I’m modding a BJ08A, and I put one of the Nanjg 2.8 amp drivers from FT into it. I had to reduce the diameter of the driver slightly by filing to get it to press-fit into the BJ08A’s pill, but the driver is in, and soldered at 2 points to the pill.
I tested with a protected TF 14500, and it works, but I’m finding that if I screw the tailcap all the way in, the light won’t come on.
If I back the tailcap out about 1/2 turn, it turns on, but I have to click the switch off then on to get it to work.
The strange thing is if the tailcap is on, and I back the tailcap out, but don’t click the switch off then on, it doesn’t turn on.
It’s like, whatever the problem is, it has some kind of memory, like “oh, I’m not going to work, even if you back the tailcap out a bit”.
Can anyone explain this behavior?
I’m charging up an unprotected battery to see if that works ok with the tailcap screwed all the way in.
Also, I’m pretty sure the negative end of the battery is bottoming out on the retaining ring inside the tailcap, so I’m wondering if I should (or if it’d be ok to) cut the inverted spring on the driver a bit to fix this problem?
Pretty sure you got a short circuit somewhere and the protection of the cell safed you from some incident. Maybe the cell is just too long, maybe the problem is somewhere else. Better figure it out before trying an unprotected cell, shorting them isnt good..
Could be that even with an unprotected cell it is putting excessive pressure on the PTC (positive temperature coefficient) resistor to make it switch to very high resistance - which it does as a safety measure to stop the battery overheating.
Thanks! That may be it. Does physical pressure on the resistor cause it to go to high-resistance? Also, is it on the spring side of the 105C drivers?
I don’t know exactly what the problem was, but I was just able to solve it.
The tailcap of the BJ08A has a threaded retaining ring to hold the spring/spring PCB in. They put a small black plastic ring that fits inside the retaining ring, presumably to prevent the spring from falling over and contacting the metallic part of the retaining ring.
The spring is one of the pyramidal type springs, plus, it’s short and kind of far away from the retaining ring, so on a whim, I removed the black plastic ring, then screwed the retaining ring back in, and then, VOILA, I can now screw the tailcap tight, and the light works!!
Now I have a fully functioning BJ08A with a triple XP-E2:
It’s driven with an FT 2.8 amp Nanjg (105C, I think), and tailcap current with an Efest IMR 14500 is 3.22 amps (so 1+ amps per emitter), so it’s bright and gets hot fast on high mode.
The PTC is usually (?always) under the positive contact of the cell, so yes it is at the driver spring end. It's a guess about pressure having an effect on the PTC - this is speculation only.
I'm not about to disassemble working cells to find out. AFAIK all lithium cells, even primary ones have (or are supposed to have) a PTC.