This is where I buy my batteries.
http://em3ev.com/store/
Paul (the owner and far smarter guy than me when it comes to this) is a great guy that will work with you and his site is full of helpful info about every part of a battery systems and even the controller you want to use to get the most from it but ya, there are a lot wires from a BMS, a pair for every serial block that monitors cell blocks plus an overall voltage, shut off circuity, and the charger hooks to it so all the charger sees is the BMS saying 'give me juice' or 'I'm done' so the charger quits sending power and the BMS proceeds to balance the pack cells after the charger changes mode.
As for 'do it yourself' race bikes sure you can cut out a lot of safety stuff and push everything to the max like anything else for that thrill but not something I would want to build to rely on for years day to day ya know?
