Today was time to do the battery. As I said, I'm using 4S2P made of Sony (actually Murata) VTC6's. I got these from Lithium Ion Wholesale for a decent price, but still not cheap! At least I know they're real and not fakes. That's the most expensive part of this whole deal since everything else is around $5 to $12.
Time to disrobe the old battery. I will be re using the BMS board and balance plug. Kind of bittersweet since this was the 2nd battery I ever built in 2017 I think for the 100W flashlight as an upgrade from the 3S2P original. It's made up of green Samsung 22F and I think LG salmon colored ones of the same capacity laptop pulls. A couple of the series cells and probably a parallel one got damaged from the 10 amp input required by that flashlight and started to read under 3.5v while the others read over 3.8. I built a 5S battery for it.
Naked! You can see some of the connections for the BMS. The balance plug sticks out the back. The blue wire is the 2S, and the red wire is the battery+ (14.8v) and the black wire is battery- (gnd). On the other side is the 3S and 1S connections.
The BMS board. 15A continuous rating and it does have reverse polarity protection unlike a lot of Chinese electronics. I got it for around $5 on eBay. On the bottom you can see the connections for the balance leads. The charging input is on the bottom oddly.
The only thing I like more than my soldering iron is my hot glue gun Laying out the cells for series/parallel configuration. I use tape to hold them together before gluing. The battery + is on the bottom left, and battery- is on the top left. The 2S is on the right side. It has an extended tab for the balance connection.
Joining the batteries. Disclaimer: I don't recommend soldering. To do it right, get a spot welder made for this. You risk overheating your batteries and damaging them (over 60C to 100C is fatal to a li-ion battery), or worse, you might get a thermal runaway, which is bad. I do it this way because a spot welder is like $250 even for a cheapish decent one that won't kill you, burn your house down, or die after 30 minutes. I use a 72W soldering iron (crank it up to 350C), high quality .032 63/37 sn/pb solder (Kester 44 eutectic) and am very careful. I let the batteries cool down between each connection and make sure they don't get hot. I even use a paper napkin soaked with rubbing alcohol to cool them off if needed. I scuff the contacts with 500 grit sandpaper for a better surface to solder on.
I make the tabs from pure nickle strips. I got a 10 ft. roll of it from eBay. 8mm wide by .15mm thick. You have to get pure nickle since it solders really nice, is ductile/flexible, corrosion proof, and holds up under high current better than steel or plated steel. I drill holes where I solder to the battery contacts. It drastically cuts down on the amount of time the heat is on the battery and it's easier to get strong joints without putting a lot of heat in the battery.
Tin the contacts of the batteries and let them cool a bit before doing the connections. Be quick! Only keep the heat on enough to wet the solder. I tin the battery-side of the strips as well.
Battery output side, B+ on the top, B- on the bottom.
Other side finished with the 1S and 3S contacts. 3S is on top, 1S on bottom.
All finished! I checked the voltages at the terminals to make sure it was all correct before soldering the connections to the BMS board.Put it on the charger to verify no shorts or bad hookups. Looks good. Charges fine.
I put electrical tape and then masking tape over areas that might short. It will me semi-permanently mounted in the host. The charging leads are exiting the left side of the BMS. Those will go to a barrel jack for charging. The balance port and charging port will come out the back of the flashlight body. The battery outputs aren't on yet because I haven't figured out how to connect them to everything yet. Next was supposed to be figuring out the reflector, but I ruined it earlier because I melted it and hogging out the melted part was a fail so I'll get another one from a donor light. I won't be fiddling with it until my spacer/centering rings get here from China sometime the first week in May (I hope) though since the edges of the die touched and melted the reflector. I decided to do the fan mounting, but the fan I got was also too big. I need a 70mm, not 80mm. Hurry up and wait...Say tuned.