3x18650 question for my tr-3t6

Batteries and Chargers Rechargeable Batteries
1

Right now i use 2x batteries in my 3t6, but have a question about using 3 batteries. I’ve read that you should use matched cells, with similar mAh.
-is that correct? Even when using protected cells?
-i bought trustfire flames 2400 mah from dx maybe 1,5 years ago, i’ve used them in my old DSLR (nikon d1h, replaced 6x ni-mh with 2x18650) but haven’t taken care of them at all, so i guess i should treat them as totally different cells from my new tff 2400 that i got from tmart yesterday?

I’m planning on building a battery tester from an arduino within a couple of months so i can test my batteries.
If i get some laptop pulls or cheap 18650 with flashlights, even if they are crappy, they should work okay in 3x setup and maybe not high mode (~1,6 a) as long as they are matched, is that a reasonable thought?

And also, if my arduino build fails, can someone link me a good budget battery capacity tester?

2

So i haven’t been here long, but here is my take on it.

Get a DMM. Digital Multimeter. Check voltages often to see they are around the same voltage. ESPECIALLY in multi cell flashlights.

DO NOT MIX CELLS. Even when they are protected. With the lower end batteries you have no idea who makes that protection circuit and how well it works… in some cases if there is even one installed.

Don’t let them go below 3.0V or above 4.2V for Li-Ion. Get a good charger.

Pulling laptop cells is tricky, and you need a DMM to ensure that no cells are completely dead, which is often the case with tossed laptop cells.

There is a lot of threads on this topic. Do a search on Li-ion safety.

Stay safe.