https://www.youtube.com/watch?time_continue=7&v=0Z5QWzSSvdI in the video this guy charges his ebike with a server power supply and Battery charger 900W DC-DC Boost Converter. you set the charger to 41 or 42 volts my pack is 36volt 20 cell total 10 cells in series and 10 cell in series both of the sets of 10 in parallel the max charge per cell is 4.2 volt x 10 cell 42 volt if you wanted to charge 1 cell set charger to 4.2 volts 4 cells to 16.8 the key is you need a charger that cuts of the voltage at your max battery pack voltage mine is 42 so you dont have a early 4 of july in this side your home. Jehu Garcia video help me out https://www.youtube.com/watch?v=ZBf_qXEYUoM Check out his electric Samba bus. His boy sell batteries on ebay he is the real deal http://stores.ebay.com/tomhasgooddeals i bought 5 sets 36v 18650 20 cell battery pacts for $125 with bms and t60 samsung all new love them! 
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hope this helps
This buck converter idea seems interesting. I’ll have to make sure that there are safety mechanisms in case of bms failure.
you set the max voltage and it will cut off for you. Dont trust BMS. I love the buck it does not come with a case and you can use and old pc power supply or better would be old serve supply used what you have on hand bro. I can charge my battery in 30 mins. The buck cost me less than $30