Like I said, with 40AH capacity, you wouldn't even need a BMS because you'd have so much capacity. Get 36v of it, buy 12 individual chargers at batteryspace or all-battery.com...the 6 amp or 2 amp ones. They will balance the pack individually every time you charge it. There you go. Only use 20 - 25 AH each time and you'll have a pack that will last 5 years or more. You'll get 25 *2 = 50 miles on the pack without pedaling...more if you pedal. That shouuld be enough for just about anybody. And you are only using 50 % so it will last a lot longer than a ping would.