hello roopeseta
you are currently experiencing CPF overwhelm load.
Jumping directly into lithium chemistry cells within your first few posts means you are really eager to get the ball rolling on the "good" stuff, But in order to really get the good stuff, you need to understand what makes a flashlight good for the use of lithium-ion rechargeable cells.
DM51 has nailed a lot of points here I won't cover again, but take these things into consideration:
So the voltage regulators will make up for the missing volts coming from the batter?
Need to address this because the answer is yes and no and it is important to the type of flashlight you decide to buy. See, it depends on the Vf (input voltage requirement of the LED at the set current drive) and depends if the regulator is designed to boost voltage, buck voltage, or both. Lets say for example, you have an LED that is going to be driven at 950mA at about 3.8V, and you have a buck circuit. Then a pair of 3.0V CR123 primary cells will maintain above that 3.8V requirement (starting around 6V and being dead around 4V) and drive the LED bright through the duration of the run till the very end where things would get pretty dim pretty fast. Lets say you take the same light and drop a 17670 cell in it. It starts off at 4.2V, so appears to have the same functionality as the 2xCR123s did because it starts off bright. Then as the voltage of the cell withers down, around 3.95V the light starts to dim (because buck regulators usually require some "overhead" voltage in order to stay in regulation)... so instead of running for like 2 hours in regulation then going basically dead like the 2xCR123 , the single 17670 might only run for 25 minutes in regulation, then start slowly dimming through the rest of the run, reaching 50% output 2-3+ hours later. A Flashlight capable of fitting an 18650 (which is a thicker cell) might do the same but run for 4+ hours to 50%... But what if you had a regulator that could both buck AND boost voltage?!?! then you could run a power source like a single 18650 and stay in regulation even when the supply voltage is above or below the Vf of the LED... there are a FEW flashlights designed like this, they are generally made specifically for the use of an 18650 cell, and they have solid 2+ hour runtimes on a rechargeable format cell that is reasonably small. Light like the Fenix P3D, which are similar in size to a 18650 powered light, will not even fit a 18650 cell, and they won't run right on a 17670, and on a pair of RCR123s only run for about 45 minutes (those cells have lower power density)..... So picking out a light that support an 18650 cell BEFORE you start sending money somewhere is a GOOD idea as you will get the most out of li-ion technology.
Ok, i thought that 18650 is the size of 2 cr123. Am i wrong again or whats the difference with 17670 and 18650?
actually a CR123 is a ~16330 size cell... take that information and .....
As Marduke was saying, look at the numbers, first 2 are diameter, next 2 are length, the 0 on the end just means "cylindrical." We're talking Millimeters here if it helps.
If you do the math you will see that neither a 17670 or a 18650 is the size of 2 CR123s, *but they are close*
