I don't think I posted this stuff previously or if I did it wasn't all together like this. Anyway, here goes. LOL!
I bought 2 of these 50 amp 20S BMS on ebay. The one on the LIPOs has been in service since I moved to 20S and the LION BMS has been in and out of the scooter a few times, but has seen 2 or 3 months of use. Before that, I balance charged the LION packs every couple of months. They work pretty well and are quite compact at about 3.5"x3.5". One is in the front compartment protecting my LIPO packs and the other is in the back battery box protecting the LION packs. Considering that they cost me a little over $60 each and that they keep the cells all balanced pretty nicely and have status LEDS for each channel, I can't complain about how well they work. I have wrapped them in Kapton tape every way I can except at the balance connectors to protect them from shorts and possible water invasion.
I have 3 LIPO packs in the front battery bay. They are nearing end of life as they are only delivering about 4000mah each and were originally 8000mah. I'll eventually replace them with laptop cells. The LION cells are doing most of the work now anyway. I took the balance wires and soldered 3 sets of 5S balance cables to them. That allows me to monitor all 3 LIPO packs via a single BMS. It's a bit of a spagetti wire mess as a result, but works reliably. I have read several postings on R/C sites and elsewhere about parallel balance charging LIPO packs and that it works quite well. The BMS is in there under those wires and it has performed quite well in keeping all 3 packs balanced. You will notice that connectors are labeled 1 to 4. It's is UBER important that the balance cables go on the correct balance connector on each battery pack. If you connect one to the wrong place, things go badly for the balance connectors. Those tiny pins in there are good for maybe 1 amp so they melt down instantly if something is plugged in wrong.
This is the back battery box. I have one of those 50A 20S BMS monitoring everything back here too. It's more wires running around in the box, but it's the exact same idea as the LIPO setup, but with three more packs. I had a couple of cells in here go south on me so I had disconnected everything so I could find which were the bad cells. Anyway, I took this picture and the above one because I had just reconnected the BMS to these packs tonight. There is a couple of disadvantages to parallel balance charging multiple cells, but anytime you have cells in parallel you have this problem. And that is the good cells mask the bad cells to some extent and a really bad cell will drain down the good cells since the bad cells may be partly shorted internally. With all those connectors on each pack, I can disconnect a pack from the whole and find the weak/bad cells. If these batteries were all welded together, that would be impossible. The BMS is in the front of the box under all that wire mess. I considered zip tieing it all up or just shortening the wires to make it all as neat as possible, but I don't really care and all of it is covered up anyway.