For testing purposes not very extensive. Have seen and will probably order some of the those quick and dirty holders from Amazon. Even from the photos on Amazon you can tell the wire sizes are inadequate for much current. I've also seen some more substantial holders that look like they are designed to be soldered to a circuit board. May order some of those as well and have a circuit board made that would allow several combinations of serial and parallel configurations.
SteveG
what I like to do with the 4 slot holders is use the 2 outside slots to hold the batteries and the 2 inside slots to hold a 3.3/5vdc regulator, that way you have a decent regulated power supply that runs on 2 18650s ... Im getting ready to make another one for an upcoming mini project I have in mind.... If I remember when Im done, Ill post pic or 2...