I believe the married battery thing goes back to mechanical mods. If one battery is weaker than the other the stronger one may over stress the weaker one. I have a Cuboid (which I never use) that is regulated and appears to monitor each cell separately and may determine the cut off voltage by which ever battery is weakest. I think this applies to batteries wired in series. There is at least one 2 cell regulated mod on the market that is wired in paralell (lower max watts) and I believe it doesn't care if the batteries are matched. Having said all that this is not my department. Somebody here will know better than me and correct me or explain it better than me.
It did start that way, then ECF proper imparted a "no stacked battery mod" policy for quite some time because there were so many problems with them.
These newer variable devices may or may not monitor the batteries individually, but just at voltage, not amperage, which is where the problem lies.
Say you've got your head up your .... one day, we've all been there so no offense meant, and grab two 25 amp batteries and an Ultrafire by accident that's rated at 7 amps. The board sees 4.2v at all three batteries and says game on! You hit it at 150w, maybe the board goes into protection mode, maybe it doesn't, but either way you are asking for close to 14 amps from a 7 amp battery! Nothing good is going to come from that instant, it might vent the battery, it might vent one of the other batteries as they try to cover the load, lots of what if's.
Now take three of the same battery, one brand new, one used hard for three months. They all, again show 4.2v to the board, but the abused/used one will not have the capacity of the newer ones. Maybe nothing happens, maybe it vents, who knows! But why risk it?
If you keep them married they all wear evenly and die evenly, not one in particular is the known weak link.
