Fenix brand 18650s

The Fenix name brand 18650s have numbers-- 2500, 2900, 3500 and now 5000.

Do these matter? Can I mix them in a light?

Yes they do matter. If you mean mix multiple 18650s in a flashlight? Not a good idea in my opinion.

There's plenty of threads about how to choose and use your 18650s.

Haven't been reading posts about new battery offerings lately but I doubt the 5000 mAh is a 18650.

I own many Fenix 18650s and they have worked well for me.

My 2 eurocents!

The numbers you quote are for the capacity of the cell. Do not mix cells of different capacities. What will happen is that the lower capacity cell will go past its safe discharge point without you knowing it, and could possibly become unstable.
It sounds like you should do some research on here before you start using these things. They can be dangerous if used inappropiately.
P

Simple rule is that for any multi-cell setup, the individual cells should be as identical (brand, voltage, capacity, state-of-charge, age, use pattern, etc, etc) as possible or practical

Actually, my simple rule is to avoid multi-cell setups whenever possible, but there ya go

archimedes said:

Simple rule is that for any multi-cell setup, the individual cells should be as identical (brand, voltage, capacity, state-of-charge, age, use pattern, etc, etc) as possible or practical

Click to expand...

Yes, basically, buy the same cell in a batch of however many you need, e.g., 4 cells of the same type. Then, always use them together as a set.

Before recharging, check the voltage levels of each with a DMM. They should all be fairly close (within 0.02v preferably). If one of the cells is showing much less voltage than the others, it's probably time to buy a new set of cells.

You can mix them in a light IF.... they are all used in parallel and at the same resting voltage and the device isn't a high drain device as the voltage across all cells in parallel should be the same if they aren't the same when put together you will have higher voltage cells trying to charge the lower voltage cells and that could be dangerous if the difference is large enough. If the light uses cells in series then you need to match them to as close as identical as possible as if one cell is stronger/more capacity than another then when the weaker cell is depleted the stronger cell will try to reverse charge it and that can lead to fire or more perhaps.
One other thing you don't want to discharge them too low as some cells can take it better than others and when they are all recharged together it could be a problem if cells are weakened they may heat up more when being charged.

I was trying to keep things simple for OP

archimedes said:

I was trying to keep things simple for OP

Click to expand...

Yup.... complicated usually means those who aren't careful can find a way to obey Murphy'S Law.

archimedes said:

I was trying to keep things simple for OP

Click to expand...

And a fine job you did.

Those of us who are electrically stupid appreciate it.