Asked by a friend to make a battery pack for his new TDI Pro. 4 cell 16.8 volt, most important thing is to buy brand name batteries from a well know source. I like Samsung, Panasonic, Molice, Epoch, Sony, Nitecore.. And most of the time I purchase from the 18650 battery store. One thing as your assembling the battery pack, keep a bucket of water and a fire extinguisher near by, Li-ion batteries can be very dangerous and can short out fast causing injuries and fire.  I know TDI Pro's are bec