one place where buying the cheapest available or buying on capacity alone is a bad idea.
Also, buy a really good charger for them. This is another area where the cheapest is not a good idea. I am partial to the Xtar chargers, the ones I have will charge any cell from CR123 up to 18650 and can also be used as a standalone USB charger with a cell in the bay and you can slow or fast charge (slow charge whenever possible).
AW brand cells are very good as well. I have several dating back to 2010 that just keep going. Japanese components but are assembled in China. Lighthound is a good place to buy. Candlepower Forums is also a wealth of information and reviews.
I think Panasonic and Sony 18650s both have Positive Temperature Coefficient (PTC) fuses and Current Interrupt Devices (CID). So if safety is your primary concern - and it should be - a battery with both a PTC and a CID should be used. A PTC fuse works on heat. Should the battery get hot for any reason it will not conduct current, the primary source of fueling thermal runaway. A CID works on pressure. Under over charge (which can happen for a number of reasons and can even happen internally or from other cells in parallel) a cell will begin to off-gas and build internal pressure. When the pressure reaches the safety level a little metal disk pops much like those old fashioned hand held clickers and the battery will no longer conduct current.
NASA usually buys one of those two 18650s for their space application batteries. They are screened over and again for reliability and the best cells go into space.
I have a couple Surefire cells that came with my light and I bought a couple Panasonic cells. I cannot discern any appreciable difference in the two and they both work fantastic. The only charger I have for them is the Surefire that came with the light.
Another good forum is budgetlightforum.com. TONS of info.
While I have a few protected cells, I've started using 18650's salvaged out of laptop batteries. Carefully pry open the battery case and you'll find 8 to 10 good cells. Usually there will be one cell that won't hold charge. Others are fine.
I took cells out of a Dell pack that are Samsung 22E. 2200mAh. Work well and you can't beat the price.