Perhaps the troublesome battery isn't making good contact with the bottom on the tank, if you seriously crank them down onto the battery. From what I've read, you might need to pull the center post of the battery up a tad to make better contact. The one thing I did after getting my Halo Triton was make sure the inside coils were tightened well, not over-cranked but just until there was resistance, both of my tank coils were loose right out of the box. Try swapping your tanks around on the batteries and see if you're getting the same performance on them. Can you check the coils on the mini tank to see if it's gunked up with residue? I have to dry burn the coils on my tanks once in awhile to get the buildup off them, actually I damp burn them and do quick pulses on my battery until it burns the crap off them. So far it's worked great.