In an upcoming build video. I'll show you how I built a dual power fume extractor (the type used to suck away fumes while soldering) The fume extractor can be operated from either 3x 18650's Or a 12v power brick. The low volt disconnect is utilized to prevent both power sources from being connected together at the same time. And it also provides protection from over-discharging the 18650s.
Check out the Low-Volt-Disconnect project here
Dual power fume extractor project: Coming soon!
Would this work as a battery 2 Battery charger ?