I am not going this route, I would just buy a BMS from Besttech for my new 18650-25R's sitting in front of me right now. Debating which way to build it with my tab welder from https://avdweb.nl/popular/spotwelder/diy-spot-welder I like 10S because it keeps the speeds down for me, but I do like the occasional twist if you know what I mean so I just might go 48V(13S) but I dont like the # 13 so I might go 14S but my Makita's are all in 5S2P so there is that, and I think 15S is too much. There is the throttle hack I can do to limit the throttle to what I want in terms of speed. Plus with the extra 5S to my existing 10S, I would get more watts but not more discharge current, because there is no increase in parallel strings, just an increase in voltage, but additional 5S would mean each 18650 can would have a lower discharge because more 18650's in series in each string. I hope I am not fuckingthat up, I am too often wrong in my daydreaming plans.
You are right though, this threads called the "Cheapest Active Cell Balancer Project" and its great for small packs, like my bicycle light, which is one Makita of 5S2P, 21V 3Ah hose clamped right to my hand bars. Usually I balance manually with RC Charger. Another option is $2 cell phone charger from the Thrift store. Which brings me to an idea I've had for months now. Buy a bunch of them cheap cell phone chargers, not sure what voltage they are and use them to balance charge to the same voltage if your cell phone chargers are all the same voltage.
I am leaning away from electronics work now. I did look into FOC controllers (better eff. and stuff, search Justin_le I read thatshit last night, forgot already ), but expensive. Next is the Sinewave but the price is higher then cheap generic. Lebowski Controller is a DIY, I've looked into, not into soldering up pcbs, I am comfortable with the noice cheap generic trap trapezoidal waveform produces. What I am into is modifying the current sense shunts. Can't believe I put up with shitty 30A's stock, upped it to 45A stock with shunt mod to nonmeasured guessimation 55-60A. 3 shunts at 45A add one in parallel so 60A.
Rambling and bored gets me chatty. Gotta ebike for some food and Home Depot.
Paralleling batteries you need to be careful, they need to be very close in voltage. I like to do it within 0.10V otherwise the equalization of current/voltage between the two battery packs will highly discharge one area of the battery to equal out the other part. Basic info for the LiPo balance charging, on them parallel boards with lots of tiny connectors on them. Hobbyking has them.Wait actually if you can parallel plug batteries on modules it should work no ? I mean : You usually use a single BMS for a whole battery pack. Can it be made ?
My battery pack I just solder on the used stuff I buy, the new pack will be tab welded so I am used to paralleling, reconfiguring and such. Have never done a BMS before, like I said manual balance every now and then with ultra stable Li-Mn chemistries, dont go too low because they unbalance.
Gotta go, rambling over.