Unless you know the batteries you are using and what they are rated for, the Ohm's of the coil and the amp load that it's putting on the batteries, you are playing with fire.
As suggested above, get an ohm meter, measure the resistance of the coil. Look up the specs on the batteries you are using and see what they are rated for, then plug the volts and ohms into an ohms law calculator and see what's really going on!
Also, are you keeping the batteries in matched pairs, meaning they are labeled then charged and discharged evenly? Or are you just grabbing batteries at random and popping them in there? If it's the later, you are heading for even more problems!
I REALLY, REALLY don't like all these new stacked battery mods that are out now! It's going to cause nothing but problems.