I keep seeing people getting confused with BMS setups. Building a new board and trying to understand why they need a BMS, which BMS?, will it be for charging and discharge?, or just charging? How do they wire it? etc etc.
You really don’t need one in most cases.
My board charges directly from a £14 lipo charging brick from ebay (2 packs of 6s 15000mah each). You can get them for 8s, 10s, 12s… I use this http://www.ebay.co.uk/itm/21-6V-22-2V-2A-DC-25-2V-Three-stages-Lithium-Battery-Charger-14500-14650-18650-/182209541446?hash=item2a6c88f546:g:0XQAAOSwNKRXjIVP
It means I can install a 3.5mm power jack on my board and simply plug in the charger and walk away. When it’s charged the LED on the charger will go from red to green.
I’ve been using lipos for years and years, well before eelctric skateboards. Unless you have a bad pack, or you regularly run your packs below 3.2 volts per cell they will stay mostly in balance. By mostly, I mean close enough it makes zero difference. If you have a bad pack then no amount of BMS balancing will make it good. If a pack has a lazy cell i.e. all at 4.2v and one at 3.95v , it will never really recover.
Set your ESC to cut off before your batteries get to 3.2v (at rest) and you can’t flatten your lipos. On most ESCS choose the middle to cautious cut off setting. If your pack doesn’t seem to run for long before cut off, measure the voltage at rest, and if it’s ok, reduce the cut off voltage, then retry. This is the key. Over discharging packs kills them. You don’t need a BMS to stop this happening, you need to use the correct low voltage cut off on your ESC.
When you charge packs, they equalise automatically. Once near full they’ll all end up at 4.2v per cell. If you have a duff cell then you can of course run into trouble as it lags way behind, but I’ve not had this happen on probably 40 packs I currenty have in use (heli’s quads, cars, esk8).
With some packs (heli/quad) I “treat” them to a balance charge once or twice a year, but no more. It takes ages, and makes near to no difference. They only real benefit is it will show if you have a duff cell.
For the low cost of lipos it’s really not worth obsessing about balancing.
Sure, if you have spent a lot if time and money making a 18650 slim pack, I’d probably install a BMS too - to protect your investment, and you are using MUCH lower Mah cells than big lipo packs, so you run a higher risk of knackering them all if one starts going down as it will go from full to empty much quicker. Plus it can be a fun project. But I’m seeing lots of people building boards with cheap lipos being told they need a BMS. You just don’t.
As a case in point , I’ve just metered my two 6S Turnigy Multistar (cheap) 15000mah packs (normally connected in parallel but unplugged for measuring voltage). Used all last year, never balanced, always charged as above.
Pack 1: 4.01 4.02 4.00 4.02 3.99 4.01 Pack 2: 3.95 3.95 3.98 4.05 4.04 4.05
Pack 1 is near perfect, Pack 2 is fine, not quite the quality of pack 1 but easily good enough - a max of 0.1v cell difference. Those packs have been sitting unused in my garage since last summer. All the cells will equalise under charge. And because I don’t run my packs under 20% of their capacity, it doesn’t matter that I’ve got a 0.1v cell difference.