Whatever you do, do NOT do this with lithium ion batteries. At low voltages, chemical side-reactions start to occur which result in dendrite formation which can pierce the membrane, shorting the battery: the result is fire. LiIon batteries generally have protection that permanently blow a fuse if the voltage goes below a minimum safe voltage, but raw 18650 cells often do not. The video above refers to NiMH cells. Afaik, this is safe to do for NiMH.