I finally built up my custom 2S3P battery pack from NCR18650BE cells pulled from (2) Xiaomi 10,000mAh Power Banks (purchased for $12/ea), a 7A 2S protection circuit, a DX 20ga cable, and a hobby charger balance lead. It wasn't all that hard to build. The nice thing is that the cells come welded together in parallel out of the power bank. I used strips of copper cut from an old Dell PC heatsink to connect the bottom of the pack pairs. Had to solder the original "strip" material a few times as it rips so easily. I had to reset the PCM circuit with a charger before I could measure output voltage; that threw me off for a second.
Photos of the "copper strips" used to connect pairs (notice the ripped stock strip, which I later soldered back together). The trick of sticking the soldering iron between the copper and battery then removing iron and pushing together quickly (I used a popsicle stick) worked great, but you have to get things aligned right or else you probably won't get it back apart!:
All Wired Up:
All wrapped up:
And testing it out after a full charge (which takes a LOOONNGG time on a wall wart "1A" charger!) on my BT40S modded to a calculated 4A:
This is the highest current measurement yet on this light! Voltage is even high enough to keep the lights battery level indicator blue instead of red.
For anyone wanting to do this "build" here is a wiring diagram that may help:
And a blow-up of the PCM board:
-Garry