Hi, most likely the answer have already been covered in forum, so pardon the question 
Question: came across these two Panasonic 18650 3400mAh here and here, but is the only difference being flat top vs button top? And seems the weight are different too?
Thank you for sharing your experience and expertise!
Your links are not working correctly but if the address bar is correct one is protected and the other is not.
Thank you very much. Just tried to correct the link hope itās Ok now.
Another question, while browsing through the battery reviews here, there seems to be concerns over the different length between 18650 cells. In actual application, does the length always present challenge in choosing the right cell for particular light?
Couple example being, KeepPower 18650 is said to be rather long and might not fit certain lights. Also some notes from dealers that SRT7 requires button top 18650 to work?
Thank you for your time 
One is protected, one is not.
Some lights, such as Eagletac, do require a Buttontop cell to make contact. It is usually so you donāt short the driver if you put it in backwards.
Protected cells are longer than unprotected because of the protection circuit added to the ends of the cell. Flattop unprotected are shorter than Buttontop unprotected. Most lights donāt have any problems, but with the advent of bigger, higher capacity cells, they seem to be getting longer and fatter all the time.
When purchasing a light, ask the dealer if it accepts the longer protected cells, or check this site. Someone here will always have your answer.
Taller the battery mean more chance it will not fit to smaller 18650 flashlight tube. Extremely tight fit sometime can damage the flashlight or battery too.
Button top generally is a better option for flashlight.
This is an example of very small tube (thereās note about battery size that fit this flashlight):
DQG 18650
HKJ 18650 battery chart. is very useful.
Yeah, adding more thrills and suspensions prior to finally making the decision to make the purchase!
Yup, the SRT7 will not work with flat top cells because of its āpatentedā design.
Thanks for the informative links Calmaja. The length does seem to be a concern. Was eyeing on this Keeppower 18650 3400mAh, but noticed that the cell was reported to be too long for SC600 (as an example). Sounds a bit scary, for someone who has been trying to get a foot into the Li-Ion world.
What cells would you be referring to?
Give me a link to the 4.35v Panasonic. The only 4.35v cells Iām aware of are Sanyos.
Stop trolling, dinogirl. ;)